Chapter 18
Design example
In order to illustrate the practical application of the principles discussed in previous
chapters we shall, in this chapter, describe a typical industrial design. The example
chosen is taken from a scheme handled in the previous author’s office some years ago. It
does embody the criteria in use today, although the design package would yield closer
limits. It is appreciated that a building services engineer will now use software design
packages. It is useful to see how the design values were arrived at. The buildings of a
disused factory were taken over by a chemical manufacturing company which proposed
to adapt them as a new works. Electric services were needed for lighting and power to
machinery.
The general plan of the buildings is shown in Figures 18.1–18.3, which also show the
main part of the lighting layout. As the design of lighting has been excluded from the
subject matter of this book it is not proposed to reproduce the lighting calculations here,
but it should be noted that after the number of lights needed in each area had been
calculated they were positioned with regard to the layout of the machinery as well as to
the need to maintain reasonable uniform levels of illumination.
The factory consists of an east building of two storeys with a basement and a threestorey
west building with a covered yard between them extending the full height of the
east building. There is a walled car park adjacent to the buildings and a new boiler-house
was to be built in this area. Since the existing buildings provided more space than was
needed for the new works, part of the west building was to left unoccupied: no services
were to be installed in this part but the installation as a whole was to be capable of
extension into this area.
The bulk of the lighting consisted of twin-tube 1500mm fluorescent luminaires with
some single-tube luminaires in passages and areas requiring lower illumination. A few
incandescent luminaires were provided in toilets and on stairs (not all of which are shown
in Figures 18.1–18.2). The covered way between the occupied and unoccupied sections
of the west building in which materials would be hoisted to the upper levels was lit by
three wall-mounted mercury lamps at ground-floor level and three at second-floor level.
Figure 18.1 Factory ground-floor
lighting layout
One end of the west building contained tall machinery on the ground floor and the firstfloor
slab was not carried across this. An area of double the normal height was thus left
and this was lit by wall-mounted mercury lamps at the lower level and high-bay industrial
mercury luminaires under the first-floor ceiling. The covered yard was lit by wallmounted
high-pressure sodium floodlights at the level of the first-floor ceilings. Four
street-lighting lanterns were provided for the car park, three of them being mounted on
columns on the roadway from the building and one on a bracket on the wall of the
building.
The first stage in the design was to arrange the lights in circuits and to arrange the
circuits in convenient groups to be served from several distribution boards. The lighting
would have to be divided in a suitable manner between the three phases to give as nearly
as possible the same loading on all three phases and this had to be borne in mind when
the lights were
Design of electrical services for buildings 266
Table 18.1 Types of lighting
Ref. Type Current (amps)
A 1500mm twin fluorescent 0.92
B 1500mm single fluorescent 0.46
C Wall-mounted 125W MBF 1.15
D Tungsten bulkhead 0.42
E High-bay industrial mercury with 250W MBF lamp 2.15
F Wall-mounted area floodlight with 250W SON lamp 3.0
G Bulkhead luminaire with 50W MBF/U lamp 0.6
H Side-entry street-lighting lantern with 35W SOX lamp 0.6
arranged into circuits. For convenience, the different types of luminaire used were listed,
as shown in Table 18.1.
It was decided that in this type of factory the lighting could be run in 2.5mm2 cable
fused at 15A. To allow a margin for safety and small alterations the circuits would be
designed to carry not more than 12A each. Although it was intended to use three different
sizes of mercury lamp it was felt that there was a possibility that at some time in the
future a works manager might change the luminaires without checking the capacity of the
wiring and it was therefore decided to design all the circuits serving luminaires with
mercury lamps to be capable of taking 250W lamps. Similarly, circuits serving singletube
fluorescent luminaires would, where appropriate, be designed to take twin-tube
luminaires so that the luminaires could at any time be replaced without alterations to the
wiring. Hence, a maximum number of luminaires on a circuit would be:
It was clearly going to be desirable to control more than this number of lights from one
switch and it was decided to do so by switching the lights through contactors. One switch
would operate a multi-pole contactor controlling several lighting circuits.
At this stage a check was made on the voltage drop in the lighting circuits. Probable
positions of distribution boards were guessed and from the drawings the average length
of a lighting circuit was estimated as 35m. At the time when this design was produced,
the maximum volt drop allowed on 230V was 9V, it was decided to build in a factor of
safety and allow a maximum of 6V, and it seemed reasonable to allow half of this in the
sub-mains and half in the final circuits, that is to say 3V. The single phase volt drop of
2.5mm2 cable is 18mV per ampere per metre.
Design of electrical services for buildings 268
This is a pessimistic value since the current in the circuit will reduce as current is
‘dropped off’ at each luminaire, and the design current Ib will not flow in the whole
length of the circuit. The designer may decide to average. However we took this value as
true.
Clearly the need to reduce voltage drop was more critical than the current rating of the
cable and the number of luminaires per circuit would have to be reduced. Acceptable
figures would be six fluorescent luminaires or two sodium (slightly over the design
value) or two mercury luminaires per circuit.
The loadings were now estimated for each area in a convenient tabulated form as
shown in Table 18.2.
This formed a preliminary guide. The number of circuits in each area was decided by
referring to the maximum number of luminaires per circuit as determined above and also
with an eye to convenient switching arrangements. At the same time, some margins were
allowed to make it possible to adjust the circuit arrangements later without major
modifications to the distribution scheme. It will be noted for example that the car-park
lights are not included in the table. This was because the design had to proceed before the
client had taken final decisions on all his requirements. The fact that last-minute
alterations would certainly be made had therefore to be kept constantly in mind.
The total load from Table 18.2 was 460.94A. It should therefore be distributed to give
about 150A per phase. An ideally equal distribution could not be hoped for but each
phase should carry between 140 and 160A and at the same time each phase should be
contained within a reasonably clear zone of the building. As a first step towards
achieving this the loads for each area were summarized from Table 18.2, as shown in
Table 18.3. They were then arranged in three groups for the three phases. After two
attempts the results shown in Table 18.4 were obtained.
This was not as good as had been hoped for. However, the process of manipulating the
figures had given the designer a feel for them and he realized that he was not likely to
achieve any further improvement at this stage. It would be possible to make some
adjustment after the distribution boards were scheduled and this was done next.
Table 18.2 Loading estimates for each area
Area Luminaire
Ref. No.
off
Amps
each
Amps
total
No. of circuits needed
Gate-house A 1 0.92 0.92
B 2 0.46 0.92
D 3 0.42 1.26
3.10 1
Design example 269
Pump house A 8 0.92 7.36 2
Boiler house A 7 0.92 6.44 2
Covered way C 6 2.15 12.9 3 controlled by 1 contactor
W bldg Grd flr A 24 0.92 21.00 3:1 switched directly, 2 controlled by
1 contactor
G 5 2.15 4.30 2
E bldg Stores A 48 0.92 44.2 8 controlled by 4 contactors
Ovens A 33 0.92 30.36 6 controlled by 4 contactors
Toilet area A 1 0.92 0.92
B 3 0.46 1.38
D 1 0.42 0.42
2.72 1
Maintenance A 8 0.92 7.36 2
Lockers A 8 0.92 7.36 2
Stairs D 1 0.42 0.42 1
1st Floor Covered
yard
F 34 3.0 102.0 16 controlled by 8 contactors
Side yard F 20 3.0 60.0 10 controlled by 4 contactors
Lockers B 12 0.46 5.52 1
W bldg 1st flr A 20 0.92 22.08 6 controlled by 2 contactors
E 12 2.15 25.80 6 controlled by 2 contactors
E bldg 1st flr A 48 0.92 44.2 8 controlled by 4 contactors
1st flr Maintenance A 7 0.92 6.44 2
1st flr Pump house A 7 0.92 6.44 2
E bldg Cellar A 12 0.92 11.04 2
W bldg 2nd flr A 30 0.92 27.6 6 controlled by 2 contactors
W bldg 3rd flr B 5 0.46 2.30 1
Table 18.3 Load summaries
Area Load in amps
W bldg grd flr 25.30
Pump house 7.36
Gate-house 3.10
Design of electrical services for buildings 270
Covered way 12.90
48.66
Boiler house 6.44
W bldg 1st flr 47.88
Covered yard (main area) 102.00
Covered yard (side area) 60.00
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
E bldg grd flr 44.20
E bldg grd flr 30.36
E bldg Toilet area 2.72
Maintenance 7.36
Lockers 7.36
Stairs 0.42
92.42
E bldg 1st flr 44.20
E bldg Maintenance 6.44
E bldg Pump house 6.44
Lockers 5.52
62.60
E bldg cellar 11.04
Table 18.4 Distribution of load across phases
(provisional)
Amps
Brown (Red) phase
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
Covered yard main area 102.0
Boiler house 6.4
138.3
Black (Yellow) phase
W bldg grd flr 48.66
Design example 271
W bldg 1st flr 47.88
East cellar 11.04
Covered yard site area 60.00
167.58
Grey (Blue) phase
E bldg grd flr 92.42
E bldg lst flr 62.60
155.02
The lights and switching were shown on drawings. In each area, the luminaires were
grouped into circuits in accordance with the maximum number of luminaires per circuit
previously determined. Clearly the luminaires on any one circuit must be in a reasonably
compact group. Also, although the luminaires on one circuit can be controlled by more
than one switch, the converse is not true: one switch cannot control luminaires on several
circuits unless a multi-pole contactor is used. The most practicable way of settling these
matters is to mark the circuits and switching groups on drawings of an adequately large
scale.
Standard distribution boards are available with 12 and 16 ways. Suitable positions
were chosen on the drawings for distribution boards to serve groups of 7 to 12 circuits to
allow a reasonable number of spare ways on each board. The positions were chosen to
keep the final circuits reasonably short and so that as far as possible each board would be
in the ‘load centre’ of the area it was serving. It became evident in. this process that the
second and third floors of the west building should be served from a single board, that the
gate-house would need its own board, that three distribution boards would conveniently
handle both parts of the covered yard, that the ground and first floors of the east building
would each need two distribution boards and that the cellar of the east building would be
most conveniently served from the gate-house. The information from the drawings was
then summarized in distribution-board schedules which are reproduced in Table 18.5. A
further table was then made in order to decide on which phase each of these boards
should be and this is given in Table 18.6.
The figures in the three right-hand columns were entered in pencil, rubbed out and
moved from column to column until by a process of trial and error quite a good balance
over the phases was obtained. The first attempt
Table 18.5 Lighting distribution boards
Board no. 1 E bldg maintenance area 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Pump house and changing rooms grd flr 15 2.5
2 4 Changing rooms grd flr 15 2.5
3 5 1st flr maintenance area 15 2.5
Design of electrical services for buildings 272
4 5 1st flr maintenance and sub-station 15 2.5
5 6 Pump house 15 2.5
6 6 Changing rooms 1st flr 15 2.5
7 5 Changing rooms 1st flr and stairs 15 2.5
8 4 Car park lights 15 2.5
9
10
11
12
Board no. 2 W bldg grd flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Bulkheads on wall 15 2.5
2 3 Production area 15 2.5
3 3 Production area 15 2.5
4 4 Production area 15 2.5
5 3 Production area 15 2.5
6 4 Production area 15 2.5
7 4 Production area 15 2.5
8 3 Hoist yard, low level 15 2.5
9 3 Hoist yard, high level 15 2.5
10 5 Rear entrance 15 2.5
11 4 Stairs 15 2.5
12
13
14
15
16
Board no. 3 W bldg 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Mercury over vats 15 2.5
2 2 Mercury over vats 15 2.5
3 2 Mercury over vats 15 2.5
Design example 273
4 2 Mercury over vats 15 2.5
5 2 Mercury over vats 15 2.5
6 2 Mercury over vats 15 2.5
7 3 Fluorescent production area 15 2.5
8 4 Fluorescent production area 15 2.5
9 4 Fluorescent production area 15 2.5
10 3 Fluorescent production area 15 2.5
11 4 Fluorescent production area 15 2.5
12 4 Fluorescent production area 15 2.5
13
14
15
16
Board no. 4 Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Production area 15 2.5
2 5 Production area 15 2.5
3 5 Production area 15 2.5
4 4 Production area 15 2.5
5 5 Production area 15 2.5
6 5 Production area 15 2.5
7 3 Laboratory and landing 15 2.5
8 5 Third floor 15 2.5
9
10
11
12
Board no. 5 E bldg grd flr stores Phase Grey (Blue) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
Design of electrical services for buildings 274
4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 6 E bldg grd flr oven area Phase Grey (Blue) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Circulation area 15 2.5
2 4 Ovens 15 2.5
3 4 Ovens 15 2.5
4 4 Ovens 15 2.5
5 4 Ovens 15 2.5
6 6 Ovens 15 2.5
7 6 Ovens 15 2.5
8 5 Maintenance area 15 2.5
9 4 Maintenance area 15 2.5
10 5 Toilets and stairs 15 2.5
11
12
13
14
15
16
Board no. 7 E bldg 1st flr stores Phase Grey (Blue) sub-main 35mm2
Design example 275
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 8 Gate-house Phase Black (Yellow) sub-main 16mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 6 Lodge and toilets 15 2.5
2 6 E bldg cellar 15 2.5
3 6 E bldg cellar 15 2.5
4
Board no.9 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 1 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 1 Covered yard 15 2.5
7 2 Covered yard 15 2.5
Design of electrical services for buildings 276
8 2 Covered yard 15 2.5
9 2 Covered yard 15 2.5
10
11
12
Board no. 10 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9 3 Covered yard 15 2.5
10 3 Covered yard 15 2.5
11
12
Board no. 11 Covered yard Phase Black (Yellow) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9
10
11
Design example 277
12
Table 18.6 Distribution of load across phases (final)
Phase
Board no. Area Amps Brown (Red) Black (Yellow) Grey (Blue)
1 E and W bldgs grd flr 24 24
2 W grd flr 32 32
3 W 1st flr 30 30
4 W 2nd and 3rd flr 31 31
5 E grd flr 48 48
6 E grd flr 45 45
7 E 1st flr 48 45
8 Gate-house 15 15
9 Covered yard 48 48
10 Covered yard 66 66
11 Covered yard 48 48
Total 145 149 138
was made on the basis of the provisional phasing decided on before the distribution
boards had been scheduled.
The size of sub-main necessary to serve these boards was next calculated. The
necessary current rating was evident from Table 18.6 but it was also necessary to
calculate the size of cable needed to give an acceptable voltage drop. The distance from
the intake to the furthest board was measured on the drawings and found to be 98m. This
was rounded off to 100m for the purpose of calculation. The current taken by the most
distant board was 31A and for the calculation this was rounded off to 30A. It had
previously been assumed that 3V would be lost in the final circuits and it was now
decided to allow a 2V drop in the sub-main. This would make the total well less than the
permissible maximum but there is no restriction on how low the voltage drop is and it
seemed prudent to allow a margin for future extensions and also for possible alterations
in the final positions of distribution boards and routes of cables.
2V=2000mV
Permissible drop is given by
Design of electrical services for buildings 278
35mm2 cable has a voltage drop of 1.25mV/A/m and is rated at 145A, when run singly.
For a 30A current we could suffer a correction factor of 30/145=0.2. We could run 12 or
more multicore PVC steel wire armoured cable on cable tray single-layer clipped
touching each other, and the correction factor would be 0.7. A larger sub-main would
seem unreasonable for the loads involved. Although the volt drop of 35mm2 cable is
higher than the calculated figure, the calculation was on the safe side and was carried out
only for the longest sub-main. The next size of cable is 50mm2 which is considerably
harder to handle and therefore more expensive to install. It would be rather unusual to use
such large cable for lighting distribution and it was therefore decided that 35mm2 cable
would be acceptable. Each of these cables would be served from a 60A switch fuse. An
exception was made for board no. 8 which would carry only 15A. By inspection and
without any calculation, it was decided that a 16mm2 cable rated at 85A clipped direct or
94A clipped to cable tray, run singly, with a volt drop of 2.8mV/A/m would be adequate
for this. It would be served from a 30A switch fuse.
Attention was now turned to the design of the power distribution. A list of the
machinery to be installed was obtained from the client and written out as shown in Table
18.7. The locations of the equipment were also obtained and are shown in Figures 18.4–
18.5. It should be noted that all power equipment was to be three-phase except for FHP
motors on rotary valves.
Figure 18.4 Factory ground-floor
equipment layout
Design example 279
Table 18.7 Machinery, assumed current demand
Running current (amps per phase)
Item No. of kW each Each Total Allow for diversity
‘A’ Agitator 4 3.73 8 32 16
Type 1 mill 1 37.3 70 70 35
Type 2 mill 1 18.65 36 36 –
‘A’ Mixers 2 3.73 8 16 8
Shakers 3 2.24 5 15 10
Extractor 13 3.73 8 104 52
‘B’ Mixers 2 11.19 22 44 22
‘G’ Mixers 10 14.92 30 300 200
‘G’ Mills 10 7.83 16 160
Rotary valves 20 0.19 3 60 15
Single-phase motors
‘A’ Pumps 1 5.60 11 11 –
‘B’ Pumps 1 3.73 8 8 8
‘C’ Pumps 3 2.98 6 18 9
‘D’ Pumps 1 3.73 8 8 8
‘E’ Pumps 3 5.60 11 33 11
‘F’ Pumps 3 5.60 11 33 11
Ovens 18 3.73 8 144 96
Hoist 1 3.73 8 8 –
‘A’ Fans 3 5.60 11 33 22
Conveyors 1 7.46 15 15 15
Dissolver 5 5.60 11 55 33
Coupling tanks 3 11.19 22 66 44
Lift 1 0.75 2 2 –
‘B’ Agitators 1 1.49 3 3 3
‘G’ Pump 1 2.24 5 5 –
‘B’ Fans 1 22.38 40 40 –
Boiler burner 1 7.46 15 15 15
‘H’ Pumps 1 11.19 22 22 22
Design of electrical services for buildings 280
‘C’ Fans 11 5.60 11 11 11
Burner auxiliary motor 1 2.24 5 5 5
‘J’ Pumps 1 3.73 5 5 5
Total amps per phase 1369 676
KvA over 3 phases 982 486
Most of it was accounted for by motors driving pumps, agitators and other mechanical
equipment: the running currents per phase were taken from standard motor performance
tables.
The allowance for diversity was based on the designer’s previous industrial experience
and his assessment of what equipment might normally be in use simultaneously. The
lighting load on the most heavily loaded phase was 149A and in view of the nature of the
building it seemed reasonable to apply
Figure 18.5 Factory first-floor
equipment layout
a diversity factor of 0.6 to this, giving an after-diversity lighting load of 90A. Addition of
this to the power load gave a total after-diversity load of 766A per phase which is
570kVA over all three phases. This could conveniently be catered for by 800A busbars at
the main intake.
A difficulty arose over this figure. The supply to the existing board came from a
315kVA transformer. If the electricity company were to be asked for a bigger supply they
Design example 281
would make a substantial charge which the factory owner wished to avoid. The client
also thought the calculated load was high but could not dispute the total installed load. He
told the designer that at an older but similar works belonging to the same company
measurements showed that the actual maximum demand was 27 per cent of the total
installed load. If the same figure were applied to the new factory the maximum demand
would be 0.27×(982kVA power load+120kVA lighting load)=298kVA which would be
within the capacity of the existing supply. The client therefore wanted this figure to be
used. Whilst unable to challenge the client’s measurements the designer felt that a
diversity factor of 27 per cent was surprisingly low. He pointed out that if the distribution
was designed on this figure and it turned out to be low it would be very difficult and
expensive subsequently to increase the capacity of the installation. He was reluctant to
work on this basis. After discussion it was agreed that 800A
Figure 18.6 Factory second-floor
equipment layout
busbars would be installed at the main intake but would be served through a 400A switch
fuse from the existing 315kVA (equivalent to 440A per phase) supply. This would make
it possible to cater for a larger load if the need arose without expensive alterations but
would not increase the initial cost very much. It therefore satisfied both points of view.
A description such as this inevitably makes the design process seem very precise
whereas in practice at each stage there are many unknown facts for which the designer
has to make a guessed allowance. In the present case the plant design was proceeding at
the same time as the electrical design and neither the ratings nor the positions of all the
equipment were finally settled. Table 18.7 is in fact based on the third attempt to draw up
such a list; it would be an unnecessary waste of space to reproduce the earlier tables
which differed only in detail. However, the element of uncertainty led to two important
decisions about the general scheme.
Design of electrical services for buildings 282
First, it was decided to use busbars with separately mounted switch fuses rather than a
cubicle-type switchboard. This would give excellent flexibility for future extensions and
also for changes and additions which might become necessary before the installation was
completed. It seemed quite likely that this would be necessary because of the uncertainty
of the final plant layout.
Second, it was decided that the design of the power installation would go only as far as
the final distribution boards. The final circuits from these to the various motors would be
settled on site after the machines were installed. In areas where there was to be a lot of
equipment horizontal busbars could be run along the building walls with tap-off boxes
spaced as required.
With these considerations in mind the load was listed again but this time area by area.
The load was summed for each area and a decision made on the size and rating of the
distribution board to serve that area, This list is
Table 18.8 Distribution of load across phases
Running current (amps per phase)
Area Item No. of Each Total After diversity
1 ‘A’ Agitators 2 8 16
E bldg N end Type 1 mills 1 70 70
‘B’ Pumps 1 8 8
‘C’ Pumps 1 6 6
Shaker 1 5 5
6 115 100
2 ‘A’ Agitator 2 8 16 16
E bldg N end ‘B’ Mixer 2 22 44 44
Type 2 mill 1 36 36
‘A’ mixer 2 2 16
‘C’ Pump 2 6 12
Shaker 2 5 10
Extractor 1 8 8
41
12 142 101
3 Per cubicle
E bldg Grinding ‘C’ Mixer 1 30
cubicles ‘C’ Mill 1 16
Rotary valve 2 6
Extractor 1 8
5 60
Design example 283
4 Ovens 18 8 144
E bldg Ovens Extractors 2 8 16
160
5 Hoist 1 8 8
W bldg 3rd flr ‘A’ Fans 3 11 33
Conveyors 1 15 15
‘D’ Pump 1 8 8
6 64 45
6 Dissolver 5 11 55 33
W bldg 2nd flr
7 Coupling tanks 3 22 66 22
W bldg 1st flr
8 ‘E’ Pump 3 11 33
W bldg Grd flr ‘F’ Pump 3 11 33
Lift 1 2 2
‘B’ Agitator 1 3 3
‘A’ Pump 1 11 11
‘G’ Pump 1 5 5
10 87 40
9 ‘B’ Fan 1 40
Boiler room Burner 1 15
‘M’ Pump 1 22
‘C’ Fan 1 11
Auxiliary motor 1 5
Pump 1 5
6 98
shown in Table 18.8, It will be noticed that some additional items not listed in the
previous table were added at this stage.
The load in areas 1 and 2 could be catered for by a 24-way 300A TPN distribution
board served from a 200A switch fuse.
A check with manufacturers’ catalogues showed that it would not be possible to get a
standard board with outgoing fuse-ways in the wide range of sizes needed, that is to say
from 5A to the 100A needed for the mills. It would therefore be necessary to use two
separate boards, one with fuse-ways from 2 to 30A and one with fuse-ways from 20 to
Design of electrical services for buildings 284
100A. The former would have fourteen items with an installed load of 97A giving about
58A after diversity and the latter would serve four items with an installed load of 150A
giving about 90A after diversity. A 20-way 60A TPN board and a 6-way 100A TPN
board would meet these requirements.
In area 3, the equipment in one cubicle only is listed. The mixer and the mill do not
run at the same time. The rotary valves run intermittently, therefore the maximum
simultaneous demand can be assessed as 30+3+8 = 41A. There are ten such cubicles
making the installed load 10×60=600A. After diversity this will be say 400A.
In this area there is a total of 10×5=50 pieces of equipment. To allow spare capacity
65 ways on a distribution board are needed.
Each bay is approximately six metres long. A busbar would have not more than six
tap-off points along this length but each bay contains two cubicles with ten pieces of
equipment. Therefore a busbar is not the most practicable method and distribution boards
should be used.
The distribution boards will be mounted on a wall. As there is a central gang-way
midway between the two facing walls the arrangement will have to be symmetrical so
that either two or four distribution boards will have to be used. This gives the possibility
of either 2 of 36-way 300A TPN boards from 200A switch fuses or 4 of 18-way 200A
TPN boards from 150 switch fuses.
At first sight the first alternative appeared cheaper but on checking with the
manufacturers it was revealed that standard boards are not made as large as this so the
second alternative had to be adopted.
Area 4 can be conveniently served by 200A TPN busbars fed by a 200A isolator at the
busbars which can in turn be served from a 150A switch fuse on the main panel.
At this stage it had been decided that the east building would require six distribution
boards and one set of busbars. All this could conveniently come from a subsidiary
distribution centre in the building. The sum of the after-diversity loads calculated for this
was 741A but allowing for diversity between the boards the maximum load on the
busbars would be less. To allow for adequate short-circuit strength and also for future
extensions it was decided to use 800A busbars for the subsidiary centre. It had already
been decided, as explained above, that the main intake would have 800A busbars with a
400A incoming switch fuse. To give discrimination, the outgoing switch fuse could not
be larger than 300A. The after-diversity load was probably still being over-estimated but
whereas switch fuses and if necessary cables can be changed later it would be very
expensive to replace the busbars. Therefore the local busbars can still be 800A but should
have an incoming isolator of a lower rating. There is a fuse at the outgoing end of the
cable from the main intake and there is no need for another fuse at its other end. As the
fuse is 300A the isolator which is protected by the fuse should have a higher rating, say
400A.
Reference was made in the last paragraph to short-circuit strength. In fact no separate
calculation was made for this design but the results of calculations on other projects were
made use of.
The busbars are rectangular copper bars supported at regular intervals. The dimensions
of the bars and the spacing of supports are given in the manufacturer’s catalogue. If the
length between two successive supports is treated as a simply-supported beam the
Design example 285
maximum permissible bending moment can be calculated from the bending stress
formula:
where
M=bending moment (Nm)
I=second moment of area (m4)
p=stress (Nm−2)
y=distance from neutral axis to outermost fibre (m).
I and y are easily calculated for a rectangular section, p is the maximum allowable
working stress of pure copper, and M is the moment to be calculated.
The bending moment for a simply-supported beam with uniform loading is
where
M=bending moment (Nm)
w=load per unit length (Nm−1)
L=distance between supports (m).
As M has been established and L is known, this enables w to be calculated to give the
maximum permissible uniform load on each bar.
When a current flows in two parallel rectangular bars the resulting mechanical force
between them is given by
where
w=force per unit length (Nm−1)
i=current (A)
s=spacing between bars (m).
If the force per unit length is taken as the maximum permissible uniform load which
has just been calculated, and the spacing between the bars is known from the
manufacturer’s catalogue, this formula allows the maximum allowable value of the
current to be calculated. This value is then the maximum current which the bars will be
just strong enough to withstand and they should not be exposed to a possible short-circuit
current higher than this.
Perhaps this seems a lengthy and somewhat circuitous piece of reasoning. It is,
however, a typical example of the way in which the various requirements for a
distribution system have to be fitted together. It has not been written as a description of
the final scheme but rather to show the process by which the scheme was arrived at.
The subsequent distribution centre in the east building could also serve a distribution
board in the maintenance area and the unit heaters for the space heating. No information
was available at this stage of the equipment which would be installed in the maintenance
Design of electrical services for buildings 286
workshop but a 12-way 60A TPN board would certainly be adequate. The area would
have ten heaters which could be served from a 12-way SPN board.
At this stage there was still some uncertainty about the exact positions of the
equipment in the lower part of the west building and indeed about how much of the
equipment planned would be installed initially and how much left for the future.
Partly for this reason and partly to give the greatest possible flexibility for the final
connections, it was decided after discussion with the client to provide a busbar under the
first floor ceiling of the west building to serve the first and ground floors. It will be
remembered that part of the first-floor slab was omitted to give a two-storey height to
part of the ground floor. This made it possible to serve the ground floor from a busbar at
high level on the first floor. Indeed in view of the lack of information available to the
electrical designer about the height of the motors on the machinery to be installed this
seemed the only reasonable thing to do.
From Table 18.8 the total amps per phase on the ground and first floors were 153
installed and 62 after diversity. Some 200A busbars served by a
Table 18.9 Summary for W building
Running current (amps per phase)
Floor Total installed After diversity
Grd 87 40
1st 66 22
2nd 55 33
3rd 64 45
272 140
200A switch fuse would be ample for this load (this is the lowest standard rating for
busbars).
The second floor needed an 8-way 60A TPN distribution board served from a 60A
switch fuse.
The third floor needed an 8-way 60A TPN distribution board served from a 60amp
switch fuse.
The loading for the various areas of the west building was then summarized as shown
in Table 18.9. It became evident that the whole of this could conveniently come from a
subsidiary distribution centre within the west building. Adequate margins and provision
for future additions suggested that a suitable size would be 400A busbars served from a
200A switch fuse.
The space heating of the west building was to be provided by four unit heaters on the
second floor and four on the first floor which all required a supply for fans and
thermostats. Much of this equipment could be conveniently served from a six-way SPN
distribution board on its own floor and these two boards could also be served from the
subsidiary distribution centre.
Design example 287
The only remaining area to be dealt with was the boiler room. This is Item 9 in Table
18.8. There would be little diversity here and it was therefore decided to provide a 150A
TPN distribution board served from a 150A switch fuse.
To save a multiplicity of sub-mains cables, it was decided that in the west building the
lighting distribution boards would be served from the subsidiary distribution centre. The
east building was nearer the main intake so that it would not be so cumbersome and
expensive to run several cables between them. Also the number of switch fuses required
on the east building distribution centre for the power boards alone was already quite high.
It was therefore decided that the east building lighting boards would be served directly
from the main intake.
The distribution scheme was now sketched, as shown in Figure 18.7. This is the most
convenient method of summarizing the decisions taken so far and checking for any
inconsistencies or omissions. In its final form it is also the
Figure 18.7 Factory distribution
diagram
clearest way of explaining the scheme to be installed to contractors and suppliers.
It now remained to decide the sizes of the various sub-main distribution cables which,
at this stage, had not been written into the scheme of Figure 18.7. The necessary current
ratings were clear from the switch fuse ratings needed on the scheme but the cables also
had to be calculated for voltage drop. It is only necessary to make sure that the voltage
drop is not excessive and one of a limited number of standard-size cables must be chosen.
The calculation was therefore simplified by taking 100m as the longest run of a sub-main
cable and using the same length in calculating all of them. It had earlier been assumed
Design of electrical services for buildings 288
that, of the total permissible voltage drop, half would be in the final circuit and half in the
sub-mains. To keep to this assumption the sub-mains had to be calculated for a drop of
4.6V (4 per cent of the Nominal Voltage/2), to take into account future extensions the
volts drop would be restricted to 3V.
In view of the number of sub-mains involved the calculation was again set out in a
tabular form as shown in Table 18.10. Column 2 gives the current rating for reference
method 11 single layer clipped to cable tray. In column 3 the drop in millivolts per amp
metre to give a drop of 1V over the assumed total length of 100m has been calculated.
This figure was then multiplied by
Table 18.10 Sizing of sub-main distribution cables
1 2 3 4 5 6 7
mV/A/m for 1 volt drop
=(1000) Cable
Board no. Amps amps×100 mV/A/m for 3 volt
drop
mm2 Amps mV/A/m
21 60 0.17 0.50 50 125 0.81
22 100 0.10 0.30 70 155 0.57
23 150 0.07 0.20 95 190 0.43
24 150 0.07 0.20 95 190 0.43
25 150 0.07 0.20 95 190 0.43
26 150 0.07 0.20 95 190 0.43
27 60 0.17 0.50 50 125 0.81
28 30 0.33 1.00 35 72 1.1
29 45 0.22 0.67 35 72 1.1
30 25 0.40 1.20 35 72 1.1
31 12 0.83 2.50 25 62 1.5
32 12 0.83 2.50 25 62 1.5
33 100 0.10 0.30 70 155 0.57
W. busbars 100 0.10 0.30 70 155 0.57
Oven
busbars
150 0.07 0.20 95 190 0.43
E. subcentre
300 0.03 0.10 300 390 0.185
W. subcentre
120 0.08 0.25 150 250 0.35
Design example 289
three and rounded off in column 4 to give the millivolts per amp metre for a 3V drop.
Columns 2 and 4 thus showed the minimum requirements of the cable: the cable chosen
to match this was then entered in columns 5, 6 and 7. The cable sizes were then entered
on the scheme which is reproduced in Figure 18.7 and this completed the design.
In this example the designer’s freedom was restricted by the limitations of an existing
building. In principle, where a new building is being designed, the designer of the
electrical services can be called in early enough to suggest arrangements which would
result in a more economical services installation. The effect of voltage drop or cable sizes
makes it desirable to have the spaces allocated for intake panels and distribution boards
as near as possible to the centre of the area being served, load centres. The provision of
false ceilings, and horizontal and vertical ducts influences the type of wiring system to be
employed. The thickness of plaster may determine whether or not cables can be buried
within it. If walls are built of a single thickness of brick it will not be practicable to chase
them for cables or conduits, and the electrical installation may have to be run on the
surface.
All these matters can be discussed by the electrical services designer and the architect
at a very early stage and should in theory influence the building design. In practice it
seems that since it is always possible to adapt the electrical installation to any building,
purely architectural considerations always override the engineering ones. Many architects
have no objection to conduit on the surface of walls even in a completely new building,
and if this is accepted the type of construction no longer matters.
In the early stages of design the architect’s ideas are very fluid and it is difficult for the
electrical designer to make suggestions which are more than vague generalities. By the
time he receives drawings on which he can start design work of his own the shape and
style of the building have been settled and can no longer be altered to accommodate or
simplify the services.
Thus the engineering designer’s influence on the overall design of the building tends
to exist more in theory than in practice.
When the work is to be put out to competitive tender it is necessary to draw up a
specification describing the quality and standard of the equipment to be used and the
standard of workmanship expected. The specification which was used for the scheme
described in this chapter is reproduced in the following pages.
It is often prudent to include in a specification descriptions of equipment which may
not be needed for the scheme as designed. Variations are frequently made during
construction, and they could introduce a piece of equipment not originally needed. If it
has not been described in the specification a short variation instruction can give rise to
different interpretations which could result in a contractual dispute.
For example, the original scheme may not require any isolators as opposed to fuse
switches and switch fuses. If an isolator is subsequently required and there is no clause in
the specification covering isolators, the variation instruction must include a complete
description or there is the possibility that an unsuitable type will be supplied. Since there
is invariably less time available for drafting variations than for the original specification,
it is better to have a few extra clauses in the specification than to risk contractual
difficulties later on.
Design of electrical services for buildings 290
Design example
In order to illustrate the practical application of the principles discussed in previous
chapters we shall, in this chapter, describe a typical industrial design. The example
chosen is taken from a scheme handled in the previous author’s office some years ago. It
does embody the criteria in use today, although the design package would yield closer
limits. It is appreciated that a building services engineer will now use software design
packages. It is useful to see how the design values were arrived at. The buildings of a
disused factory were taken over by a chemical manufacturing company which proposed
to adapt them as a new works. Electric services were needed for lighting and power to
machinery.
The general plan of the buildings is shown in Figures 18.1–18.3, which also show the
main part of the lighting layout. As the design of lighting has been excluded from the
subject matter of this book it is not proposed to reproduce the lighting calculations here,
but it should be noted that after the number of lights needed in each area had been
calculated they were positioned with regard to the layout of the machinery as well as to
the need to maintain reasonable uniform levels of illumination.
The factory consists of an east building of two storeys with a basement and a threestorey
west building with a covered yard between them extending the full height of the
east building. There is a walled car park adjacent to the buildings and a new boiler-house
was to be built in this area. Since the existing buildings provided more space than was
needed for the new works, part of the west building was to left unoccupied: no services
were to be installed in this part but the installation as a whole was to be capable of
extension into this area.
The bulk of the lighting consisted of twin-tube 1500mm fluorescent luminaires with
some single-tube luminaires in passages and areas requiring lower illumination. A few
incandescent luminaires were provided in toilets and on stairs (not all of which are shown
in Figures 18.1–18.2). The covered way between the occupied and unoccupied sections
of the west building in which materials would be hoisted to the upper levels was lit by
three wall-mounted mercury lamps at ground-floor level and three at second-floor level.
Figure 18.1 Factory ground-floor
lighting layout
One end of the west building contained tall machinery on the ground floor and the firstfloor
slab was not carried across this. An area of double the normal height was thus left
and this was lit by wall-mounted mercury lamps at the lower level and high-bay industrial
mercury luminaires under the first-floor ceiling. The covered yard was lit by wallmounted
high-pressure sodium floodlights at the level of the first-floor ceilings. Four
street-lighting lanterns were provided for the car park, three of them being mounted on
columns on the roadway from the building and one on a bracket on the wall of the
building.
The first stage in the design was to arrange the lights in circuits and to arrange the
circuits in convenient groups to be served from several distribution boards. The lighting
would have to be divided in a suitable manner between the three phases to give as nearly
as possible the same loading on all three phases and this had to be borne in mind when
the lights were
Design of electrical services for buildings 266
Table 18.1 Types of lighting
Ref. Type Current (amps)
A 1500mm twin fluorescent 0.92
B 1500mm single fluorescent 0.46
C Wall-mounted 125W MBF 1.15
D Tungsten bulkhead 0.42
E High-bay industrial mercury with 250W MBF lamp 2.15
F Wall-mounted area floodlight with 250W SON lamp 3.0
G Bulkhead luminaire with 50W MBF/U lamp 0.6
H Side-entry street-lighting lantern with 35W SOX lamp 0.6
arranged into circuits. For convenience, the different types of luminaire used were listed,
as shown in Table 18.1.
It was decided that in this type of factory the lighting could be run in 2.5mm2 cable
fused at 15A. To allow a margin for safety and small alterations the circuits would be
designed to carry not more than 12A each. Although it was intended to use three different
sizes of mercury lamp it was felt that there was a possibility that at some time in the
future a works manager might change the luminaires without checking the capacity of the
wiring and it was therefore decided to design all the circuits serving luminaires with
mercury lamps to be capable of taking 250W lamps. Similarly, circuits serving singletube
fluorescent luminaires would, where appropriate, be designed to take twin-tube
luminaires so that the luminaires could at any time be replaced without alterations to the
wiring. Hence, a maximum number of luminaires on a circuit would be:
It was clearly going to be desirable to control more than this number of lights from one
switch and it was decided to do so by switching the lights through contactors. One switch
would operate a multi-pole contactor controlling several lighting circuits.
At this stage a check was made on the voltage drop in the lighting circuits. Probable
positions of distribution boards were guessed and from the drawings the average length
of a lighting circuit was estimated as 35m. At the time when this design was produced,
the maximum volt drop allowed on 230V was 9V, it was decided to build in a factor of
safety and allow a maximum of 6V, and it seemed reasonable to allow half of this in the
sub-mains and half in the final circuits, that is to say 3V. The single phase volt drop of
2.5mm2 cable is 18mV per ampere per metre.
Design of electrical services for buildings 268
This is a pessimistic value since the current in the circuit will reduce as current is
‘dropped off’ at each luminaire, and the design current Ib will not flow in the whole
length of the circuit. The designer may decide to average. However we took this value as
true.
Clearly the need to reduce voltage drop was more critical than the current rating of the
cable and the number of luminaires per circuit would have to be reduced. Acceptable
figures would be six fluorescent luminaires or two sodium (slightly over the design
value) or two mercury luminaires per circuit.
The loadings were now estimated for each area in a convenient tabulated form as
shown in Table 18.2.
This formed a preliminary guide. The number of circuits in each area was decided by
referring to the maximum number of luminaires per circuit as determined above and also
with an eye to convenient switching arrangements. At the same time, some margins were
allowed to make it possible to adjust the circuit arrangements later without major
modifications to the distribution scheme. It will be noted for example that the car-park
lights are not included in the table. This was because the design had to proceed before the
client had taken final decisions on all his requirements. The fact that last-minute
alterations would certainly be made had therefore to be kept constantly in mind.
The total load from Table 18.2 was 460.94A. It should therefore be distributed to give
about 150A per phase. An ideally equal distribution could not be hoped for but each
phase should carry between 140 and 160A and at the same time each phase should be
contained within a reasonably clear zone of the building. As a first step towards
achieving this the loads for each area were summarized from Table 18.2, as shown in
Table 18.3. They were then arranged in three groups for the three phases. After two
attempts the results shown in Table 18.4 were obtained.
This was not as good as had been hoped for. However, the process of manipulating the
figures had given the designer a feel for them and he realized that he was not likely to
achieve any further improvement at this stage. It would be possible to make some
adjustment after the distribution boards were scheduled and this was done next.
Table 18.2 Loading estimates for each area
Area Luminaire
Ref. No.
off
Amps
each
Amps
total
No. of circuits needed
Gate-house A 1 0.92 0.92
B 2 0.46 0.92
D 3 0.42 1.26
3.10 1
Design example 269
Pump house A 8 0.92 7.36 2
Boiler house A 7 0.92 6.44 2
Covered way C 6 2.15 12.9 3 controlled by 1 contactor
W bldg Grd flr A 24 0.92 21.00 3:1 switched directly, 2 controlled by
1 contactor
G 5 2.15 4.30 2
E bldg Stores A 48 0.92 44.2 8 controlled by 4 contactors
Ovens A 33 0.92 30.36 6 controlled by 4 contactors
Toilet area A 1 0.92 0.92
B 3 0.46 1.38
D 1 0.42 0.42
2.72 1
Maintenance A 8 0.92 7.36 2
Lockers A 8 0.92 7.36 2
Stairs D 1 0.42 0.42 1
1st Floor Covered
yard
F 34 3.0 102.0 16 controlled by 8 contactors
Side yard F 20 3.0 60.0 10 controlled by 4 contactors
Lockers B 12 0.46 5.52 1
W bldg 1st flr A 20 0.92 22.08 6 controlled by 2 contactors
E 12 2.15 25.80 6 controlled by 2 contactors
E bldg 1st flr A 48 0.92 44.2 8 controlled by 4 contactors
1st flr Maintenance A 7 0.92 6.44 2
1st flr Pump house A 7 0.92 6.44 2
E bldg Cellar A 12 0.92 11.04 2
W bldg 2nd flr A 30 0.92 27.6 6 controlled by 2 contactors
W bldg 3rd flr B 5 0.46 2.30 1
Table 18.3 Load summaries
Area Load in amps
W bldg grd flr 25.30
Pump house 7.36
Gate-house 3.10
Design of electrical services for buildings 270
Covered way 12.90
48.66
Boiler house 6.44
W bldg 1st flr 47.88
Covered yard (main area) 102.00
Covered yard (side area) 60.00
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
E bldg grd flr 44.20
E bldg grd flr 30.36
E bldg Toilet area 2.72
Maintenance 7.36
Lockers 7.36
Stairs 0.42
92.42
E bldg 1st flr 44.20
E bldg Maintenance 6.44
E bldg Pump house 6.44
Lockers 5.52
62.60
E bldg cellar 11.04
Table 18.4 Distribution of load across phases
(provisional)
Amps
Brown (Red) phase
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
Covered yard main area 102.0
Boiler house 6.4
138.3
Black (Yellow) phase
W bldg grd flr 48.66
Design example 271
W bldg 1st flr 47.88
East cellar 11.04
Covered yard site area 60.00
167.58
Grey (Blue) phase
E bldg grd flr 92.42
E bldg lst flr 62.60
155.02
The lights and switching were shown on drawings. In each area, the luminaires were
grouped into circuits in accordance with the maximum number of luminaires per circuit
previously determined. Clearly the luminaires on any one circuit must be in a reasonably
compact group. Also, although the luminaires on one circuit can be controlled by more
than one switch, the converse is not true: one switch cannot control luminaires on several
circuits unless a multi-pole contactor is used. The most practicable way of settling these
matters is to mark the circuits and switching groups on drawings of an adequately large
scale.
Standard distribution boards are available with 12 and 16 ways. Suitable positions
were chosen on the drawings for distribution boards to serve groups of 7 to 12 circuits to
allow a reasonable number of spare ways on each board. The positions were chosen to
keep the final circuits reasonably short and so that as far as possible each board would be
in the ‘load centre’ of the area it was serving. It became evident in. this process that the
second and third floors of the west building should be served from a single board, that the
gate-house would need its own board, that three distribution boards would conveniently
handle both parts of the covered yard, that the ground and first floors of the east building
would each need two distribution boards and that the cellar of the east building would be
most conveniently served from the gate-house. The information from the drawings was
then summarized in distribution-board schedules which are reproduced in Table 18.5. A
further table was then made in order to decide on which phase each of these boards
should be and this is given in Table 18.6.
The figures in the three right-hand columns were entered in pencil, rubbed out and
moved from column to column until by a process of trial and error quite a good balance
over the phases was obtained. The first attempt
Table 18.5 Lighting distribution boards
Board no. 1 E bldg maintenance area 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Pump house and changing rooms grd flr 15 2.5
2 4 Changing rooms grd flr 15 2.5
3 5 1st flr maintenance area 15 2.5
Design of electrical services for buildings 272
4 5 1st flr maintenance and sub-station 15 2.5
5 6 Pump house 15 2.5
6 6 Changing rooms 1st flr 15 2.5
7 5 Changing rooms 1st flr and stairs 15 2.5
8 4 Car park lights 15 2.5
9
10
11
12
Board no. 2 W bldg grd flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Bulkheads on wall 15 2.5
2 3 Production area 15 2.5
3 3 Production area 15 2.5
4 4 Production area 15 2.5
5 3 Production area 15 2.5
6 4 Production area 15 2.5
7 4 Production area 15 2.5
8 3 Hoist yard, low level 15 2.5
9 3 Hoist yard, high level 15 2.5
10 5 Rear entrance 15 2.5
11 4 Stairs 15 2.5
12
13
14
15
16
Board no. 3 W bldg 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Mercury over vats 15 2.5
2 2 Mercury over vats 15 2.5
3 2 Mercury over vats 15 2.5
Design example 273
4 2 Mercury over vats 15 2.5
5 2 Mercury over vats 15 2.5
6 2 Mercury over vats 15 2.5
7 3 Fluorescent production area 15 2.5
8 4 Fluorescent production area 15 2.5
9 4 Fluorescent production area 15 2.5
10 3 Fluorescent production area 15 2.5
11 4 Fluorescent production area 15 2.5
12 4 Fluorescent production area 15 2.5
13
14
15
16
Board no. 4 Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Production area 15 2.5
2 5 Production area 15 2.5
3 5 Production area 15 2.5
4 4 Production area 15 2.5
5 5 Production area 15 2.5
6 5 Production area 15 2.5
7 3 Laboratory and landing 15 2.5
8 5 Third floor 15 2.5
9
10
11
12
Board no. 5 E bldg grd flr stores Phase Grey (Blue) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
Design of electrical services for buildings 274
4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 6 E bldg grd flr oven area Phase Grey (Blue) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Circulation area 15 2.5
2 4 Ovens 15 2.5
3 4 Ovens 15 2.5
4 4 Ovens 15 2.5
5 4 Ovens 15 2.5
6 6 Ovens 15 2.5
7 6 Ovens 15 2.5
8 5 Maintenance area 15 2.5
9 4 Maintenance area 15 2.5
10 5 Toilets and stairs 15 2.5
11
12
13
14
15
16
Board no. 7 E bldg 1st flr stores Phase Grey (Blue) sub-main 35mm2
Design example 275
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 8 Gate-house Phase Black (Yellow) sub-main 16mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 6 Lodge and toilets 15 2.5
2 6 E bldg cellar 15 2.5
3 6 E bldg cellar 15 2.5
4
Board no.9 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 1 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 1 Covered yard 15 2.5
7 2 Covered yard 15 2.5
Design of electrical services for buildings 276
8 2 Covered yard 15 2.5
9 2 Covered yard 15 2.5
10
11
12
Board no. 10 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9 3 Covered yard 15 2.5
10 3 Covered yard 15 2.5
11
12
Board no. 11 Covered yard Phase Black (Yellow) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9
10
11
Design example 277
12
Table 18.6 Distribution of load across phases (final)
Phase
Board no. Area Amps Brown (Red) Black (Yellow) Grey (Blue)
1 E and W bldgs grd flr 24 24
2 W grd flr 32 32
3 W 1st flr 30 30
4 W 2nd and 3rd flr 31 31
5 E grd flr 48 48
6 E grd flr 45 45
7 E 1st flr 48 45
8 Gate-house 15 15
9 Covered yard 48 48
10 Covered yard 66 66
11 Covered yard 48 48
Total 145 149 138
was made on the basis of the provisional phasing decided on before the distribution
boards had been scheduled.
The size of sub-main necessary to serve these boards was next calculated. The
necessary current rating was evident from Table 18.6 but it was also necessary to
calculate the size of cable needed to give an acceptable voltage drop. The distance from
the intake to the furthest board was measured on the drawings and found to be 98m. This
was rounded off to 100m for the purpose of calculation. The current taken by the most
distant board was 31A and for the calculation this was rounded off to 30A. It had
previously been assumed that 3V would be lost in the final circuits and it was now
decided to allow a 2V drop in the sub-main. This would make the total well less than the
permissible maximum but there is no restriction on how low the voltage drop is and it
seemed prudent to allow a margin for future extensions and also for possible alterations
in the final positions of distribution boards and routes of cables.
2V=2000mV
Permissible drop is given by
Design of electrical services for buildings 278
35mm2 cable has a voltage drop of 1.25mV/A/m and is rated at 145A, when run singly.
For a 30A current we could suffer a correction factor of 30/145=0.2. We could run 12 or
more multicore PVC steel wire armoured cable on cable tray single-layer clipped
touching each other, and the correction factor would be 0.7. A larger sub-main would
seem unreasonable for the loads involved. Although the volt drop of 35mm2 cable is
higher than the calculated figure, the calculation was on the safe side and was carried out
only for the longest sub-main. The next size of cable is 50mm2 which is considerably
harder to handle and therefore more expensive to install. It would be rather unusual to use
such large cable for lighting distribution and it was therefore decided that 35mm2 cable
would be acceptable. Each of these cables would be served from a 60A switch fuse. An
exception was made for board no. 8 which would carry only 15A. By inspection and
without any calculation, it was decided that a 16mm2 cable rated at 85A clipped direct or
94A clipped to cable tray, run singly, with a volt drop of 2.8mV/A/m would be adequate
for this. It would be served from a 30A switch fuse.
Attention was now turned to the design of the power distribution. A list of the
machinery to be installed was obtained from the client and written out as shown in Table
18.7. The locations of the equipment were also obtained and are shown in Figures 18.4–
18.5. It should be noted that all power equipment was to be three-phase except for FHP
motors on rotary valves.
Figure 18.4 Factory ground-floor
equipment layout
Design example 279
Table 18.7 Machinery, assumed current demand
Running current (amps per phase)
Item No. of kW each Each Total Allow for diversity
‘A’ Agitator 4 3.73 8 32 16
Type 1 mill 1 37.3 70 70 35
Type 2 mill 1 18.65 36 36 –
‘A’ Mixers 2 3.73 8 16 8
Shakers 3 2.24 5 15 10
Extractor 13 3.73 8 104 52
‘B’ Mixers 2 11.19 22 44 22
‘G’ Mixers 10 14.92 30 300 200
‘G’ Mills 10 7.83 16 160
Rotary valves 20 0.19 3 60 15
Single-phase motors
‘A’ Pumps 1 5.60 11 11 –
‘B’ Pumps 1 3.73 8 8 8
‘C’ Pumps 3 2.98 6 18 9
‘D’ Pumps 1 3.73 8 8 8
‘E’ Pumps 3 5.60 11 33 11
‘F’ Pumps 3 5.60 11 33 11
Ovens 18 3.73 8 144 96
Hoist 1 3.73 8 8 –
‘A’ Fans 3 5.60 11 33 22
Conveyors 1 7.46 15 15 15
Dissolver 5 5.60 11 55 33
Coupling tanks 3 11.19 22 66 44
Lift 1 0.75 2 2 –
‘B’ Agitators 1 1.49 3 3 3
‘G’ Pump 1 2.24 5 5 –
‘B’ Fans 1 22.38 40 40 –
Boiler burner 1 7.46 15 15 15
‘H’ Pumps 1 11.19 22 22 22
Design of electrical services for buildings 280
‘C’ Fans 11 5.60 11 11 11
Burner auxiliary motor 1 2.24 5 5 5
‘J’ Pumps 1 3.73 5 5 5
Total amps per phase 1369 676
KvA over 3 phases 982 486
Most of it was accounted for by motors driving pumps, agitators and other mechanical
equipment: the running currents per phase were taken from standard motor performance
tables.
The allowance for diversity was based on the designer’s previous industrial experience
and his assessment of what equipment might normally be in use simultaneously. The
lighting load on the most heavily loaded phase was 149A and in view of the nature of the
building it seemed reasonable to apply
Figure 18.5 Factory first-floor
equipment layout
a diversity factor of 0.6 to this, giving an after-diversity lighting load of 90A. Addition of
this to the power load gave a total after-diversity load of 766A per phase which is
570kVA over all three phases. This could conveniently be catered for by 800A busbars at
the main intake.
A difficulty arose over this figure. The supply to the existing board came from a
315kVA transformer. If the electricity company were to be asked for a bigger supply they
Design example 281
would make a substantial charge which the factory owner wished to avoid. The client
also thought the calculated load was high but could not dispute the total installed load. He
told the designer that at an older but similar works belonging to the same company
measurements showed that the actual maximum demand was 27 per cent of the total
installed load. If the same figure were applied to the new factory the maximum demand
would be 0.27×(982kVA power load+120kVA lighting load)=298kVA which would be
within the capacity of the existing supply. The client therefore wanted this figure to be
used. Whilst unable to challenge the client’s measurements the designer felt that a
diversity factor of 27 per cent was surprisingly low. He pointed out that if the distribution
was designed on this figure and it turned out to be low it would be very difficult and
expensive subsequently to increase the capacity of the installation. He was reluctant to
work on this basis. After discussion it was agreed that 800A
Figure 18.6 Factory second-floor
equipment layout
busbars would be installed at the main intake but would be served through a 400A switch
fuse from the existing 315kVA (equivalent to 440A per phase) supply. This would make
it possible to cater for a larger load if the need arose without expensive alterations but
would not increase the initial cost very much. It therefore satisfied both points of view.
A description such as this inevitably makes the design process seem very precise
whereas in practice at each stage there are many unknown facts for which the designer
has to make a guessed allowance. In the present case the plant design was proceeding at
the same time as the electrical design and neither the ratings nor the positions of all the
equipment were finally settled. Table 18.7 is in fact based on the third attempt to draw up
such a list; it would be an unnecessary waste of space to reproduce the earlier tables
which differed only in detail. However, the element of uncertainty led to two important
decisions about the general scheme.
Design of electrical services for buildings 282
First, it was decided to use busbars with separately mounted switch fuses rather than a
cubicle-type switchboard. This would give excellent flexibility for future extensions and
also for changes and additions which might become necessary before the installation was
completed. It seemed quite likely that this would be necessary because of the uncertainty
of the final plant layout.
Second, it was decided that the design of the power installation would go only as far as
the final distribution boards. The final circuits from these to the various motors would be
settled on site after the machines were installed. In areas where there was to be a lot of
equipment horizontal busbars could be run along the building walls with tap-off boxes
spaced as required.
With these considerations in mind the load was listed again but this time area by area.
The load was summed for each area and a decision made on the size and rating of the
distribution board to serve that area, This list is
Table 18.8 Distribution of load across phases
Running current (amps per phase)
Area Item No. of Each Total After diversity
1 ‘A’ Agitators 2 8 16
E bldg N end Type 1 mills 1 70 70
‘B’ Pumps 1 8 8
‘C’ Pumps 1 6 6
Shaker 1 5 5
6 115 100
2 ‘A’ Agitator 2 8 16 16
E bldg N end ‘B’ Mixer 2 22 44 44
Type 2 mill 1 36 36
‘A’ mixer 2 2 16
‘C’ Pump 2 6 12
Shaker 2 5 10
Extractor 1 8 8
41
12 142 101
3 Per cubicle
E bldg Grinding ‘C’ Mixer 1 30
cubicles ‘C’ Mill 1 16
Rotary valve 2 6
Extractor 1 8
5 60
Design example 283
4 Ovens 18 8 144
E bldg Ovens Extractors 2 8 16
160
5 Hoist 1 8 8
W bldg 3rd flr ‘A’ Fans 3 11 33
Conveyors 1 15 15
‘D’ Pump 1 8 8
6 64 45
6 Dissolver 5 11 55 33
W bldg 2nd flr
7 Coupling tanks 3 22 66 22
W bldg 1st flr
8 ‘E’ Pump 3 11 33
W bldg Grd flr ‘F’ Pump 3 11 33
Lift 1 2 2
‘B’ Agitator 1 3 3
‘A’ Pump 1 11 11
‘G’ Pump 1 5 5
10 87 40
9 ‘B’ Fan 1 40
Boiler room Burner 1 15
‘M’ Pump 1 22
‘C’ Fan 1 11
Auxiliary motor 1 5
Pump 1 5
6 98
shown in Table 18.8, It will be noticed that some additional items not listed in the
previous table were added at this stage.
The load in areas 1 and 2 could be catered for by a 24-way 300A TPN distribution
board served from a 200A switch fuse.
A check with manufacturers’ catalogues showed that it would not be possible to get a
standard board with outgoing fuse-ways in the wide range of sizes needed, that is to say
from 5A to the 100A needed for the mills. It would therefore be necessary to use two
separate boards, one with fuse-ways from 2 to 30A and one with fuse-ways from 20 to
Design of electrical services for buildings 284
100A. The former would have fourteen items with an installed load of 97A giving about
58A after diversity and the latter would serve four items with an installed load of 150A
giving about 90A after diversity. A 20-way 60A TPN board and a 6-way 100A TPN
board would meet these requirements.
In area 3, the equipment in one cubicle only is listed. The mixer and the mill do not
run at the same time. The rotary valves run intermittently, therefore the maximum
simultaneous demand can be assessed as 30+3+8 = 41A. There are ten such cubicles
making the installed load 10×60=600A. After diversity this will be say 400A.
In this area there is a total of 10×5=50 pieces of equipment. To allow spare capacity
65 ways on a distribution board are needed.
Each bay is approximately six metres long. A busbar would have not more than six
tap-off points along this length but each bay contains two cubicles with ten pieces of
equipment. Therefore a busbar is not the most practicable method and distribution boards
should be used.
The distribution boards will be mounted on a wall. As there is a central gang-way
midway between the two facing walls the arrangement will have to be symmetrical so
that either two or four distribution boards will have to be used. This gives the possibility
of either 2 of 36-way 300A TPN boards from 200A switch fuses or 4 of 18-way 200A
TPN boards from 150 switch fuses.
At first sight the first alternative appeared cheaper but on checking with the
manufacturers it was revealed that standard boards are not made as large as this so the
second alternative had to be adopted.
Area 4 can be conveniently served by 200A TPN busbars fed by a 200A isolator at the
busbars which can in turn be served from a 150A switch fuse on the main panel.
At this stage it had been decided that the east building would require six distribution
boards and one set of busbars. All this could conveniently come from a subsidiary
distribution centre in the building. The sum of the after-diversity loads calculated for this
was 741A but allowing for diversity between the boards the maximum load on the
busbars would be less. To allow for adequate short-circuit strength and also for future
extensions it was decided to use 800A busbars for the subsidiary centre. It had already
been decided, as explained above, that the main intake would have 800A busbars with a
400A incoming switch fuse. To give discrimination, the outgoing switch fuse could not
be larger than 300A. The after-diversity load was probably still being over-estimated but
whereas switch fuses and if necessary cables can be changed later it would be very
expensive to replace the busbars. Therefore the local busbars can still be 800A but should
have an incoming isolator of a lower rating. There is a fuse at the outgoing end of the
cable from the main intake and there is no need for another fuse at its other end. As the
fuse is 300A the isolator which is protected by the fuse should have a higher rating, say
400A.
Reference was made in the last paragraph to short-circuit strength. In fact no separate
calculation was made for this design but the results of calculations on other projects were
made use of.
The busbars are rectangular copper bars supported at regular intervals. The dimensions
of the bars and the spacing of supports are given in the manufacturer’s catalogue. If the
length between two successive supports is treated as a simply-supported beam the
Design example 285
maximum permissible bending moment can be calculated from the bending stress
formula:
where
M=bending moment (Nm)
I=second moment of area (m4)
p=stress (Nm−2)
y=distance from neutral axis to outermost fibre (m).
I and y are easily calculated for a rectangular section, p is the maximum allowable
working stress of pure copper, and M is the moment to be calculated.
The bending moment for a simply-supported beam with uniform loading is
where
M=bending moment (Nm)
w=load per unit length (Nm−1)
L=distance between supports (m).
As M has been established and L is known, this enables w to be calculated to give the
maximum permissible uniform load on each bar.
When a current flows in two parallel rectangular bars the resulting mechanical force
between them is given by
where
w=force per unit length (Nm−1)
i=current (A)
s=spacing between bars (m).
If the force per unit length is taken as the maximum permissible uniform load which
has just been calculated, and the spacing between the bars is known from the
manufacturer’s catalogue, this formula allows the maximum allowable value of the
current to be calculated. This value is then the maximum current which the bars will be
just strong enough to withstand and they should not be exposed to a possible short-circuit
current higher than this.
Perhaps this seems a lengthy and somewhat circuitous piece of reasoning. It is,
however, a typical example of the way in which the various requirements for a
distribution system have to be fitted together. It has not been written as a description of
the final scheme but rather to show the process by which the scheme was arrived at.
The subsequent distribution centre in the east building could also serve a distribution
board in the maintenance area and the unit heaters for the space heating. No information
was available at this stage of the equipment which would be installed in the maintenance
Design of electrical services for buildings 286
workshop but a 12-way 60A TPN board would certainly be adequate. The area would
have ten heaters which could be served from a 12-way SPN board.
At this stage there was still some uncertainty about the exact positions of the
equipment in the lower part of the west building and indeed about how much of the
equipment planned would be installed initially and how much left for the future.
Partly for this reason and partly to give the greatest possible flexibility for the final
connections, it was decided after discussion with the client to provide a busbar under the
first floor ceiling of the west building to serve the first and ground floors. It will be
remembered that part of the first-floor slab was omitted to give a two-storey height to
part of the ground floor. This made it possible to serve the ground floor from a busbar at
high level on the first floor. Indeed in view of the lack of information available to the
electrical designer about the height of the motors on the machinery to be installed this
seemed the only reasonable thing to do.
From Table 18.8 the total amps per phase on the ground and first floors were 153
installed and 62 after diversity. Some 200A busbars served by a
Table 18.9 Summary for W building
Running current (amps per phase)
Floor Total installed After diversity
Grd 87 40
1st 66 22
2nd 55 33
3rd 64 45
272 140
200A switch fuse would be ample for this load (this is the lowest standard rating for
busbars).
The second floor needed an 8-way 60A TPN distribution board served from a 60A
switch fuse.
The third floor needed an 8-way 60A TPN distribution board served from a 60amp
switch fuse.
The loading for the various areas of the west building was then summarized as shown
in Table 18.9. It became evident that the whole of this could conveniently come from a
subsidiary distribution centre within the west building. Adequate margins and provision
for future additions suggested that a suitable size would be 400A busbars served from a
200A switch fuse.
The space heating of the west building was to be provided by four unit heaters on the
second floor and four on the first floor which all required a supply for fans and
thermostats. Much of this equipment could be conveniently served from a six-way SPN
distribution board on its own floor and these two boards could also be served from the
subsidiary distribution centre.
Design example 287
The only remaining area to be dealt with was the boiler room. This is Item 9 in Table
18.8. There would be little diversity here and it was therefore decided to provide a 150A
TPN distribution board served from a 150A switch fuse.
To save a multiplicity of sub-mains cables, it was decided that in the west building the
lighting distribution boards would be served from the subsidiary distribution centre. The
east building was nearer the main intake so that it would not be so cumbersome and
expensive to run several cables between them. Also the number of switch fuses required
on the east building distribution centre for the power boards alone was already quite high.
It was therefore decided that the east building lighting boards would be served directly
from the main intake.
The distribution scheme was now sketched, as shown in Figure 18.7. This is the most
convenient method of summarizing the decisions taken so far and checking for any
inconsistencies or omissions. In its final form it is also the
Figure 18.7 Factory distribution
diagram
clearest way of explaining the scheme to be installed to contractors and suppliers.
It now remained to decide the sizes of the various sub-main distribution cables which,
at this stage, had not been written into the scheme of Figure 18.7. The necessary current
ratings were clear from the switch fuse ratings needed on the scheme but the cables also
had to be calculated for voltage drop. It is only necessary to make sure that the voltage
drop is not excessive and one of a limited number of standard-size cables must be chosen.
The calculation was therefore simplified by taking 100m as the longest run of a sub-main
cable and using the same length in calculating all of them. It had earlier been assumed
Design of electrical services for buildings 288
that, of the total permissible voltage drop, half would be in the final circuit and half in the
sub-mains. To keep to this assumption the sub-mains had to be calculated for a drop of
4.6V (4 per cent of the Nominal Voltage/2), to take into account future extensions the
volts drop would be restricted to 3V.
In view of the number of sub-mains involved the calculation was again set out in a
tabular form as shown in Table 18.10. Column 2 gives the current rating for reference
method 11 single layer clipped to cable tray. In column 3 the drop in millivolts per amp
metre to give a drop of 1V over the assumed total length of 100m has been calculated.
This figure was then multiplied by
Table 18.10 Sizing of sub-main distribution cables
1 2 3 4 5 6 7
mV/A/m for 1 volt drop
=(1000) Cable
Board no. Amps amps×100 mV/A/m for 3 volt
drop
mm2 Amps mV/A/m
21 60 0.17 0.50 50 125 0.81
22 100 0.10 0.30 70 155 0.57
23 150 0.07 0.20 95 190 0.43
24 150 0.07 0.20 95 190 0.43
25 150 0.07 0.20 95 190 0.43
26 150 0.07 0.20 95 190 0.43
27 60 0.17 0.50 50 125 0.81
28 30 0.33 1.00 35 72 1.1
29 45 0.22 0.67 35 72 1.1
30 25 0.40 1.20 35 72 1.1
31 12 0.83 2.50 25 62 1.5
32 12 0.83 2.50 25 62 1.5
33 100 0.10 0.30 70 155 0.57
W. busbars 100 0.10 0.30 70 155 0.57
Oven
busbars
150 0.07 0.20 95 190 0.43
E. subcentre
300 0.03 0.10 300 390 0.185
W. subcentre
120 0.08 0.25 150 250 0.35
Design example 289
three and rounded off in column 4 to give the millivolts per amp metre for a 3V drop.
Columns 2 and 4 thus showed the minimum requirements of the cable: the cable chosen
to match this was then entered in columns 5, 6 and 7. The cable sizes were then entered
on the scheme which is reproduced in Figure 18.7 and this completed the design.
In this example the designer’s freedom was restricted by the limitations of an existing
building. In principle, where a new building is being designed, the designer of the
electrical services can be called in early enough to suggest arrangements which would
result in a more economical services installation. The effect of voltage drop or cable sizes
makes it desirable to have the spaces allocated for intake panels and distribution boards
as near as possible to the centre of the area being served, load centres. The provision of
false ceilings, and horizontal and vertical ducts influences the type of wiring system to be
employed. The thickness of plaster may determine whether or not cables can be buried
within it. If walls are built of a single thickness of brick it will not be practicable to chase
them for cables or conduits, and the electrical installation may have to be run on the
surface.
All these matters can be discussed by the electrical services designer and the architect
at a very early stage and should in theory influence the building design. In practice it
seems that since it is always possible to adapt the electrical installation to any building,
purely architectural considerations always override the engineering ones. Many architects
have no objection to conduit on the surface of walls even in a completely new building,
and if this is accepted the type of construction no longer matters.
In the early stages of design the architect’s ideas are very fluid and it is difficult for the
electrical designer to make suggestions which are more than vague generalities. By the
time he receives drawings on which he can start design work of his own the shape and
style of the building have been settled and can no longer be altered to accommodate or
simplify the services.
Thus the engineering designer’s influence on the overall design of the building tends
to exist more in theory than in practice.
When the work is to be put out to competitive tender it is necessary to draw up a
specification describing the quality and standard of the equipment to be used and the
standard of workmanship expected. The specification which was used for the scheme
described in this chapter is reproduced in the following pages.
It is often prudent to include in a specification descriptions of equipment which may
not be needed for the scheme as designed. Variations are frequently made during
construction, and they could introduce a piece of equipment not originally needed. If it
has not been described in the specification a short variation instruction can give rise to
different interpretations which could result in a contractual dispute.
For example, the original scheme may not require any isolators as opposed to fuse
switches and switch fuses. If an isolator is subsequently required and there is no clause in
the specification covering isolators, the variation instruction must include a complete
description or there is the possibility that an unsuitable type will be supplied. Since there
is invariably less time available for drafting variations than for the original specification,
it is better to have a few extra clauses in the specification than to risk contractual
difficulties later on.
Design of electrical services for buildings 290
No comments:
Post a Comment