Conduit Wiring: The Best System of Wiring | Electrical Engineering

Conduit Wiring is the best system of wiring. There are two methods for laying conduits. In one method conduit is laid on the surface of the wall, ceiling etc. This is called surface wiring. The other method is to lay the conduit recessed in wall or ceiling, and this system is known as concealed wiring.

Conduit may be rigid or flexible. Rigid conduit is used for general work, while flexible conduit is used for short runs. Various types of fittings are used for jointing and terminating conduit. Conduit and its accessories must be galvanised or enamelled in order that these are weather proof and immune to rust.

Wooden plugs are grouted in the wall at an interval of not more than 1 m. Saddles of suitable sizes are used for fixing conduit pipes secured to these wooden plugs by means of wooden screws. At all outlets or junction boxes rigid conduit should be provided with bushings so that the possibility of abrasion of wire insulation is avoided.

Different Types of Conduit:

Various types of conduit are in use. But in wiring the practice is to use heavy gauge conduits only. This conduit will be solid drawn or lap-welded and its outer surface will be stove enamelled or galvanised. Every conduit must be threaded at two ends. For the purpose of laying recessed in wall, a kind of oval shaped conduit is available now-a-days.

Three categories of conduit are available in the market. These are thin wall conduits, rigid conduits and flexible conduits.

Thin wall conduit may be close-joint type or brazed type. Close-joint conduit is made of thin strips of steel bent so as to form, a tube. There is no mechanical adhesion between its two edges. It is the cheapest type of conduit and not used for quality work. Brazed conduit is made similar to that of close-joint conduit with its two ends brazed together. As a result the brazing material remains projected inside the conduit, and this makes the drawing of wires through the pipes very difficult.

Rigid conduit is made of heavy gauge steel with its two edges electrically welded together. It may also be drawn from solid and has no joint throughout its length. These are the heaviest and best type of conduits and are available in lengths of about 3m. Rigid conduits are either black enamelled or galvanised.

Flexible conduits are classified into three categories. These are concaved double strip type, flat double strip type and single strip type. These conduits are made of galvanised steel strips specially wound upon each other. Flexible conduit is suitable for installations where certain amount of flexibility is required, e.g. wiring of motors having sliding bases. It is costlier than rigid conduit. Since this type of conduit is not moisture proof, it is not used in concealed wiring.

Size of Conduit:

The size of a conduit for the purpose of wiring should be such that the required number of wires of required sizes can be easily drawn through it. Negligence in this respect leads to excessive strain in drawing cables through it, and also there is risk of damaging the insulation of cables.

Besides, during future extension it will be impossible to accommodate higher sizes of cables in replacement of lower sizes or one or two more cables in the conduit. There will then be no other alternative but to draw out all cables from the old conduit and to draw them through a conduit of larger size. For this reason conduit wiring is to be designed with the idea of future expansion in mind. That is why the outside diameter of the narrowest conduit used in a wiring system is usually 19 mm (3/4″).

A rough estimate about the sizes of conduits required for specific purposes may be obtained from table nos. 17 and 18. Where there are more bends and/or twists, such number of cables should be drawn as would leave a space not less than 60 per cent of the total space inside the conduit. In such cases conduit used should be one size higher. Again, where the run is straight and more than 4m long, a conduit of one size higher should also be used there.

Points to be Observed in a Conduit Wiring:

All metal conduits should be well earthed so that electrical continuity between two sides of a joint is maintained. Each joint should be strong and lasting. If the conduit itself is used as earth continuity conductor, its resistance in d.c. circuit and impedance (overall) in a.c. circuit must not exceed 1 ohm.

If alternating current flows through the cables inside a steel conduit, all the phase wires as well as the neutral wire (if any) shall be drawn through the same conduit.

In countries where wind carries dust, arrangement must be provided to prevent entry of dust particle into the conduit.

Conduit Accessories: Conduit Box:

Conduits are always terminated at outlets into a box. This is known as Conduit box. There are different types of boxes used for various types of work. Conduit boxes may be two-way through, two-way angle, three-way or four-way. For long and straight runs of wiring, it becomes necessary to join one piece of conduit with another piece. The device by which two conduit pieces are joined together is known as Coupler. The function of a conduit coupler is the same as that of a socket for a water pipe of a gas pipe.

In general good quality of conduit boxes are made of malleable cast iron. But in order to reduce the cost of production ordinary grey cast iron boxes are also available in the market, and in many wiring system these boxes are used. The trouble with these boxes is that, grey iron being weaker than the malleable iron, the screw threads on these boxes are less durable. As a result the fittings cannot be tightly fixed with these boxes.

Conduit boxes of ordinary grey cast iron are available with either a hole or a piece of tube at the end. One such box is shown in fig. 148.

Where there is a box with a hole, the end of a conduit is inserted into the hole and a coupler is fixed on its outside threads. The inside thread of the coupler is tightly engaged at the other end with a hexagonal bush (fig. 144) made of brass. In this way a conduit is tightened with a conduit box and the fitting.

The conduit is at first fixed on the wall and then insulated wires are drawn through its inside space. As there are various bends and/or turns in a conduit line, the conduit box is very useful for drawing wires. There is a cover on each conduit box (see fig. 146). By opening these covers wires can be easily drawn through conduits.

In place of conduit boxes shown in fig. 143 and in fig. 145 elbow or bend may be used for drawing conduit line, but the angle box has many extra advantages. If at all elbow or bend is to be used (and conduit line is generally drawn with these accessories), this should be partly open and covered with a lid.

The purpose of this opening is that, cables can be drawn as easily as through an angle box. Such an opening is called inspection hole. Often a coupler is also provided with an inspection hole. Solid elbow and solid bend should not be used except in a free space. These may be used just behind a fitting or at the termination of a conduit.

Method of Fixing Conduits:

The roughness, if any, of the inner surface of conduit, coupler, box, bend etc. shall at first be smoothened, otherwise, being rubbed against this roughness, the insulation of wires may be damaged when drawn through conduit and its accessories. The inner surface of a conduit end or edges of a conduit box or bend should be properly filed and made smooth. It is always essential to provide brass or ebonite bushes at both ends of a conduit line. Bushes made of rubber or fibre are also available in the market. These are used in cheap work.

The next step is to cut the conduit to measure by means of very fine hack-saw and to make threads on its outer surface at two ends in order to fix it with box and/or other accessories. After the boxes and/or other accessories have been fitted with the conduit, extra length of its thread must be painted. Electrical continuity of conduit line will not be properly maintained if the threads are loosely cut.

Hence, special care should be taken in this respect. While fixing a conduit with a conduit box, care must be taken to see that the threads on both are clean. Otherwise this also adversely affects the electrical continuity.

The oil used at the time of cutting the threads should be carefully wiped out and the conduit is tightly fixed so that water etc. may not get any access into the conduit at a later date. The joint is tightened after applying aluminum paint on the threads. Red lead or putty should not be used. Wherever there is a joint, the paint must be applied there even at a latter date.

Besides, the conduit line shall not come in contact with a water pipe or a gas pipe. Throughout the entire length of a conduit line wherever there is a turning point, a bend with a radius of curvature not less than 75 mm should be used there. For this reason use of elbow is prohibited at many points. Where elbow is a necessity, it should be provided with an inspection hole. This helps to draw wires easily at a corner point.

In a good installation sometimes for drawing wires use of gas pipes is insisted in place of conduit pipes. Such an arrangement is adopted where it is necessary to keep the wires well-protected and where wiring line is drawn along the outer wall of a house. But in such cases it should be borne in mind that the gas pipes may rust as its inner surface is not stove enamelled and smooth as that of a rigid conduit. For this reason it is better to draw lead covered cables through the pipes, and cost of wiring should not be the main consideration in such cases.

In this connection another point is also to be noted. Wherever there is a turning point, inspection bend, inspection tee, etc. are to be used. Otherwise it will not be easy to draw lead covered cables through the pipes. Farther, a brass adapter is to be fitted at each point where a conduit is joined with a gas pipe. This is because the nature of threads in a conduit is not the same as that of a gas pipe.

The inner surface of the adapter is gas-threaded which fits with the gas pipe and its outer surface is electrical-threaded to match with the conduit. The size (outer diameter) of the gas pipe fits with the inner diameter of the adapter at one end, while the inner diameter of the conduit fits with outer diameter of the adapter at the other end. This is an essential point to be noted.

In case a conduit is drawn through floor, roof or wall, its continuity is tested immediately after installation and before completion of floor, roof or wall to see whether its resistance is as low as desired. If the installation work is properly done, almost in all cases the resistance is found to be 0.5 ohm or even less.

Conduit is installed by means of spacing saddles fixed on wooden plugs which are grouted into the wall at an intervals of about one metre or less. At the time of installation it should be observed that there is some clearance between conduit and the wall. It is for this clearance that the spacing saddle is at first placed on the wooden plugs and then the conduit is fixed over it (fig. 153).

Spacing saddle has many utilities. First, the hole through which the base of the saddle remains fixed on the wall with the help of screws is not round but oval in shape enabling us to move the saddle a bit this way or that way when necessary. This facilitates right and straight installation of conduit line. Second, the two holes through which the screws are driven to fix the saddle with the base are split (fig. 150). This helps to take out the saddle from the base easily.

Also little adjustment here and there is possible without any difficulty. Moreover the screws need not be fully unscrewed; only a little loosening is enough to remove or to re-set the saddle.

Often (specially in cheap work) saddles shown in fig. 151 are used for conduit installation. But this keeps the conduits in contact with the wall. Hence, use of such type of saddles should be avoided as far as possible. The type of saddle used to install two or more conduits side by side together is known as Multiple Saddle. Such a Saddle in shown in fig. 152.

At the time of fixing conduit, switch box, joint box, ceiling rose and other accessories should be fixed simultaneously in order to complete the conduit installation. Wooden block should not be used in conduit wiring. Where a conduit is to pass through a wall, it should at first be well-fixed with the wall so that switch, bracket and other fittings may not become loosened afterwards.

If there are too many bends and turns in an ordinary small room, it is the usual practice to draw a piece of long and narrow steel wire through the conduit along with the progress of fixing bends switch-box etc. This is not P.V.C. cable; it is an ordinary galvanished iron wire. Later on as many P.V.C. cables as required are tied up to the lead of the G.I. wire and the other end of the G.I. wire is slowly pulled.

All the cables are now easily drawn up through the conduit. The piece of narrow steel wire is called Pulling-in-Wire or Fish-Wire. Since the drawing of cables through a straight run of conduit line is comparatively easy, often G.I. wire is used only near a bend.

In case there is possibility of drawing a few more cables in future, a conduit pipe of larger diameter shall be used and along with single-core P.V.C. or V.I.R. wires, a pulling-in-wire is also to be drawn through it. The leads of the additional cables will be tied up with the pulling-in-wire in future and the cables will be drawn through the conduit.

Fig. 156 shows how the leads of the cables are tied up with a pulling-in-wire. The insulation of cables is cut off to some length and all wires are twisted together at one end. This end is then bent into a ring. The remaining bare wires are twisted together and tightly wound round the loose side of the wires so that the ring does not yield at the time of pulling. Later, all these should be so nicely taped that the bundled end becomes tapering.

As the slippery condition is convenient for pulling in, the bundled end is well smeared with French chalk or paraffin wax. At last the end of the pulling-in-wire is tightly twisted with the ring before pulling the cables. The precaution to be observed at the time of drawing the cables is that inside the conduit the cables do not intertwine themselves. This will make not only drawing of cables difficult on account of less space inside the conduit, but also it will become an impossible job to draw out one cable from the conduit when required.

While drawing cables through a conduit, it becomes more troublesome to draw unbraided P.V.C. cables than to draw braided V.I.R. cables. For this reason it is necessary to use more conduit boxes. Moreover, drawing of cables becomes easier if a little Vaseline is applied on their outer surface.

Before drawing cables through a conduit, a piece of wet cloth or jute should be drawn through it. This would remove any dirt or other thing from inside the conduit.

For a.c. wiring all live and neutral wires should pass through each part of the conduit line, otherwise there is possibility for the conduits becoming heated. This is a good practice for d.c. wiring also, although such precautionary measure is not necessary there.

Cables for Conduit Wiring:

Stranded cables are to be used in conduit wiring. In case of a.c. circuit single-core cable of size greater than 0.013 cm² (0.002 inch²) must not be drawn through any conduit.

Accumulation of Water inside a Conduit:

In tropical countries where summer is hotter and winter is colder and in countries where humidity is high (e.g. Bengal), water naturally collects inside a conduit. It is for this reason that the conduit wiring has to be carried out with great care in those countries and for the same reason sometimes opinions are against conduit wiring.

If a conduit line is to be drawn in an open space, it should not touch the wall; it should be taken over small pieces of wood fixed to wooden plugs. This facilitates good ventilation all round the conduit, thus reducing the water accumulation. It is better if these pieces of wood are dipped in creozote before fixing.

There should also be arrangement for ventilation through the space inside a conduit. Covering lids of switch box, tee-joint etc. are to be provided with vent holes. At the highest point of a conduit line a tee or some other device may be attached so that the conduit remains open upwards and thus ventilation is ensured.

In order, however, to prevent white ants or other insects to get an access inside the conduit and to spoil cables, all the openings in a conduit line should be covered by means of fine wire gauze. Where the conduit line goes a long way horizontally, it is necessary to allow a little incline so that one end of the line is at a bit higher position than the other end.

At the lower end arrangement must be provided for drainage of water accumulated inside the pipes. In open space care must be taken to see that rain water does not get access into the pipe. No part of a horizontal conduit line should sag; otherwise there is every possibility that water would accumulate at this part of the line.

There are, however, different views on this point. Some moisture is always present in the air. With the change of temperature volume of air also changes. As a result heated air comes out from the pipe and cool air gets into the pipe from outside. This cannot be prevented. Therefore, some persons hold the view that, instead of providing suitable openings for ventilation in the conduit line, the switch-box, ceiling rose-box etc. at the end points of the line may be filled up with plastic insulating compound.

This would prevent entry of cool air into the conduit and accumulation of water inside the pipes. The ends of the conduits near the fuse-board etc. should also be similarly filled with compound. This compound is available in the market. But in practice such type of precautions are seldom adopted.

In case of concealed wiring where conduit lines remain embedded into the wall, ceiling or floor, if wires are drawn through the conduit before it is completely dry along with wall, ceiling etc., there is possibility of the presence of accumulated water inside the pipe. One of the main causes of this water accumulation is that, during construction of roof or floor huge quantity of water is used everywhere and this water is quite likely to get an easy access into the threads of coupler bend etc. For this reason cables should never be drawn through conduit line until floor, roof etc. as well as conduit become perfectly dry.

Only one size of conduit should be used all throughout a conduit line. Even for a branch line or for a line connecting a lamp point, the size of conduit used should be the same as the sub- main line.

Where there is possibility of water accumulation, it is better to use cables of 650-volt grade there. After installation conduit line shall be properly earthed.

Now-a-days at many places, rigid polythene insulated pipe is used in place of steel conduit. This pipe may be fixed on the wall with saddles screwed to wooden plugs like steel conduit wiring. If concealed wiring is adopted, this pipe may be embedded under plaster of the wall.

Rigid polythene pipe shall be manufactured as per Indian Code of Practice No. I.S.: 2509-1963 and approved by the Electrical Inspector. Moreover, it is necessary to observe few more points at the time of wiring.

These are as follows:

(i) Polythene pipe should be thick and made of heavy gauze when used in concealed wiring, otherwise it may yield to the weight of concrete when roof etc. are cast.

(ii) While joining pipes and fixing ceiling rose, bracket etc., boxes made of P.V.C should be used.

(iii) Where ambient temperature-is about 50°C, great care must be taken to use this pipe. If the temperature is 60°C or more, polythene pipes must not be used.

(iv) When wiring is done with 250-volt, insulated cable, specified number of cables of specified size should be drawn through the specified size of pipe according to the table for non-metallic conduit pipe (Table No. 19).

(v) For outdoor wiring earth wire is to be drawn along the pipe line.