Running electrical cables

The hardest part of the average electrical job is running the cables: it takes up a lot of time and a lot of effort. But there are certain techniques used by professional electricians which can make it much easier.

running electrical cables

Before you get involved in the details of how to install the wiring, there are few questions you must answer. Does it matter if the cable runs show? Is it safe ? Does it comply with the Electrical Regulations? This is because there are only two approaches to the job of running cable. Either you fix the cable to the surface of the wall, or you conceal it. The first option is far quicker and easier but doesn’t look particularly attractive; it’s good enough for use. in, say, an understairs cupboard. For a neater finish, using this method, you can smarten up the cable runs by boxing them in with some trunking. Many people, however, prefer to conceal the wiring completely by taking it under the floor, over the ceiling, or in walls.

Planning the route

With concealed wiring, the position is more complicated. When running cable under a floor or above a ceiling, you must allow for the direction in which the joists run – normally at right angles to the floorboards – and use an indirect route, taking it parallel to the joists and/or at right angles to them. When running cable within a wall, the cable should always run vertically or horizontally from whatever it supplies, never diagonally.

Concealing cables in walls

There are two ways to conceal cable in a wall. With a solid wall, chop a channel (called a ‘chase’) out of the plaster using a club hammer and bolster chisel, carefully continuing this behind any skirting boards, picture rails, and coverings.However, to give the cable some protection, it is better to fit a length of PVC conduit into the chase and run the cable through this before replastering.

To continue the run either above the ceiling or through the floor before you position the conduit, use a long drill bit so you can drill through the floor behind the skirting board. If a joist blocks the hole, angle the drill sufficiently to avoid it. With a hollow internal partition wall, the job is rather easier, because you can run the cable within the cavity. First drill a hole in the wall where the cable is to emerge, making sure you go right through into the cavity. Your next step is to gain access to the timber ‘plate’ at the very top of the wail, either by going up into the loft, or by lifting floorboards in the room above.

Drill a 19mm (3/4in) hole through the plate, at a point vertically above the first hole, or as near vertically above it as possible. All that remains is to tie the cable you wish to run to a length of stout ‘draw’ wire – singlecore earth cable is often used – and then to tie the free end of this wire to a length of string. To the free end of the string, tie a small weight, and drop the weight through the hole at the top of the wall. Then all you do is make a hook in a piece of stout wire, insert it in the cavity, catch hold of the string and pull it (and in turn the draw wire and cable) through the hole in the room below.

What are the snags?

There are two. You may find that, at some point between the two holes, the cavity is blocked by a horizontal timber called a noggin. If this happens, try to reach the noggin from above with a long auger bit (you should be able to hire one) and drill through it. Failing that, chisel through the wall surface, cut a notch in the side of the noggin, pass the cable through the notch, and then make good.

The second snag is that you may not be able to reach the top plate to drill it. In which case, either give up the ideas of having concealed wiring, or try a variation on the second method used to run cable into the cavity from below the floor. Here, it is sometimes possible to lift a couple of floorboards and drill up through the plate forming the bottom of the wall. Failing that you have to take a very long drill bit, drill through the wall into the cavity, then continue drilling through into the timber plate. You can now use the weighted string trick to feed the cable in through the hole in the wall, and out under the floor.

Running cable beneath a floor

The technique for running cable beneath a suspended timber floor depends on whether the floor is on an upper storey and so has a ceiling underneath, or is on a ground floor with empty space below. If it’s a ground floor, it may be possible to crawl underneath and secure the cable to the sides of the joists with cable clips, or to pass it through 19mm (3/4in) diameter holes drilled in the joists at least 50mm (2in) below their top edge. This prevents anyone nailing into the floor and hitting the cable.

If you cannot crawl underneath, then the cable can be left loose in the void. But how do you run it without lifting the entire floor? The answer is you use another trick, called ‘fishing’. For this, you need a piece of stiff but reasonably flexible galvanised wire, say 14 standard wire gauge (swg), rather longer than the intended cable run, and a piece of thicker, more rigid wire, about 1m in length. Each piece should have one end bent to form a hook; Lift a floorboard at each end of the room and use a push and pull action to get the electrical cable in place.

Hollow internal partition wall

Drill a hole in the top or bottom plate, then drill another in the wall where the cable is to emerge. Drop a weighted piece of string through one of the holes and hook it out through the other. Use this to pull through a stout draw wire which is attached to the cable.
• if the weighted piece of string gets obstructed by a noggin or its way to the hole in the wall, use a long auger bit to drill through the noggin.
• don’t pull the cable through with the weighted string – the string tends to snap
• never run cable down the cavity of an external wall – treat these as solid walls.

Under floors

Use a technique known as fishing:
• lift the floorboards at either end of the run
• thread stiff wire beneath the floor through one hole and hook it out of the other with another piece of wire • use the longer piece of wire to pull the cable through.

• if there’s a gap beneath a ground floor you can ‘fish’ the cable diagonally across the room under the joist
• if the gap under the joists is large enough you can crawl in the space clipping the cable to the joists
• where the cable crosses the joists at right angles, run it through holes drilled 50mm (2in) below their top edges.

Over ceilings
If you can get above the ceiling into a loft, you can clip the cables to the joists. Otherwise you’ll have to ‘fish’ the cable across.

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