Electrical Materials

First fix electrics happen once. Once the walls are plastered you cannot easily add a circuit or move a back box, so understanding what your electrician is installing, why it matters, and how it is meant to be laid out is one of the most valuable things you can do as a project manager.

These guides cover the materials your electrician will use during a domestic extension: the cables that carry power around your home, the consumer unit and protective devices that keep it safe, and the accessories that end up on your finished walls. Each page explains what the component does, how to read a specification, and the warning signs that something has been done wrong.

Where to start

If you only read three of these before first fix, make them the cable sizing guide, back boxes and the consumer unit. Cable size is fixed the moment it is buried in a wall, back box depth decides which faceplates you can fit later, and whether your existing consumer unit can take the new circuits often decides the size of the electrician's quote.

The guides, grouped by job

Cable. 1.5mm twin and earth for lighting, 2.5mm for socket circuits, and 4mm, 6mm and 10mm for long runs, cookers and showers. Outside the house you need SWA cable; for fire alarm and emergency circuits there is MICC. Low-voltage runs are covered in data cable and speaker cable, which are far cheaper to pull in at first fix than to chase in afterwards.

Containment and termination. Conduit, conduit clips, cable trunking, earth sleeving and heat-shrink tubing. These are cheap items, and a missing one is a common reason for a failed inspection.

Protection at the board. The consumer unit and the devices inside it: MCBs, RCBOs, surge protection (SPDs) and AFDDs. Read these before you accept a quote that includes a board change.

Kitchen and appliance circuits. Cooker connection units, fused connection units and the switch fuse for fixed appliances, plus the immersion heater if you have a hot water cylinder.

What ends up on the walls and ceilings. Socket faceplates, switch faceplates, USB sockets, weatherproof sockets, LED downlights, pendant fittings, ceiling roses and under-cabinet LED strips. You choose these at second fix, but most of them need the right cable or box depth left for them at first fix.

Where these fit in the build

The layout is decided in electrical layout planning, the cable and boxes go in at first fix electrics, and the faceplates, fittings and board connections happen at second fix electrics. New circuits in an extension are notifiable work, so expect your electrician to register it and hand you an Electrical Installation Certificate at the end.