You want sockets for a new home office at the end of a run, and the ring is already stretched. A spur looks like the easy answer. The number of sockets you can hang off it depends on which kind of spur you use.

Unfused or fused: the two spur types

The rules sit in Appendix 15 of BS 7671, the Wiring Regulations published by the IET. See the IET BS 7671 page for the standard itself.

Unfused spur rules

• One single or one twin socket per spur, or one fixed appliance.
• No spur off another spur.
• The number of unfused spurs cannot exceed the number of sockets and fixed appliances on the ring itself.
• Cable is the same size as the ring, normally 2.5mm².

That last rule is the one that catches people out. A ring with six sockets can carry at most six unfused spurs.

Fused spur rules

A fused spur starts at an FCU to BS 1363-4, either switched or unswitched. The fuse in it is 13A at the most. ELECSA and other technical guides describe the number of sockets on a fused spur as unlimited in principle, with the load, after diversity, deciding what is sensible.

On cable, fit 2.5mm² and give it a miss on thinner. Some guides allow 1.5mm² after a 13A fuse, but cable rating depends on how it is clipped and what it runs through, so 2.5mm² is the safe default. Voltage drop matters on long runs.

A worked example: where the 13A fuse bites

A 13A fuse at 230V gives roughly 2,990W. Say your spur feeds three twin sockets in a home office in a Victorian terrace. A PC at 300W, a monitor at 60W and a printer at 500W peak come to about 860W, well inside the limit.

Plug a 2kW fan heater and a 1.2kW kettle into the same spur and you are at 3.2kW. The fuse blows, which is the fuse doing its job. The lesson is to put heavy loads on the ring, not the spur.

Picking the fused connection unit

Browse our fused connection units. The BG NBS50 brushed steel 13A switched fused spur unit is £5.97 and the BG NFB50 matt black is also £5.97 (prices checked 6 October 2026). For a neon indicator, the BG FFB52 flatplate in matt black is £8.13. Matching sockets sit in the British General range and the BG Evolve range.

Not sure whether you need switched or unswitched? Read fused connection units: switched vs unswitched. For the ring itself, see ring or radial circuits. Adding a spur to an existing circuit is non-notifiable in England outside a bath or shower room, as covered in our Part P guide, but the work must still meet BS 7671.

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