What size cable for sockets?
Most UK socket circuits are a 32 A ring final in 2.5 mm² twin and earth, a 20 A radial in 2.5 mm², or a 32 A radial in 4 mm². All need 30 mA RCD protection.
- V – Voltage
- I – Current
The standard socket circuits
| Circuit | Breaker | Twin and earth | Floor area served |
|---|---|---|---|
| Ring final | 30 or 32 A | 2.5 mm² (1.5 mm² CPC) | Up to 100 m² |
| Radial | 20 A | 2.5 mm² | Up to 50 m² |
| Radial | 32 A | 4 mm² | Up to 75 m² |
These come from the standard circuit arrangements in BS 7671 Appendix 15 and the On-Site Guide, and assume normal conditions. A ring works because its two legs share the load, so 2.5 mm² cable can sit on a 32 A breaker.
How long can a radial be?
For the worst case, with the full current drawn at the far end, the 5 % voltage drop limit gives:
- 20 A radial, 2.5 mm²: up to about 31 m.
- 32 A radial, 4 mm²: up to about 32 m.
Real loads are spread along the circuit, so actual drops are lower. Use the voltage drop calculator with your real figures. 2.5 mm² is rated 27 A clipped direct, but only 17 A above more than 100 mm of loft insulation and 13.5 A fully inside an insulated stud wall. In those spots a 20 A radial needs bigger cable, so run the cable size calculator.
Spurs off a ring
- An unfused spur can feed one single or one twin socket, or one item of fixed equipment.
- Don't have more unfused spurs than sockets on the ring itself, and never spur off a spur.
- More than that needs a fused spur through a fused connection unit (13 A maximum).
RCD protection
BS 7671 requires 30 mA RCD additional protection for socket-outlets rated up to 32 A. Cables buried less than 50 mm deep in a wall must run in the safe zones and be RCD protected too.
Assumes 70 °C flat twin and earth (6242Y), 230 V, an MCB or RCBO, 30 °C ambient, the circuit not bunched with others, and no thermal insulation round a clipped cable. Voltage drop limit 5 % (11.5 V), BS 7671 Table 4D5 values. Grouping, heat or insulation can need a bigger cable: check with the cable size calculator. For study and estimating only; the installing electrician must design and certify the circuit.