How to chase walls and run cable safely

Helpful video reference. The video above is from "Electrical Chasing and cutting in boxes" by GSH Electrical. The channel is run by Gary Hayers, a college lecturer and former installation electrician, and the content is squarely aimed at UK practice. The video covers finishing chases ready for plastering, which is exactly the stage this guide is about.

1. Locate hidden services before any cutting

A cable and pipe detector costs around £20 to buy or can be hired cheaply. Run it slowly along every line you intend to cut. Mark any hits with a pencil cross. Some detectors will find both live cables and metal pipes; higher-end models also find timber noggins.

If you get a hit from a cable detector, do not assume it is at the surface. Old wiring was sometimes buried deeper, and some systems use steel conduit that the detector reads from a distance. When in doubt, chisel carefully by hand rather than grinding through blind.

2. Plan the route in a safe zone

The IET Guidance Note on cable installation defines safe zones for concealed cables: directly above or below a socket, switch or consumer unit, within 150 mm horizontally of either side of the accessory. Horizontal runs are also permitted within 150 mm of the ceiling or floor.

Outside those zones, any concealed cable must either be mechanically protected (steel conduit, for example) or be deep enough that a normal fixing cannot reach it. In practice, this means running vertically from sockets up to a ceiling void or down to a floor void, then horizontal through the void, then back down vertically to the next point. Diagonal runs across a wall outside safe zones are not acceptable without mechanical protection.

Mark the full route with a pencil and steel rule or batten. Double-check it makes sense from the consumer unit or source circuit all the way to the destination. Measure twice.

3. Isolate and confirm dead

Switch off the relevant circuit breaker or pull the fuse. Lock the board if you have a hasp, or use sticky tape over the switch with a note. Test at both ends of the proposed cable run with a voltage tester to confirm dead. Do this even if you think you know which circuit is involved, because circuits are not always labelled correctly.

4. Cut the channel

For plaster over brick or blockwork, a wall chaser with two diamond discs set at the correct separation cuts two parallel lines quickly and cleanly. An angle grinder with a single diamond disc needs two passes. The SDS drill with a flat chisel then breaks out the material between the cuts. Work in short sections: cut and clear as you go rather than cutting the whole run then digging out.

For plasterboard (stud walls), chasing is rarely done: instead, you thread cable through the void behind the board, or surface-run in oval conduit on the face. Cutting a plasterboard chase is possible but the board weakens around it and needs careful making good.

Keep the depth consistent: 20 to 25 mm is typically enough for 2.5 mm twin and earth in oval conduit, leaving the cable 6 mm below the finished plaster surface. Chasing into solid brick more than 30 mm can begin to compromise the wall's structural integrity, especially at mortar courses.

5. Fit back boxes and thread cable

Use the SDS and flat chisel to open recesses for back boxes. A 25 mm single socket back box in brick or block needs care: knock in the centre with the chisel and open outwards to avoid cracking the surrounding material.

Feed the twin and earth cable through the chase. Use oval conduit where required or where the cable needs protection as it passes through timber noggins or is within 50 mm of the wall surface. Clip the cable lightly at the back of the chase if it needs to stay in position before filling. Do not stretch the cable tight or create sharp bends at corners.

6. Protect and make good before second fix

Fill the chase with bonding compound or plaster, working it in so there are no voids around the cable. Let it set and skim flat before second fix. Take a photograph of the cable positions before filling: that image is useful for anyone doing maintenance or alterations later, and a responsible electrician will keep a record.

Where cable passes through structural timber (floor joists) within a zone that could be hit by fixing screws, drill through the centre of the joist to give at least 50 mm of timber above and below the cable. If less than 25 mm of timber covers the hole, fit steel notch plates.

When to call us

If you are running cable for a new circuit, that is Part P work and requires either a registered contractor or local authority notification. Richard does first-fix and second-fix wiring in Sandwich and east Kent, including chasing walls on rewires, extensions and kitchen or bathroom refurbishments.