Summary: Learn how to run cables under floors, run cables parallel to joists, run cable at right-angles to joists and run cables in stud walls.
* Notifiable project requiring Building Control approval.
Electrical cables can be surface-mounted, fixed to skirting boards or architraves with clips or fixed to walls and concealed with self-adhesive mini-trunking. To conceal wiring completely, you will have to bury it behind walls or under the floor.
Planning the route
When deciding where to put power sockets and fittings, trace the path of existing cables and plumbing pipes using a cable-detector and pipe-detector. You may need to gain access to the joists to locate the main circuit cable.
Cables should be run along the ceiling void or under the floor to a point directly above or below the switch or appliance outlet and never run diagonally across walls to reach the switch.
Light fittings and junction boxes will need to be positioned on one side of a joist or stud to secure them to the timber structure but switches, sockets etc. must be between joists or studs.
If you have concrete floors you must conceal cable in the walls, as cutting into the floor risks damaging the damp proofing and any buried pipes.
Running cables under floors
There are two methods of securing cable to the joists: you can either run the cable parallel with the joists, fixed to the side of the joists or drill holes in the joists and feed the cable through them at right-angles.
Running cable parallel to joists
- Lift every third floorboard along the cable route with a crowbar.
- Wind thick wire around the end of the cable like a mouse string, and use it to thread the cable underneath the floorboards to a position above or below the outlet or appliance.
- Fix the exposed cable to the joist at regular intervals by hammering in clips 50mm (2in) below the top of the joist.
- To lead the cable to a lighting point between joists, fix a batten between the joists and run the cable along the batten, and secure as before.
- Drill an exit hole in the ceiling or floor and feed the cable through.
- Replace the floorboards and mark them to show where the cable has been laid.
Running cable at right-angles to joists
- Using a crowbar, lift the floorboards along the path the cable will take to gain access to the joists.
- Reach down between the joists and drill a hole in the centre of each joist to enable you to feed cable through the joists. Use a flat wood bit about 10mm (½ in) wide.
- Thread the cable loosely through the holes.
- Drill an exit hole in the ceiling or floor for the cable.
- Replace the floorboards and mark them to show where the cable has been laid.
Running cables in stud walls
If you are building a new stud wall, you can run cable between the sheets of plasterboard. To protect the cable, you can thread it into a PVC conduit as used in masonry walls.
- With one side of the wall plasterboarded, pull the cable through a hole in the head or sole plate.
- Use a flat wood bit about 10mm (½ in) thick to drill holes in the timber studs.
- Feed the cable through the holes to the exit point.
- An alternative to drilling holes in the studs is to cut notches in the front face of the studs, which the cable can rest in, then fix the cable to the studs with clips.
- Plasterboard the other side of the stud wall, cutting a hole in the sheet of plasterboard where the cable needs to exit.
Dropping a cable behind existing stud walls may be possible if there are no horizontal noggings or blanket insulation to block the path of the cable.
- At a position above the intended outlet, gain access to the ceiling by removing the floorboards with a crowbar.
- Remove any insulation under the floorboard to gain access to the head plate.
- Using a10mm (½ in) drill bit, drill a hole through the head plate and feed the cable through the hole and down behind the stud wall.
- Cut a hole in the stud wall to retrieve the cable.
- To draw the cable up from below, drill an exit hole for the cable in the floor and a mounting box recess in the wall, and pull the cable up behind the plaster to the mounting box recess with a mouse string.
Running cables in masonry walls
A groove cut into a masonry wall to accommodate wiring is called a chase. Before you cut out a chase to house the cable, use a cable detector and pipe detector to locate any other cables and pipework behind the walls. If existing wiring is near to where you want to run a new cable, cut off the power supply at the consumer unit before carefully cutting out a chase.
- Draw 2 lines 25mm (1in) apart with a spirit level.
- Drill overlapping holes down the length of both lines.
- Wearing safety goggles, place a bolster chisel in the groove made by the holes and tap it with a club hammer to chop out a chase. Alternatively hire an electric chasing machine.
- The electrical cables must be protected in a PVC conduit cut to the length of the chase.
- Feed the cables into the conduit and push the conduit into the chase at least 5mm (1/5in) from the surface to allow for plaster on top. The conduit should terminate at the mounting box and below the top of the skirting. See the materials section for advice on joining and cutting PVC conduit.
- Secure the conduit with galvanised nails hammered in either side of the conduit.
- Follow the directions below to cut a recess for a mounting box.
Fixing a mounting box
Whether your new cable route is to supply a switch, socket or other outlet, you will need to fix a mounting box or plate to the wall to house the connections. To make a hole in the ceiling for a ceiling light flex or to install a light fitting mounting box, see the section on installing light fittings.
- Hold the mounting box level against the wall at least 150mm (6in) above the floor or worktop and draw around it.
- If you are installing a number of sockets, consider purchasing a box chaser attachment for your drill, which cuts out holes for mounting boxes quickly and neatly.
- If the wall is solid, fit a masonry bit into your drill and set the drill to the depth required by the mounting box.
- Drill holes around and inside the outline.
- Cut out the recess with a club hammer and cold chisel or brick bolster.
- Vacuum the hole and surrounding area to remove the dust.
- If the wall is plasterboard, drill a starter hole and insert a padsaw blade into the hole to cut out the marked area.
- Push out the tab in the back of the mounting box covering the cable entry hole. If the mounting box is metal, place a grommet around the entry hole to protect the wires.
- Feed the cables though the hole in the back of the mounting box and fit the box in the hole. Drylining boxes for hollow walls have spring-loaded lugs, which lock the box in place when it’s pushed into position.
- If you are fitting a mounting box to a masonry wall, mark the position of the screw holes, remove the box to drill and plug the holes.
- Feed the cables through the hole at the back of the mounting box and screw the mounting box to the wall.
- Plaster over the chase and around the mounting box area following the guidelines in the section on patch plastering.
Tools Needed
- Cable/pipe detector
- Hammers
- Power drill
- Masonry bit
- Wood bit
- Crowbar (to lift floorboards)
- Brick bolster and stone chisel
- Club hammer
- Padsaw
Materials needed
- Mini-trunking
- Sockets, switches, junction boxes
- Cable clips
- Timber batten
- PVC conduit
- Galvanised nails
- Wall plugs
- Screws
- Hammer
Important note:
The colour-coding system for electric cable changed in 2006, make sure you read Electric cable and flex to view the new information.