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How to run a new electrical cable

Introduction

If you want to add a new electrical outlet or light fitting, it'll almost certainly involve running cable from a power source. You can simply clip cables to the surface of your wall, but it's safer and neater to conceal them. You can run them above your ceiling, below the floor or behind walls. Where it's practical, try to mark where you run new cables for future reference.

For your safety, electrical products must be installed in accordance with local Building Regulations. If in any doubt, or where required by the law, consult a competent person who is registered with an electrical self-certification scheme. Further information is available online or from your Local Authority.

Never take risks with electrical safety. Before you start any type of electrical work, you must follow these following safety precautions:

  • Switch off the main power at the consumer unit/fuse box. Isolate the circuit you plan to work on by removing the circuit fuse. Put this in your pocket to avoid accidental replacement
  • Or switch off the breaker and lock it if you can
  • Attach a note to the unit to advise you are working on the circuit
  • Check the circuit is dead with a socket tester or voltage tester/meter for lighting circuits

If you want to run cable between floors, here's what you should do in specific areas of your home.

 

Concrete floors

If a concrete floor has a timber sub-floor on top, you can lay cables on the surface of the concrete. But they should only run through a concrete floor if you pass them through conduit fitted when the floor was laid - not through channels cut into the concrete afterwards.

 

Clipping cable to a joist

Run the cable along the centre of one side of the joist, and space the clips at roughly 300mm intervals. Try to buy clips that match the size of your cable.

 

Drilling through a joist

Always be careful when you're drilling or cutting joists, as it's very important not to weaken the structure. Drill a hole that allows a 25% air gap around the cable - but no bigger than that. Keep to the central two thirds of the joist's length and drill at the centre of its depth (or at least 50mm from either top or bottom) to minimise weakening and ensure you're clear of any nails or fixings.

 

Ground floors

If you can get access to the underside of the floor, it's best to clip the cables beneath the joists.

 

Intermediate floors/ceiling voids

Clip the cables to the sides of joists, lay them on the ceiling or route them through holes drilled through the joists.

 

Lofts

You can lay cables across the top of the joists that run around the edges of your loft, well away from where anyone is likely to walk. You could also clip them to the sides of the joists - but keep them well above any thermal insulation, which might make them overheat. Polystyrene insulation can also react with cable sheathing.

You can simply mount cables on the surface of your wall with clips, or run them through plastic trunking that you can nail or screw to the wall (some types are even self-adhesive). It's a good idea to route them along skirting boards and around door and window frames so they won't be conspicuous. But for a really neat finish, you should conceal them. In a solid wall, you need to cut a channel (or 'chase') in the plaster and run the cable through an oval plastic conduit or, for longer lengths, 'top-hat' capping.

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Step 1

Always run cable vertically to a fitting in a solid wall so that you will know its approximate position when the work is finished. Plan the route of the channel using a spirit level or plumb line and mark it in pencil. Check it with a cable detector to ensure you won't accidentally damage an existing cable or pipe. Wearing thick gloves and safety goggles, cut the channel with a bolster and club hammer, making it 6mm wider than the conduit or capping. Cut away to the depth of the conduit, plus about 3mm.

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Step 2

Cut the plastic capping or conduit to length with a hacksaw and feed in the cable. Capping is fixed to the wall with masonry nails; conduit snaps into clips nailed to the wall, or can be simply held with masonry nails driven in each side. Either way, the cable will be held firm by the plaster when you fill the channel.

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Step 3

Fill the channel with patching plaster or filler to within about 3mm of the wall's surface. When it has set, add another layer flush with the wall surface. When the filler has set, lightly sand it for a perfect finish.

Before you begin, plan your layout. Work out where your inlet/outlet pieces and other accessories need to go, and use them as a template to draw around in the correct places.

 

Top tip - Blending with your decor

You can paint plastic trunking with gloss or emulsion paint to make sure it blends well with your home's decor. To get the best results, scuff your trunking with fine sandpaper to key the surface before you paint it.

 

Step 1

Measure the distances between each accessory to find out how long your lengths of trunking need to be. Then use a mitre block and fine blade saw to cut your lengths, taking care to make a straight cut.

 

Step 2

After checking your surfaces are clean and dry, attach your trunking using the self-adhesive tape already attached to the back of it. Make sure the lengths match up with your pencil position marks.

 

Step 3

If you're using trunking that's 40mm wide or more, you'll also need to screw it into the wall. Make sure you use fixings that work with the type of surface you're screwing into.

 

Step 4

Lay your cables inside the trunking, insert the accessories over the cable and close the trunking lid.

 

Step 5

When you're fitting corner accessories on quadrant profiles, it's important that both lengths of trunking butt up to each other, but don't overlap. Check this before you insert your corner accessories.

A stud partition wall is perfect for concealing cables. But it's much easier to install one while the wall is being built, when you can easily drill holes through the various parts of the framework.

 

In existing walls, you might be able to drill the head or sole plate and feed the cable in, then fish it out at the appropriate spot with stiff wire. However, there may be a horizontal nogging (bricks that fill in the open spaces in a wall between studs) blocking the way. One solution is to drill a passage for the cable through the plasterboard and nogging.

 

Safety first

If you over-bend cable, it will damage its cores. So be sure to cut off the bent section before making the connections.

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Step 1

Bend the end of the cable before you drop it into the wall, as this will help you retrieve it. If you come up against a noggin, you can either find its exact position with a stud finder, or by tapping the face of the wall with a hammer handle (the sound will be more solid over timber). Find the edges of the nogging by pushing a bradawl through the plasterboard. Then, after checking there are no hidden pipes or cables, use a 20mm wood bit to bore at a shallow angle through the plasterboard into the nogging from above and below. The holes should meet, which will allow you to pass the cable through.

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Step 2

Bend the end of a piece of stiff wire (like a coat hanger) into a hook shape. Then fish for the cable through the hole above the nogging. (Don't worry, this isn't as tricky as it sounds.) Then pull the end of the cable out through the hole.

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Step 3

Feed the end of the cable back into the upper hole and through the holes you've bored in the nogging, until it comes out through the lower hole.

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Step 4

Pull enough cable through the lower hole to reach the fitting, taking care not to damage the outer sheathing. (If you do, cut off and throw away the damaged length). Then feed the cable back into the lower hole and down into the wall. Fish it out of the fitting's mounting box hole. Finally, fill the holes and sand-smooth them when they're dry.

Another method of running a cable past a nogging is to remove a square of plasterboard, and then cutting a shallow notch in the face of the nogging to hold the cable. To stop yourself accidentally cutting the cable with a nail or screw, you can either fix a flat steel plate over it or hammer on a purpose-designed cable safe plate.

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Step 1

Before dropping your cable into the wall from above, mark its route in pencil using a spirit level or plumb line. Look for noggings with a stud detector, or by tapping the wall with a hammer handle and listening for a more solid tone. If you find a nogging, mark out a 125mm square at that position. Check for hidden pipes or cables, then drill holes in diagonally opposite corners. Insert the blade of a pad saw or plasterboard saw, and cut out the square of plasterboard.

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Step 2

Using a chisel and mallet, cut a notch across the face of the exposed nogging about 13mm deep and 20mm wide. Then feed the cable into the wall. You can protect it as it passes through the notch by screwing on a square steel plate, or hammering on a cable safe plate.

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Step 3

Cut a plasterboard repair patch to fit the hole (or use the old piece if it wasn't damaged) and nail it to the nogging at each side. Fill the gaps around the edges and sand the filler when it's dry.