Electrical first fix: planning cable runs before the walls go up

Helpful video reference. The video above is from eFIXX — a UK electrical training channel with more than 280,000 subscribers, producing content for electricians and electrical apprentices — "Award Winning Electrician Reveals First Fix Secrets". The presenter talks to an experienced UK electrician about the habits and approaches that separate a well-executed first fix from a sloppy one. Worth watching before you start, because the details covered here — labelling, tail length, back box depth — are things you cannot easily correct once the walls are closed. Watch on YouTube.

1. Agree every position before a single back box is fixed

The most expensive mistake in first fix is fixing things in the wrong place. Walk the job with the homeowner before any stud walls go up or any concrete is poured. Agree the exact position and height of every socket, light switch, ceiling rose and any dedicated circuit (cooker, shower, EV charger). Write it down or photograph it with a tape measure in shot.

Changes at the planning stage cost nothing. Changes after plastering mean cutting, patching, repainting, and sometimes re-running cable through a closed wall — none of which is cheap or fast.

Note any requirements that deviate from standard heights: lower sockets for wheelchair users, specific positions dictated by a kitchen design, or locations where the builder has put a steel post that limits where you can drill.

2. Mark positions on the structure before boarding

With positions agreed, mark them on the timber frame or blockwork before any plasterboard goes up. Standard heights to mark:

• Socket outlets: 450 mm from finished floor level to the bottom of the faceplate
• Light switches: 1200 mm from finished floor level (sometimes 1000 mm in homes with young children)
• Telephone and data points: 300 mm from floor level where desk-mounted, higher on the wall where wall-mounted
• TV points: varies by room layout, agree with the homeowner

Use a chalk line or laser level to keep all positions on the same height across a long wall. A run of sockets at slightly different heights is obvious once the faceplates are on.

3. Fix back boxes to the structure

For stud walls that will be plasterboarded, fix galvanised steel boxes to studs or noggings. A back box fixed to a nogging — a horizontal timber added between the studs at the right height — is far more secure than one on plasterboard alone. Set the back box face at a depth that accounts for the plasterboard thickness plus the skim coat: typically 12.5 mm board plus 2–3 mm skim, so the box face should sit 14–16 mm proud of the timber stud face.

For solid masonry walls being re-chased, use a plaster-depth gauge to set the flush-fitting boxes correctly. If the plastering contractor is fitting the plaster after the back boxes are in, agree the plaster depth with them — a discrepancy here means either a proud back box or a sunken one, neither of which looks right.

For dry-lined walls with metal stud, use dry lining boxes with expanding lugs that grip the plasterboard from behind.

4. Plan cable routes in cable safe zones

All cables concealed in walls must run within the prescribed cable safe zones defined in BS 7671. The zones are:

• Vertically above or below an accessory, within 150 mm of the accessory edge
• Horizontally from the accessory to the corner of a wall
• Within 150 mm of a corner (a junction of two walls)

Cables running within these zones are where any future trade would expect to find them — and would therefore avoid them when drilling. A cable run diagonally across a wall, or at a random height that makes it impossible to predict, is a hazard for any future owner who drills a picture hook or fixes a shelf bracket.

If the cable cannot be run in a safe zone — for example, a long horizontal run between two distant sockets — protect it in steel conduit or use a wiring accessory fitted with channel for the cable.

5. Run circuit cables from the consumer unit to each position

Pull each circuit cable from the consumer unit position through the building structure to its destination. Twin and earth cable is usually pulled on the drum — a cable tray or drum stand makes long pulls much easier than wrestling with a free-standing drum.

Pull each cable in one length where possible. Joints in hidden cable runs are not prohibited but they need to be in an accessible junction box, not buried behind a wall. Keeping runs as single lengths avoids this complication and gives the cleanest installation.

Leave at least 1 m of tail at the consumer unit end of every circuit. If the board is in a corner cupboard or an awkward position, leave more — 1.5 m is not excessive. At the accessory end, leave 200–300 mm hanging out of each back box. This gives enough to strip, terminate and fold back comfortably without pulling the cable tight in the wall.

6. Secure cable at correct spacing

Cable clips must be spaced at:

• 250 mm on horizontal runs
• 400 mm on vertical runs
• Within 300 mm of each change of direction

Cables should be held firmly without deforming the sheath. A clip tightened so hard that it flattens the oval section of twin and earth cable is applying mechanical stress to the conductor insulation — back off until the cable is held without being crushed.

Where cables pass through timber, use grommets in drilled holes. Where they pass over a joist surface, protect the cable against the sharp timber edge with a cable clip at the point of contact, not just either side of it.

7. Label every cable before the walls close

This is the step that gets skipped when a job runs late. Do not skip it. Label every cable at both ends with the circuit it belongs to before any plasterboard goes on or any plaster is applied. Use tape flags, cable markers, or simply a strip of electrical tape with the circuit name written in marker pen.

At the consumer unit end, a labelled bundle of cables is what allows a competent second-fix electrician (who may not be the same person as the first-fix electrician) to work quickly and without mistakes. At the accessory end, the label tells the second-fix operative which cable connects to which location without tracing every run back to the board.

Photograph the labelling with the floorplan or schedule in shot. File the photograph with the job documentation — it will be needed if an EICR is carried out in ten years' time.

When to call us

First fix is skilled work. The physical act of clipping cable is simple enough, but the planning — which cable size, which route, which circuit breaker, how to sequence the work — is where errors get made. Richard works on first fix for extensions and rewires across east Kent and can take the job from planning through to test and certification.