Cable in before the walls close

Zones, depth and protection

A cable buried in a wall or partition at less than fifty millimetres from the finished surface has to be somewhere a person would reasonably expect it to be, or else be protected so that finding it with a nail does not matter. In practice that means keeping the run inside the permitted zones — within a hand's width of the top of the wall, of an internal corner, or running straight up or across from an accessory — and giving it the additional protection of a thirty milliamp residual current device. Where a run cannot live in a zone it gets mechanical protection or earthed metallic containment instead.

The alternative is worth naming, because it is the one that gets skipped: a cable taken diagonally across a wall to save two metres is outside any zone, invisible from the room, and in a wall somebody will one day fix a shelf to. Two metres of cable costs less than that conversation.

Through the structure

Holes through joists are drilled on the neutral axis, in the part of the span where the published structural guidance allows them, and sized to that guidance rather than to the diameter of the cable in hand. Those limits belong to the person responsible for the structure, not to an electrician's preference, and on a job with an engineer involved they are confirmed rather than assumed. Every hole a conductor crosses gets a grommet, because the sharp edge of a drilled metal stud and the sheath of a twin-and-earth cable are a slow puncture waiting for a hot day.

Runs are kept clear of flues, hot pipes and the backs of radiators. Where power and data share a route they are separated — different containment, a sensible gap — so that a mains cable is never lying along the length of a data cable for the whole run.

What first fix leaves behind

The end of first fix is a shell full of cable that somebody else is about to cover up. What makes the next two stages straightforward is entirely what was done about identification before that happened.

Boxes to the finished line

Back boxes are set to the depth of the finished wall, not the blockwork, so an accessory face finishes flat instead of proud or sunk.

Both ends marked

Every cable carries the same identification at the consumer unit end and at the accessory end, applied before anything is covered.

Tails long enough twice

Conductors are left long enough to be cut back and terminated a second time, because the first termination is not always the last one.

Photographs of the runs

Each run is photographed against something that fixes its position, a stud centre or a door opening, so a drill can go into that wall later without a guess.

Data drops in place

Outlet positions for data and telecoms are pulled on the same pass, in their own containment, terminated later at second fix.

Earthing and bonding

The consumer unit position is fixed, with tails, the main earthing terminal and the bonding to incoming services already in place.

Next stage: second fix, accessories on and terminated.