Spurring Off a Socket: Rules, Limits and BS 7671 Requirements

Quick Answer: A spur is a branch taken from a ring final circuit to feed an additional point. Per BS 7671 Appendix 15 and the IET On-Site Guide, one non-fused spur may feed ONE single or one twin socket-outlet (or one fused connection unit) and is taken from a socket terminal or junction box on the ring. A fused spur, via an FCU with a BS 1362 fuse rated ≤13A, can feed multiple points. Standard spur cable is 2.5mm² — and you must never spur off another spur.

Summary

Adding a socket by spurring off an existing one is the single most common alteration a domestic sparky makes, and the rules are simple once you separate the two kinds of spur. A non-fused spur is a direct tap off the ring: the new outlet is still protected by the ring's 32A device, so it is limited to one twin socket (or one single, or one FCU) to keep the 2.5mm² spur cable within safe loading. A fused spur puts a fused connection unit (FCU) in the way; the BS 1362 fuse (13A or less) protects everything downstream, so you can run several outlets, a fused spur for fixed appliances, lighting, and so on.

The limits exist because of the cable. On a ring, every socket is fed from two directions, so the loading on any one leg is shared. A non-fused spur is fed from one direction only — a single 2.5mm² conductor behind a 32A device. That is acceptable for one twin socket (where the BS 1363 plug-top fuses limit the total draw and diversity does the rest), but not for a string of outlets, which is why the unfused spur is capped at one accessory. Put an FCU in and the 13A fuse becomes the limiting device, so the downstream cable only ever has to carry 13A — and now multiple points are fine.

Two rules trip people up. First: you take a non-fused spur from a point on the ring — a socket's terminals or a junction box in the ring — never from a point that is itself a spur. Spurring off a spur double-branches a single unprotected leg and is not permitted. Second: count your existing spurs. The historic "number of spurs must not exceed the number of points on the ring" guidance is about not overloading the ring with too many one-directional branches; in practice, keep spurs to a minimum and fuse them where in doubt.

Key Facts

• Non-fused spur limit — one single OR one twin socket-outlet, OR one fused connection unit, per non-fused spur (Appendix 15 / On-Site Guide).
• Fused spur — via an FCU with a BS 1362 fuse rated ≤13A; can feed multiple outlets/points downstream of the fuse.
• Spur cable — 2.5mm² twin-and-earth to match a 2.5mm² ring; this is the standard.
• Take-off point — a non-fused spur comes from a socket terminal on the ring, or a junction box (BS 5733 maintenance-free or accessible) in the ring.
• Never spur off a spur — a non-fused spur must originate at the ring, not at another spur.
• Protection — a non-fused spur is protected by the ring's 32A device; a fused spur is protected by its own FCU fuse (≤13A).
• FCU types — switched or unswitched, with or without a neon/flex outlet; the fuse is the protective element for the load.
• Spurs vs ring points — keep total non-fused spurs modest; historic guidance limited them to no more than the number of points on the ring proper.
• Twin socket counts as one point — a twin (double) socket on a non-fused spur is permitted and counts as the single accessory.
• RCD additional protection — the spur inherits the ring's 30mA RCD protection (Regulation 411.3.3); if the ring lacks it, the alteration must not make matters worse and additional protection should be considered.
• Cable in walls — spur cable in walls follows the same RCD/zone/depth rules as any other (Regulation 522.6); 30mA RCD required for cables <50mm deep without earthed mechanical protection.
• Junction box — if using a JB on a ring, it must be accessible for inspection or be a maintenance-free type to BS 5733.
• Fixed appliances — feed a fixed appliance (e.g. towel rail, boiler) from a fused spur/FCU, not a non-fused spur off the ring.

Quick Reference Table

Detailed Guidance

Non-fused vs fused — decision context

The question to ask before adding any point is: "How much can this branch ever draw, and what protects the cable?"

• One extra twin socket near an existing ring socket → non-fused spur. Tap the existing socket's terminals (you will have the two ring cables plus your new spur cable in each terminal), run 2.5mm² to the new twin socket. The ring's 32A device protects it; the single twin socket is the only accessory allowed.
• Several new outlets, a run of sockets in a study, or a fixed appliance → fused spur. Fit an FCU on the ring, fuse it appropriately (13A for general sockets, lower for a small fixed load), and run 2.5mm² to your outlets. The FCU fuse now limits the load, so multiple points are fine.
• A fixed appliance (towel rail, extractor, boiler, under-sink heater) → FCU with the correct fuse for that appliance, flex out to the appliance.

If you are unsure whether a non-fused spur is appropriate, fit an FCU. It is the conservative choice and removes any argument about loading the spur cable.

Taking the spur off correctly

A non-fused spur is taken from the ring at a socket (its terminals already hold the two ring cables; your spur is the third conductor into each terminal) or from a junction box installed in the ring. Crucially, the take-off point must be on the ring itself. If you tap into a socket that is already a spur, you have created a spur off a spur — not allowed, because two one-directional accessories now hang off a single 2.5mm² leg behind a 32A device. Before you connect, confirm the socket you are tapping is genuinely on the ring (a ring continuity test, or at least confirming two cables already present and tracing them) — do not assume.

Counting spurs on the ring

The total number of non-fused spurs should be kept modest. The long-standing guidance is that the number of non-fused spurs should not exceed the number of socket-outlets and points connected directly in the ring. The reasoning is the same as the per-spur limit: each non-fused spur is a one-directional branch that does not benefit from the ring's shared loading, so a ring overloaded with spurs starts to behave like a tangle of stub radials. If a ring is already heavily spurred, add new outlets via an FCU instead.

RCD, zones and cable protection

A spur inherits the protection of the ring it comes from. If the ring is on a 30mA RCD (as required for socket-outlets under Regulation 411.3.3), the spur is covered. If the existing ring has no RCD, your alteration must not make the situation worse, and Part P / BS 7671 best practice is to provide 30mA additional protection — flag this to the customer. Spur cable buried in a wall less than 50mm deep, without earthed mechanical protection, must also be on a 30mA RCD (Regulation 522.6.202/203) and run in the permitted zones.

Testing after adding a spur

Treat the addition as Minor Works: confirm polarity at the new accessory, prove continuity of the CPC (R1+R2 or R2), test insulation resistance, verify Zs at the new point meets the disconnection time, and confirm the RCD operates. A spur added to a ring should not change the ring's healthy continuity readings dramatically, but the spurred socket will read higher line-to-CPC in the cross-connected ring test because it is fed from one direction — that is expected and is how a spur is identified. Issue a Minor Electrical Installation Works Certificate.

Frequently Asked Questions

Can I put two sockets on one non-fused spur?

No. A single non-fused spur can feed only one single socket, one twin (double) socket, or one fused connection unit. If you want more than that from a branch, fit an FCU and take a fused spur — the 13A fuse then allows multiple points downstream.

Can I spur off a spur?

No. A non-fused spur must be taken from the ring itself — a socket on the ring or a junction box in the ring. Tapping a point that is already a spur creates a spur off a spur, which BS 7671 / On-Site Guide arrangements do not permit.

What cable do I use for a spur?

2.5mm² twin-and-earth, matching a standard 2.5mm² ring. Downstream of an FCU, the load is limited to 13A by the fuse, but 2.5mm² is still the normal choice up to the FCU and commonly beyond.

How do I add a socket for a fixed appliance like a towel rail?

Use a fused connection unit (FCU) as a fused spur off the ring, with the fuse rated for the appliance, and run flex from the FCU to the appliance. Don't feed a fixed appliance from a non-fused spur or a plug-and-socket where a permanent connection is intended.

Does the spur need its own RCD?

Not its own — it inherits the ring circuit's protection. Socket-outlets require 30mA RCD additional protection (Regulation 411.3.3), and spur cable buried shallow in a wall also needs 30mA RCD cover (Regulation 522.6). If the existing ring has no RCD, recommend providing it as part of the work.

Regulations & Standards

• BS 7671:2018+A2:2022 (IET Wiring Regulations, 18th Edition) — Appendix 15 (ring/radial circuits and spurs), Regulation 411.3.3 (30mA RCD for socket-outlets), Regulation 522.6 (cables in walls/partitions).
• BS 1363 — 13A socket-outlets and connection units (including FCUs).
• BS 1362 — cartridge fuses (≤13A) for FCUs and plugs.
• BS 5733 — general requirements for electrical accessories, including maintenance-free junction boxes.
• IET On-Site Guide — spur rules, take-off points and the number-of-spurs guidance.
• IET Guidance Note 3 (Inspection & Testing) — test requirements for additions (Minor Works).