Single-Phase Socket Outlet Wiring
Ring finals vs radials, correct terminations, spurs, and the checks before you screw it back to the wall.
BS 7671:2018+A2:2022IET On-Site Guide
Socket circuits are the bread and butter of domestic work, and also where small mistakes — a loose cpc, a broken ring, an over-loaded spur — cause the most call-backs. This covers the two circuit types, how to terminate correctly, and the rules for spurs.
Ring final vs radial
| Circuit | Cable | Protective device | Notes |
|---|---|---|---|
| Ring final | 2.5 mm² T&E | 32 A | Both ends return to the same MCB. Floor area historically ≤ 100 m². |
| Radial (2.5 mm²) | 2.5 mm² | 20 A | One cable out, no return. Simpler to fault-find. |
| Radial (4 mm²) | 4 mm² | 32 A | Larger radial for bigger areas; no unlimited length rule but volt drop applies. |
A ring shares load across two paths, so 2.5 mm² can be protected at 32 A. That only holds while the ring is continuous — a break turns it into a long radial that's under-protected. This is exactly what ring continuity testing exists to catch.
Terminating a socket
1
Prepare the conductors
Strip only as much insulation as the terminal needs; no bare copper should show below the terminal. Sleeve every cpc in green/yellow — bare cpcs in a back box are a common fail.
2
Match by function, not colour habit
Brown line → L, blue neutral → N, green/yellow cpc → E. On a ring, two conductors go into each terminal — make sure both are captured and tight.
3
Torque the terminals
Tighten to the maker's figure. Under-tight terminals arc and overheat; over-tight can shear a stranded conductor. Give each conductor a gentle tug to confirm it's held.
4
Dress and fit
Fold conductors neatly into the box without trapping them behind the screws or the accessory. Check the earth tail to the box (metal boxes) is connected.
The cpc to a metal back box
A metal back box needs an earth connection — a fly-lead from the socket's earth terminal to the box's earth terminal, or a fixed lug. A plastic accessory on a metal box with no earth to the box is a defect.
Spurs — the rules people break
- A non-fused spur off a ring may supply one single or one double socket (or one fixed appliance). One spur per socket on the ring.
- For more than that, use a fused connection unit (FCU) — the fuse (usually 13 A) protects the thinner spur cable.
- Don't spur off a spur. If you need to extend, take it from the ring or fit an FCU.
- Kitchen appliances behind units are best on FCUs so they can be isolated without pulling the appliance out.
Before you sign it off
- Ring continuity: prove the ring is unbroken (the three-step r1/rn/r2 method).
- Insulation resistance: dead test, ≥ 1 MΩ.
- Polarity: line to the switched/fused terminal everywhere; no reversed L–N.
- Zs and RCD operation: confirm disconnection times and 30 mA additional protection.
Reference material for qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.