Physical Plan of the Electrical Installation

The layout plan

Position of intake point

An important factor to consider is the situation of the intake point. One place to avoid is the cupboard under the stairs, which is likely to get filled with things like pushchairs and tennis racquets, so that the meter reader has great difficulty in getting to the meter, and if a fuse blows, confusion in the resulting darkness may become even greater as the householder first has to force his way through old trunks and then fumble around with fusewire.

It should be realised that the area electricity boards only allow a certain distance of ‘free’ cable, from the mains in the street, or from the pole line, to the supply point. This is often of the order of 10 metres. If a supply point position is chosen so that this distance is exceeded, the consumer must be prepared to pay for the extra cable involved.

A garage is often found to be a convenient place for the intake point, but care must be taken to situate the fuses, etc., in a position agreed with the area electricity board, and to allow ample room for cabling runs to the rest of the house, and for extensions to the consumer unit assembly.

Many new houses are fitted with a cloakroom or lavatory adjacent to the hall, and this is another place where the intake point may conveniently be situated, perhaps in a cupboard. A new development is the placing of meters on the outside wall of the premises in a special lockable cabinet, so that the meter reader can take the readings even when the household is unoccupied.

How many socket-outlets are needed?

The number of socket-outlets to be installed, and their positions, must be a matter for the owner of the installation, but the electrician should try and persuade him to be as generous as possible with outlets. In every household the usage of electrical appliances is always increasing.

In general socket-outlets should be installed in living rooms at about 500mm from the floor level, to obviate the need to stoop down to insert the plug. In the kitchen, socket-outlets may be at table-top level, for convenience in using irons, food mixers, kettles and the like.

It is desirable to install at least one socket-outlet on the landing and another in the hall, so that vacuum-cleaner connections may be made without difficulty.

It should always be remembered that much time and material can be saved by careful planning and measuring-up before commencing work. The frenzied rush to the electrical shop for one more reel of cable, just as it closes, is a sign of bad planning.

There is another advantage in taking great care to prepare a drawn-out plan of the installation. It is almost inevitable that extensions will be needed some time in the future, and it is of course not impossible that a fault will occur in some part of the wiring. If the original wiring plan is available, both the work of extension and that of tracing and rectifying a fault will be greatly simplified.

Identifying tags attached to cables under floors and behind consumer units will be found useful if work is to be carried out in the future.

Size of cable

Having decided on the circuit layout, the next problem relates to the physical installation of the wiring.

The first point to decide is the size and type of the cabling. The size of cable to be used is determined by several factors.

Lighting circuits

Each lampholder or fluorescent lighting fitting is usually taken as consuming 100 W. So if there are six lighting points on a circuit, the power that must be allowed for is 600 W. This is equivalent to 2.5 A, at 240 V.

For lighting circuits it is usual to arrange the points so that the circuits are wired using 1.00 mm2 cable with PVC insulation, in the form of twin sheathed PVC cable with protective conductor included. These circuits are controlled by 5 A fuses, and care should be taken to ensure that this current is not exceeded.

Permanently connected appliances such as cookers, water heaters and radiators For appliances that consume a considerable amount of current, for example a cooker, the total wattage of the appliance must first be ascertained – that is, the loading with all parts switched on to their highest loadings.

Suppose this comes to 6kW. The current necessary is therefore 25 A.

According to the wire tables published in the Regulations for Electrical Installations, where twin sheathed cable is used, the 6 mm2 conductor size is suitable, when supplied from a 30 A fuse or MCB. But when considering appliances like cookers, it should always be realised that loadings continually increase, and when a new cooker is installed it may be very costly and inconvenient to have to rewire the circuit. Therefore it may pay to allow for a larger loading than is strictly necessary.

Investigations into the usage of cookers and other heavy-current appliances have shown that in normal domestic use a diversity factor may be applied. For example, a cooker whose total loading, with all plates and oven elements switched on full, is 13 kW, needing about 54 A, may in fact be connected by means of a 6mm2 cable, protected by a 30 A fuse because diversity is applied to take into account the fact that all the load will not be used at the same time.

The operation of diversity of demand is such that most cookers used in domestic premises may be supplied from a 30 A fuse, that allows for over 7kW of load, however a 45 A fuse is available should a very large double oven cooker be used.

Ring circuit

The ring circuit is permitted, under the Regulations, only for use with 13 A socket-outlets fitted with fused plugs. The number of socket-outlets that may be connected has been set out earlier in a preceding post.

Mineral-insulated cables

Mineral-insulated copper-sheathed cables employ single wires, not stranded conductors, and since the current-carrying capacity of any conductor is related to the heating effect caused by the passage of the current, and the mineral insulation differs from the PVC type of insulation in regard to its heat resisting properties, higher operating temperature and the ability to dissipate heat, different cable size rules apply. Table 4.1 shows the ratings of the sizes most commonly used.

Flexible cords

Flexible cords should always be used as little as possible and should be kept as short as possible. They should never be used for permanent wiring. Four types of flexible cord are commonly used: for light-current work for heavy-current work for portable tools, etc. for the connection of hot appliances.

For light-current work such as suspending lamps and connecting up lighting fittings and radio sets, where the current does not exceed 3 A, the type of flexible cord generally used is twin, plastic-insulated cord, with 0.5 mm2 conductors.

For heavy-current appliances such as radiators, three-core plastic-insulated cord, sheathed or covered with braiding, is used, with 1.5mm2 conductors.

For portable tools, a three-core, plastic-insulated cable, with 1.5 mm2 conductors, is suitable.

For connections to such appliances as irons, water heaters and any other appliance that can become hot, a butyl rubber or silicone rubber cable, of three-core construction, with special heat-resisting qualities, and conductors of 1.5 mm2 size, is used.

Note: in using these tables, it is important to note that they are not extracts from the Regulations but are derived from them and are intended for approximate guidance only, the full text of the Regulations for Electrical Installations should be consulted before they are applied to anything but the simplest domestic installation. There are important qualifications in the Regulations that must be observed in certain cases; for example the cables must be derated if the ambient air temperature is over 30° C. Conversely the ratings may be increased if the ambient temperature is below 30° C.

Wire tables

Table 4.2 shows the current-carrying capacities and weight-supporting loadings applicable to commonly used flexible cables used in domestic and small industrial installations.

Type of cabling

The conduit system

If the heavy gauge steel conduit system is adopted, the system must be electrically continuous throughout: that means that all sections of conduit must be screwed together, with screwed sleeves to join each section of straight pipe, and properly screwed and terminated lengths running into metal junction boxes and the boxes that receive switches, ceiling roses, socket-outlets, clock connector boxes and all other fittings.

The conduit must be carefully measured before screwing, and after the thread has been cut with the proper dies, the ends must be reamered out to remove all sharp edges.

Steel conduit is made in various sizes, and Table 4.5 gives the mechanical characteristics of the sizes most commonly used in domestic installations.

It is laid down in the Regulations that all conduit installations must be completely erected before any wires are drawn in.

In deciding how many wires can be pulled into conduit systems, it must be realised that the friction caused by the pulling in of wires running close together can damage the PVC insulation.

The Regulations for Electrical Installations lay down the number of separate single-core cables that can be drawn into various sizes of conduit, and it is assumed that the reader who contemplates installing a system of this kind will have consulted the Regulations.

The method used to determine the cable capacities of conduits is described in Regulation 529-7 and Appendix 12. Each cable size is allocated a factor. The sum of all factors for the cables intended to be run in the same conduit is compared with a Table, and a suitable size of conduit is selected, this takes into account any bends or sets that are to be included in the installation.

Installing conduit

If conduit wiring is to be installed in a new house, the conduit must be placed in position at exactly the right time in relation to the other building work, or else there will be a great deal of expensive reinstatement of plaster work that has had to be cut into to run the conduit. This stage is usually called the carcassing stage.

When the the shell of the house is built, and the roof is on, but before the floorboards are laid and of course before plastering commences, the electrician has the ideal conditions for his work.

For extensions to existing installations it is usually necessary to chase out a trough in the plaster and bricks for the conduit to lie partly below the surface, otherwise, with a plaster depth of only 10 mm, the conduit would stand proud of the wall. For the outlet boxes, into which the actual socket-outlets are to be fitted, a deeper hole must be cut in the wall.

Right-angle elbows should be avoided wherever possible, because of the obstacle they provide to easy drawing-in of the wires. Bends are preferred, and these should have an internal radius of not less than 2.5 times the overall diameter of the conduit, e.g. overall diameter of conduit = 16mm, minimum internal radius of bend = 16 x 2.5 = 40 mm.

Although a conduit system is in theory completely sealed, from end to end, air can gain access at the socket-outlets and switches, and moisture-carrying air brings with it the danger of condensation. This could mean that small quantities of water could run down into switches and other fittings, and give rise to corrosion. To prevent this, conduit systems should have drain holes situated at low points.

Conduit runs should always be kept clear of gas pipes, water pipes, and any other wiring. This is because of the possible dangers of bimetallic corrosion through contact between different metals, and also because of possible sparking dangers, if a fault should occur on the electrical installation and fault current flows through the conduit itself.

As erection proceeds, draw wires should be inserted from junction box to junction box. These wires will greatly facilitate drawing-in. The Regulations state that all the conduit must be complete before any wires are drawn in.

When pulling in the cables, the best method is to bare all the ends of the conductors, twist them together and bend them over to form an eye and attach the end of the fish wire or tape to this eye. Care must be taken later to cut away the whole of the conductors used for this purpose, as they may have become damaged, before making them off into the fittings.

Always make certain that enough cable is pulled through. It is very unwise to leave only the bare minimum protruding from the box in the wall. If any slight slip is made during making off, there will be an extremely arduous business of pulling new wire in.

As a general rule, always try to avoid joints, even though they can be made in proper joint boxes, if there is no alternative. This means, of course, that short lengths of cable will get left on drums, and the electrician may think he has been wasteful. But he will have the satisfaction of knowing he has provided a sound job, and he will have avoided the laborious business of making joints in joint boxes which may – and often do – find themselves in difficult positions.

When it is necessary to pull in a number of cables, nasty kinks and knots may be avoided by ‘combing’ the cables through a piece of wood or stiff cardboard, with an appropriate number of holes, before entering them into the conduit. This will ensure that the cables are reasonably straight and will ease the drawing in process.

Sheathed wiring systems

PVC insulated and sheathed cables are by far the most commonly used materials in domestic wiring systems, whether for new installations or for extensions to existing wiring.

All wiring systems, as we have seen, must have proper earthing arrangements. With properly screwed metal conduit the conduit itself may act as the protective conductor, but where sheathed cable is used there is usually an uninsulated wire lying between the red and black wires within the sheath, to provide earth continuity.

The main points to watch when installing sheathed wiring systems are these: (1) The greatest possible care must be taken to ensure that the cables are protected against mechanical damage. (2) The cables have only a limited ability to resist heat, and must not be installed, for example, in the close vicinity of hot-water pipes or near flues.

If cabling has to be run in hot situations, the best choice is mineral-insulated copper-sheathed cable.

Preventing mechanical damage

Returning to the question of preventing mechanical damage, the installer must always try to visualise what might happen: for example, running cable in a trough cut into the plaster down a wall to reach a switch may be all right while the present occupier inhabits the room, but some future occupier might easily decide to fit a bookcase or a mirror on that part of the wall and drive nails or fixing screws right through the wire.

The best way of running sheathed wiring in plaster is to install a round or oval conduit for all runs, or at the least to chase out a trough for the wiring so that it can be covered with a metal channel. The use of conduit for runs to socket-outlets and switches has the advantage of enabling the cable to be pulled out if it has to be replaced, without breaking away the wall.

Alternatively, metal channelling fitted above the cable before making good the plaster will partly protect the cable, although complete protection is not ensured. A cautious person driving a nail or drilling for a wall plug would probably detect the metallic contact, if the channelling was in the line of fixing. The channelling will also protect the cable during plastering.

Similarly, if the floorboards are removed to run cabling beneath, it is rather tempting, when running at right angles to the joists, to cut a nick or groove in each joist, and run the cable across them in this way, afterwards replacing the floorboards. The proper method is to drill generously sized holes in the joist, at least 50 mm down each joist, and thread the cable through. 20 mm holes will allow three 2.5 mm2 cables to pass through.

In general, if it is difficult to conceal the cables by running through confined spaces, or where thick stone walls are encountered, or if it is very difficult or impossible to obtain access below floors, it is better to run the cabling on the surface. People are not likely to damage a cable they can see. It is quite possible to run the usual cables in domestic premises so neatly that when painted to match the surroundings they become almost invisible, but are still apparent to anyone proposing to drill or drive nails.

Where the wiring need not be covered for aesthetic reasons, such as in a garage, it is better to run it in such a way that it can be clearly seen. By using this method damage to the wiring will be less likely than if it is indifferently concealed.

Cables must always be supported by some form of clasp, usually taking the form of plastic clips or buckle clips, the normal spacing being 250mm apart. Saddles may be used to embrace several cables.

Where a number of cables run along a joist, half-way down, an easy way of fixing is to use saddles from old sheath ends, secured with tacks of at least 15 mm in length.

Sheathed cables should never be allowed to run unsupported for a distance much exceeding 300 mm. The reason for this is that if they are at any time subjected to overheating – and this may be due not only to the passage of excess current, but also through the proximity of a flue, or a hot-water pipe – the sagging cable may become distorted so that the PVC insulation tends to drop away from the conductors, and might even leave them bare.

It is always bad practice to tie or tape sheathed cables to existing water or gas pipes – especially the latter. Wood battens should be run along the path to be taken by the cables, well removed from the piping, and the cables secured by means of saddle clips. Running sheathed cable through walls: protective conduit should be employed, it need not be earthed

Where sheathed cables have to pass through a wall, or through any partition, the best method is to use some lengths of protective conduit.

Running out the cable

The best system to employ, when running out sheathed cables, is to arrange some sort of spindle for the drums, such as a broomstick supported on wooden frames. If the cable runs off the drum as it lies flat on the floor, there will be a danger of producing a twisted mess of cable that will take much time to disentangle and may in any case result in kinks and untidy twists in the cable, preventing a neat appearance in the finished job.

To secure the neatest job, experiment with two short lengths of cable, say 300 mm each, on a piece of wood. See how near to each other the cables will lie, and then apply two sets of plastic clips, 250 mm apart. If the cables are now clipped in, and lie as close together as possible, measure the spacing between the clips at each point, and adopt that measurement along the runs of cable where more than one circuit is to be laid. Nothing looks worse than wiring with two or three cables untidily running along at varying distances apart.

When running out sheathed cable, which comes in 50 or 100 m drums, great care must be taken to avoid twists and kinks, as mentioned earlier. These may harm the cable if, when kinked, it is pulled too tight. The best way of smoothing out the cable is to clamp a smooth round object, of at least 30mm diameter, in a vice and pull the cable tightly round it, one hand opposing the other.

PVC sheathed cable tends to be stiff to handle in cold weather. This problem can be overcome to some extent by storing the reels in a warm room before operations commence.

Assuming the installation is to use sheathed cable run beneath the plaster, the first process is to mark out, as accurately as possible, the cable runs.

The cable could be covered with steel channelling, and this comes in standard sizes. A run commonly used is two 2.5 mm2 cables, as employed on a ring circuit, and is about 45 mm wide overall. If this size of channelling is used, a chase, or trough, about 50 mm wide needs to be cut in the plaster.

The best way to cut the plaster without bringing away more surface than is required is to employ a wide flat chisel and cut diagonally inwards at both sides of the chase. The plaster can then be levered out with a knife.

The cable will terminate in a box, on which will be mounted the switch or socket-outlet.

Fitting steel boxes

Steel boxes are made in different depths. The shallow boxes are 16 mm deep, and do not allow much room for more than one cable and the resultant interconnecting wiring. These are eminently suitable for switches. A deeper box is 47 mm deep, and allows ample room for PVC connectors, when a joint has to be made in a cable, or for the three sets of wires that result, say, from the entry and exit of ring circuit connections and also the departure of a spur connection.

These boxes have knockouts on all sides, and when the knockout has been removed, a rubber grommet must be inserted in the hole.

The brickwork must be cut away to receive the deeper type of box. Over the whole area to be cut away holes are drilled in the brick with a size 10 or 12 masonry drill, and then a sharp chisel will soon clear away the honeycombed brick.

The box must be sunk in so that its top edge is about 5 mm below the general plaster level of the wall.

Holes for screws will be found in the base of the box, two of them usually being oval to allow for adjustment. Holes must now be drilled in the wall, a wall plug inserted, and the box screwed firmly to the brick.

At this point a small spirit-level is useful, because the levelling of the box will ensure that the front plate of the switch or socket-outlet will also come exactly level. If one switch-plate slopes one way and an adjacent switch slopes in the opposite direction, the result clearly indicates bad workmanship. The spirit-level may be used to level the box, using the tolerance provided by the oval holes, before the screws are finally tightened.

Fitting cables and channelling

The next step is to run the cables in the chase cut in the plaster. The cables are naturally springy, and it is not easy to get them to be flat ready to receive the metal channelling. It has been found worthwhile to plug the brick in one or two places is made available, making off may be difficult, and this may lead to the physical impossibility of tightening all the terminals as they should be tightened.

Loosely tightened terminals can easily cause internal sparking and overheating. A high resistance is introduced into the circuit at this point, and insulation may become charred and dangerous, and springs in switch contacts may become overheated and lose their tension, and so provoke further trouble.

As each fitting – switch, ceiling rose, or socket-outlet – is completed, it should be gently pushed into position, the electrician taking every opportunity to make absolutely certain that as the fitting goes into its box none of the wires is being twisted too sharply, or compressed against a sharp edge, or brought into contact with the live terminals.