Conduit

In a conduit system the cables are drawn into tubing called conduit . The conduit can be steel or plastic . Steel conduit is made in both light gauge and heavy gauge , of which heavy gauge is much more frequently used . In both cases , it can be made either by extrusion or by rolling sheet and welding it along the longitudinal joint . The latter is specified as welded conduit and the former as seamless . Seamless conduit is generally regarded as the better quality . The different sizes of conduit are identified by their nominal bore and in the case of electrical conduit the nominal bore is always the same as the outside diameter of the tue . Thus 20mm light and heavy gauge conduits both have the same outsid diameter and consequently must have slightly different inside diameter . This is the opposite of the convention used for pipes of mechanical engineering in whichthe nominal bore usually corresponds more closely to the inside than the outside diameter . Electrical conduit is specially annealed so that it may be readily bent or set without breaking , splitting or kinking .
  Heavy gauge conduit is normally joined together by screwed fittings : there is a standard elecrical thead which is different from other treads of the same nominal diameter . A screwed connection between two lengths of conduit is shown in Figure 3.1  . A male electrical thread is cut on the ends of both lengths of conduit to be joined and a standard coupler with a female electrical thread is screwed over them . A lock nut , which has been previously threaded well up out of the way on one of the male threads , is then wound down and tightened against the coupler . The screwed connection is relied on for continuity of the earth path and the lock nut is essential to prevent the socket working its way along the threads until it engages nmore on one conduit than on the other . The reason for wanting an earth path is discussed in chapter 9 . method  of jointing conduit to boxes of the kind described  in Chapter 1 are shown in figure 3.2 A bush of some sort must always be used to provide a smooth entry into the box , to avoid sharp corners which could damage the cable insulation , and in certain cases to maintaine earth continuity .