There are t types of construction in modem buildings, one where the floors are of wood and supported by wooden joists and the other where the floors and ceilings are solid. Conduit in Solid Floors If an installation is being done in a building under construction, then with solid floors and ceilings the conduit should always be erected and completed before the cement is laid.
It is obviously a waste of time to install draw-in or inspection boxes in a solid floor, since they would be covered over and sealed off by the cement. If no conventional type of draw-in box can be used, this could lead to difficulty in wiring, so another type of conduit fitting can be used. This is called a loop-in conduit box and can be of various designs, but generally has 2, 3 or 4 holes in the back to enable the conduit to be “looped” from one box to the next. This means that these boxes can be used as draw — in and looping — in points for the cables.
To attach conduit to this box, the correct method is to screw a socket on to the end of the conduit and then clamp the box on by fitting a hexagon brass bush inside. This should then be tightened by means of a spanner.
Inspection tees or elbows are also of no use under a solid surface, so it is better to set the bends in concealed conduit runs than to fit made up bends. It is recommended that as few conduit joints as possible should be below the surface in a concrete floor, because when pouring, liquid cement might enter the conduit and form a solid block inside.
Some types of solid floors are too shallow to use concealed conduit, since a very sharp set would have to be made in the conduit when it leave the loop-in box in order to keep it below the surface. With larger sizes of cable, the radius of the set could then be less than the maximum allowable for the cable and so concealed conduit could not be used.
A typical layout of the concealed conduit system in a small house is given in fig using loop-in conduit boxes. At the ceiling points, where the loop-in conduit boxes are used, or other points where junction boxes are fitted, it sometimes happens that after plastering the box is recessed below the surface. To overcome this, extension rings can be obtained to give a flush fit with the plaster.
If socket outlets are being fed from concealed conduit, it is good practice to run the conduit from the floor above rather than from the floor below. This prevents moisture condensing in the traps that would be formed by the conduit if run below the floor.
When concealed conduit is run to switch and other positions in walls, it is usual to cut a chase in the wall to take the conduit. The chase must be deep enough to allow at least in of cement and plaster covering, otherwise rust from the conduit may come through to the surface of the wall.
It is also advisable to give conduit buried in plaster a coat of protective paint. The conduit should be secured in the chase by pipe-hooks or saddles at intervals of 4ft. along the conduit, and within 4 in. of each fitting or accessory.
Very often surface mounted accessories such as distribution fuse board are used with concealed conduit, and difficulty arises when running the conduit to the board. The recommended method is to fit a flush adaptable box in the wall behind the distribution fuse board and run the concealed conduits straight to it. The cables can holes. This presents a far neater appearance than the common practice of setting the conduits out from the wall to enter the distribution fuse board. A compromise method could be to sink a metal box partly into the wall so that the conduit can still be run concealed to it and then attach the distribution board directly to the exposed surface of the box. Drawing Cables into Conduit On new installations and those in humid conditions, moisture tends to accumulate inside the conduit. Once the conduit system has been completely erected, it must be made sure that the bore of the conduit is dry and free from condensation.
This is achieved by drawing a swab through the whole system just before the cables are due to be draw in. In a large installation it is better to start drawing in cables from a mid point in the conduit system, so as to cut down the length of cable which has to be drawn in.
A steel tape should be used from one draw-in point to another in order to pull through the draw wire. The steel tape should not be used to draw in the cables directly since it can get easily damaged. When the steel tape has been pushed through the conduit the draw wire should then be pulled through by the steel tape and the ends of the cables then attached to the draw wire. The recommended method is to bare the ends of the cables for approximately 2 in. and thread them through the loop at the end of the draw wire.
When drawing in cables, the reel or drum on which they are wound should be allowed to run freely and should be in an upright position, NOT laid on its side on the ground. If it is flat on the ground the cable spirals off the reel and is drawn into the conduit twisted. If a number of cables are being drawn in, as soon as they enter the conduit they all try to form the same shape as they were on the reels.
This results in cables taking up a screw like shape inside the bore and the insulation being worn away by friction as they are drawn down the conduit. This is quite a common fault in installation. The correct method if a number of cables are being drawn in at the same time would be to make up a simple rack on which the reels can revolve easily. If only two or three cables are being drawn in, then the reels can be supported on the short length of conduit held by the electrician’s assistant. By these methods the cables are not twisted and no trouble with drawing in is experienced.
As the cables are pulled in from one end of the conduit run, they must be fed very carefully into the conduit at the other end, making sure that the cables do not get tangled together, or that they are not pulled against the side of the draw-in box. Sometimes French chalk will help the cables to be drawn in more easily, especially in the hot weather. Oil or grease should not be used as lubricants.
Some slack cable should always be left in all drawn-in boxes in order to assist later wiring.
As stated previously, runs of cheated cable are usually at high level, so no protection to the cables in necessary. If however, the level of run in below about 8ft. e.g. at a switch drop, the cables must be protected, preferably with insulated material, because a length of steel conduit or pipe would have to be earthed. Where single core cables carrying heavy currents run in cleats go into metal boxes, e.g. metal casing for a switch, terminal box or similar equipment, they should, where possible, pass together through a single hole. If this is not possible, then individual holes must be made in the metal casing, but the space between these holes should be slotted. This prevents losses due to circulating currents in the metal.
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