Circuits

Chapter 5
Circuits
The final outlets of the electrical system in a building are lighting points, socket outlets
and fixed equipment. The wiring to each of these comes from an excess current
protection device (fuse or circuit breaker) in a distribution board, but one fuse or CB can
serve several outlets. If the circuit supplies current using equipment, wiring from one fuse
or CB is known as the final circuit, and all the outlets fed from the same fuse or CB are
on the same final circuit. The fuse or CB must be large enough to carry the largest steady
current ever taken at any one instant by the whole of the equipment on that final circuit.
Since the fuse or CB protects the cables, no cable forming part of the circuit may have a
current carrying capacity less than that of the fuse, unless the characteristics of the load or
supply are such that an overcurrent cannot occur. The size of both the fuse or CB and
cable is, therefore, governed by the number and type of outlets on the circuit.
It is unusual to have a fuse of more than 45A in a final distribution board in domestic
premises, and the cables normally used for final circuits are 1.5mm2, 2.5mm2 and
6.0mm2, according to the nature of the circuit. Lighting is almost invariably carried out in
1.5mm2 cable and power circuits to socket outlets in 2.5mm2. 6.0mm2 and 10mm2 cable
is used for circuits to cookers, instantaneous water heaters, showers, and other large
current-using equipment, such as machine tools in workshops. These sizes are so usual
that it is better for the designer to restrict the number of outlets on each final circuit to
keep within the capacity of these cables than to specify larger cables. If he does choose
the latter course there is a real danger that the site electrician will install the cables he is
used to, instead of complying with the designer’s specification. These considerations will
not, of course, apply in a factory in which individual machines can take very heavy
currents.
It is worth noting here that a single phase 1hp motor takes a running current of about
6A and a starting current of something under 30A. These currents would also be taken by
each phase of a three-phase 3hp machine. Therefore, quite large machine tools impose no
bigger a load than a cooker, and can be served from a fuseway on a standard distribution
board. The distribution in a workshop or medium sized factory can follow the same
principles as that in a domestic or commercial building. Indeed, in a technical college, the
electrical load in the metal workshops can well be lower and impose fewer problems than
that in the domestic science rooms with their many cookers.
For factories with heavier machinery, distribution boards with 60A or 100A fuses or
CBs can be used, with correspondingly larger cables. For factories with very large loads,

the normal type of distribution board ceases to be practicable and other means of
arranging the connections to the machinery are adopted.
We have said that the fuse or CB must be rated for the largest current taken at any one
instant by all the equipment on the circuit. This is not necessarily the sum of the
maximum currents taken by all the equipment on the circuit, since it may not happen that
all the equipment is on at the same time. One can apply a diversity factor to the total
installed load to arrive at the maximum simultaneous load. To do this, one needs an
accurate knowledge of how the premises are going to be used, which one can get by a
combination of factual knowledge and intuition. IEE Guidance Note 1 and the IEE On-
Site Guide include a scheme for applying the diversity factor, and circuits designed in
accordance with this should comply with the regulations; nevertheless the author of this
book thinks that a capable designer will not rely on such a rigid guide to the total
exclusion of his own judgement. A general knowledge of life and how different buildings
are used may be of more help than theoretical principles.
It will be easier to understand the ideas underlying the use of circuits intended for
fused plugs if we first consider the limitations of other circuits.
The fuse or CB in a final circuit may not have a rating greater than that of the cables in
the circuit. If it did, it would not protect the cables against overloads falling between the
capacity of the cables and the normal current of the fuse. However, neither the designer
of the building wiring nor the installer has any control over the sizes of flexible cables
attached to portable appliances which will be plugged in at socket outlets, although BS
7671 is directed to fixed installations only. When electricity was first introduced for
domestic use, the system adopted for house wiring in the UK was that 2A socket outlets
were provided to serve radios and portable lamps which would have small flexible cable,
5A outlets were provided for larger equipment and 15A sockets for the heaviest domestic
appliances such as 3kW fires, which would be supplied with substantial flexible cables. If
carefully designed and properly used, such a system would give reasonable protection,
not only to the permanent wiring of the building, but also to the flexibles and the portable
appliances plugged in at the socket outlets. Unfortunately, the multiplicity of plugs and
sockets made life difficult for the householder and tempted him to use multi-way
adapters, which totally defeated the object of having different sized outlets. Also, the use
of electricity has greatly increased since its first introduction to domestic and commercial
premises, so that it has become necessary to have a large number of socket outlets in each
dwelling and in every office. With the original system of wiring, it would have been
necessary to have many more fuses, the fuseboards would have become larger, more
numerous, or both, and the cost of the installation would have increased rapidly.
It was to overcome these difficulties that 13A socket outlets with fused plugs were
introduced a few years after the Second World War. The socket outlets are made to BS
1363 and the fuses that go in the plugs to BS 1362. The fuse in the plug protects the
flexible cable. Any fault in the appliance or any damage to the flexible cable will blow
the fuse in the plug; provided this fuse is correctly rated to protect the appliance, it does
not matter if the fuse in the permanent wiring has a higher rating. This latter fuse now has
to protect only the permanent wiring up to the socket outlet. The permanent wiring can,
therefore, be designed without consideration for protection of appliances, which have
been given their own protection. There is no longer any need to have different outlets for
Design of electrical services for buildings 78


different classes of appliance, and it is possible to standardize on one type of socket and
one type of plug.
This system depends on matching the fuse in the plug to the appliance. Fuses for these
plugs are made to BS 1362 in ratings of 3A, 5A, 10A and 13A. Modern appliances are
usually supplied with a moulded plug, and are properly fused by the manufacturer.
Fused plugs bring a further advantage. Whereas a 15A socket had to be assumed to be
feeding something taking 15A, a 13A socket with a fused plug may well be feeding
equipment taking 2A or less. Therefore, where there are several 13A outlets, it becomes
permissible to make use of a diversity factor in deciding on the circuit loading.
This is taken into account in the standard circuit arrangements given in the IEE
Guidance Note 1 or the IEE On-Site Guide, which should be used wherever possible.
A ring circuit with socket outlets for 13A fused plugs cabled in 2.5mm2 PVC cable
and protected by a fuse or circuit breaker rated at 30A or 32A can serve any number of
outlets but the floor area covered must not be more than 100m2. A radial circuit for this
type of outlet can serve a floor area of 50m2 if it is cabled in 4mm2 cable and protected by
a 30/32A HRC fuse or circuit breaker. If it is cabled in 2.5mm2 cable and protected by
any type of fuse or circuit breaker rated at 20A, it is restricted to a floor area of 20m2. In
either case there can be any number of outlets within this area.
Any number of fused spurs may be taken from any of these three circuits, but the
number of unfused spurs is limited to the number of socket outlets on the circuit. Water
heaters having a capacity of over 15 l, and comprehensive space heating schemes must be
provided with their own circuit.
The cable sizes quoted are for copper conductors with PVC insulation. IEE Guidance
Note 1 and the On-Site-Guide give different sizes for MICC cables.
Socket outlets of 15A and 5A must be assumed to supply appliances taking 15A and
5A respectively, and no diversity may be applied to circuits containing such outlets. The
circuit may, however, contain any number of such outlets provided the rating of the
protective device is equal to the sum of the ratings of the outlets on the circuit. Thus a
circuit with a 15A fuse may feed one 15A socket or three 5A sockets. The framers of
these regulations had chiefly in mind domestic installations, and the arrangement may be
departed from in non-domestic installations if the exact usage is known. To illustrate this
we can take as an example a hospital ward in which a socket outlet next to each bed is
needed for portable cardiographs or X-ray equipment. It is also desirable to ensure that
only certain equipment can be connected to these sockets. This may be done by installing
15A outlets and providing 15A plugs only for the equipment which is to use these special
outlets. It may be known that although there is to be one of these outlets next to each bed,
only one patient in a ward will be treated at one time. In such a situation it is clear that
twenty socket outlets could be put on one circuit and yet that circuit would never carry as
much as 15A, and an exception from the standard circuit could therefore be made or,
alternatively, sockets which are similar to the BS 1363 pattern but with the earth pin
turned through 90°. The fuse advantage of the plug is, therefore, maintained, but only
dedicated equipment may be plugged in.
A cooker, whether served through a control switch or through a cooker control unit,
should be on a circuit of its own. The cooker control unit may incorporate a socket outlet,
which will be on the same circuit. The circuit rating should be that of the cooker. Two
cookers in the same room may be on a single circuit provided its rating does not exceed
Circuits 79


50A. For domestic cooker circuits, IEE Guidance Note 1 and the IEE On-Site Guide give
the diversity to be applied.
We have referred to a ring circuit and we must now consider what this is. As its name
implies, a ring circuit is one which forms a closed ring; it starts at one of the ways of a
distribution board, runs to a number of outlets one after another, and returns to the
distribution board it started from. This is illustrated in Figure 5.1. The advantage of this
arrangement is that current can flow from the fuseway to the outlets along both halves of
the ring, so that at any one point the cable carries only part of the total current being
taken by the whole circuit. It is this feature which makes it possible for the fuse rating to
be greater than the cable current rating. The fuse carries the sum of the currents in the two
halves of the ring and will blow when the current in one part of the ring is about half the
fusing current of the fuse. For 2.5mm2 PVC thermoplastic cable to be used for a 30/32A
ring, its current carrying capacity after applying rating factors must not be less than 20A.
A circuit which runs only from the fuseway to the outlets it serves without returning to
the fuse, is called a radial circuit, to distinguish it from the ring
Figure 5.1


circuit which we have just described. Every circuit is necessarily either radial or ring.
In explaining the rules for the number of outlets on a ring circuit, we have spoke of
spurs, and we should pause to explain what is meant by this. Ideally the outlets on a ring
are placed so that the cable can run from the first to the second and from the second to the
third without doubling back on itself. If one outlet is a long way from the others, this
doubling back may be expensive in cable and it may be cheaper to serve the odd outlet by
a radial branch or spur from the ring. The reasoning which applies to the choice of cable
and fuse size for the ring does not apply to the cable in the spur. This length of cable must
be protected against overload and short circuit. The maximum load which can be
connected to a non-fused spur is a twin 23A socket, the maximum load is therefore 26A.
The cable must be rated accordingly. Alternatively, a fused connection unit is a
convenient device for providing fusing. The maximum fuse in this case is a 13A fuse. It
will be seen from the rules given above that a considerable number of spurs may be taken
from one ring, but in practice this is very seldom done.
Fused connection units are also used for connecting fixed appliances to ring circuits
even when they are close to the line of the ring. The fuse in the connection unit performs
the same function as the fuse in the plug of a portable appliance and protects the short
length of cable between the outlet and the appliance.
Although the ring circuit was developed for domestic premises, it is equally useful for
commercial premises and is frequently used for the power wiring of offices and shops. In
housing, it is standard practice to put all the socket outlets on one floor of a house on one
circuit. Consideration should be given to a separate circuit for a domestic kitchen. A little
more thought is clearly needed in the layout of the circuits in commercial premises. The
number of outlets on a ring is ultimately limited by the rating of the fuse. For premises
other than factories, it is almost universal to run ring circuits in 2.5mm2 PVC cable and to
fuse them at 30/32A. The designer must assess the maximum current likely to be taken at
any one time and plan such a number of separate circuits that none of them will be
required to supply more than 30/32A at a time. When doing this, it should be
remembered that the use of electricity has increased enormously in the past few decades
and is likely to go on increasing. It is, therefore, possible that within the lifetime of an
installation more appliances will be plugged in simultaneously than is usual today, and
also that individual appliances may be heavier users of current than the appliances in
common use today. Some allowance must be made for a future increase of use, and in the
absence of any other way of doing it, the author feels that it is prudent to restrict the
present maximum current to say 15–20A per ring circuit.
In a school or college workshop, it is often desirable for an emergency stop button to
switch off all machines. The usual arrangement is that prominent stop buttons are fixed at
two or three easily accessible places in the workshop so that in the event of any pupil
having an accident, the machine may be stopped quickly wherever the person in charge
happens to be at the time. All machines have to be controlled together so that if an
emergency button is pushed, they all stop. A further requirement for safety is that the
circuit must be such that the machines will not start again until the emergency stop has
been reset. There are basically two ways in which an emergency stop circuit to meet these
requirements can be carried out.
If the machine tools are large and each takes a large current, it will be better to feed
each one on a separate radial circuit on its own final circuit. There will then have to be a
Circuits 81

distribution board with a number of fuseways/CBs serving a number of machines. The
incoming supply to this board can conveniently be taken through a contactor which is
normally open but is held shut when the operating coil is energized. The circuit of the
operating coil is taken around the workshop and goes through as many emergency stop
buttons as are needed. The result is that when any one of these buttons is struck, the
operating coil is de-energized, the contactor opens, and the entire fuseboard is deenergized.
Everything fed from that board then stops.
A large workshop may have so many machines that it requires two or three
distribution boards to supply them all. The emergency circuit must then shut off the
supply to all of these boards. It would be possible to take the sub-mains to several
distribution boards through a multipole contactor with one pole for each phase and each
neutral, but such contactors are not readily available, and it is better for each distribution
board to be fed through its own contactor. The emergency stop circuit can contain a relay
with one pole for each contactor, and the operating coil circuit of each contactor is then
broken by the relay when the relay itself is de-energized on the interruption of the
emergency circuit. This is a simple arrangement, but needs an extra circuit for the relay,
and this circuit cannot come from any of the distribution boards which the relay controls.
One way of avoiding the extra circuit and the relay would be to cable the operating coils
of the contactors in parallel so that they were all part of a single circuit. This would have
the disadvantage that at least one of the contactors must have its coil and main contacts
fed from two different sources, so that it would be possible for the coil to be live when
the main feed to the contactor had been disconnected. Such an arrangement can cause
damage to unwary maintenance electricians and is not recommended. It is safer to pay for
the relay.
Similar circuits can be adopted whenever it is necessary to control a large number of
points together or from a remote place. The external lights of a hotel or public building
may, for example, be sufficiently extensive to require several ways of a six or eight-way
distribution board. If the distribution board is controlled by a contactor, all the lights can
be switched together on one switch, which is in the operating coil circuit of the contactor.
The second method of providing an emergency stop circuit is appropriate for smaller
workshops in which it may be cheaper and quite satisfactory to serve all machines from a
single-ring circuit. The outlets on the ring main take the form of fused isolators, and each
machine is connected locally to its own fused isolator. The emergency circuit still works
a contactor, but in this case, the contactor is on the load side of the fuseboard. The supply
is taken from the fuse to the contactor, and the ring starts from and comes back to the
contactor. The operation of the emergency stop cuts out everything fed from this one
fuseway but leaves in operation circuits from all the other ways of the distribution board.
For a small workshop, this saves the expense of a separate distribution board for the
machines only.
We have so far discussed mainly power circuits and must now say something about
lighting circuits. It is usually necessary to have several lights on one circuit with each
light controlled by its own switch. Figure 5.2 shows the wiring arrangement used to
achieve this. It also shows circuits for two-way and for two-way and intermediate
switching.
Design of electrical services for buildings 82

Examination of these diagrams will reveal that the flexible cord to the lampholder is
protected by the fuse in the whole lighting circuit. It is usual that this fuse does not have
higher rating than the smallest flexible cord used on that circuit, although, strictly
speaking it does not need overload protection since the maximum lamp size that can be
connected to the bayonet lampholder is 200W. The flexible cord does need short circuit
protection. In domestic premises flexible cords will almost inevitably be 0.75mm2 rated
at 6A and so domestic lighting circuits should not be fused at more than 6A. Now a
150W tungsten bulb takes 0.65A and consequently a 6A circuit can have nine of these on
simultaneously. In an average house, the 6A limit will not be exceeded if there is one
lighting circuit for upstairs and one for downstairs, but in a large house it may be
necessary to have more lighting circuits than this.
In commercial and industrial premises far too many circuits would be needed if they
were all restricted to 5A. There is no objection to lighting circuits being rated at 10 or
even 15A, but the designer and installer must make sure that all pendant cords to lights
are of the same rating as the rest of the circuit.
It has been explained in Chapter 1 that in the wiring of buildings, cable joints are not
made by soldering but by mechanical connectors, or crimping. It is a help to maintenance
if loose connector blocks can be avoided and cables joined together only at the various
outlets. Furthermore, each connector adds a small joint resistance and it is advantageous
to keep the number of these down to a minimum. Mechanical connections cannot be
avoided at the outlets, but the number of joints can be reduced if no connections are made
except at the outlets. This method of joining cables only at outlets is known as the
‘looping in’ system, and the diagrams in Figures 5.1 and 5.2 show how it is achieved.
One piece of cable or three single cables for cables in conduit or trunking run from the
distribution board to the first outlet (socket outlet or ceiling rose as the case may be) to
the second outlet. If the wiring is properly planned and carried out there need be no joint
in this length. A second run of cable runs from the second outlet to the third, and so on.
Both these cables connect to the same terminal at the second outlet and thus the circuit
becomes continuous. No joints are needed except at the outlets. This means that the joints
are easily accessible.
The looping in system is the reason for having the third terminal on a ceiling rose,
which we described in Chapter 1. It can be seen from Figure 5.2 that the third terminal
(excluding the earth terminal) is needed to join the incoming and outgoing phase cables.
It remains live when the light attached to the rose is switched off, and can give an
unpleasant surprise to the home handyman replacing a luminaire. All circuits should be
effectively isolated before work is carried out. All good ceiling roses have this terminal
shrouded to prevent accidental contact, but nevertheless, some specifications insist that
only two terminal roses be used. The looping could be done at the switch, as
Design of electrical services for buildings 84


Figure 5.3 Looping at switch
shown in Figure 5.3, but it is difficult to visualize a building in which this scheme would
not require very much more cable than that of Figure 5.2.
We have described the way in which outlets are conventionally grouped and arranged
in final circuits. Each of these sub-circuits is fed from a fuse on a distribution or
fuseboard and the next step in describing a complete electrical system is to show from
where the distribution board obtains its supply. This we shall do in the next chapter.
IEE Wiring Regulations particularly applicable to this chapter are:
Section 314
Section 463
Appendix 4
Circuits 85