Spur Socket Regulations: Fused and Unfused Spur Guide UK

A spur is a branch cable that extends from an existing circuit to supply one or more additional outlets or items of equipment. It connects to the main circuit at a socket outlet, junction box, or the terminals of a fused connection unit (FCU), and runs to the new outlet or appliance.

Spurs are the most common method of adding socket outlets or fixed equipment connections to an existing ring circuit or radial circuit without running a completely new circuit from the distribution board. They are quick to install, require minimal disruption, and — when done correctly — are fully compliant with BS 7671.

There are two fundamental types: fused spurs and unfused spurs. The rules governing each type are different, and getting them wrong is one of the most common errors found during EICR inspections. Understanding the difference is essential for every electrician.

The critical distinction is protection. An unfused spur relies entirely on the main circuit protective device (the MCB at the distribution board) for overcurrent protection. This means the spur cable must be capable of carrying the full rating of the MCB without damage — so it must be the same size as the circuit cable. A fused spur has its own overcurrent protection (the fuse in the FCU), which means the cable after the FCU only needs to be rated for the fuse size, not the full circuit MCB rating.

A common scenario: a homeowner wants a new double socket in a bedroom. If there is an existing socket on the ring circuit in the same room, an unfused spur using 2.5mm²cable from that socket to the new double socket is the simplest solution. But if the homeowner also wants a permanently connected towel rail in the en-suite, a fused spur via a switched FCU with a 3A fuse is the correct approach.

A spur can be connected to the existing circuit at three points:

For all connection methods, the cable connections must be mechanically sound and electrically reliable. Use the correct terminal screws, strip the correct length of conductor insulation, and ensure no bare copper is visible outside the terminal. If using Wago connectors or similar maintenance-free connectors, ensure they are rated for the circuit current and installed in an accessible enclosure.

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The cable size for a spur depends on whether it is fused or unfused, and the rating of the fuse (if fused).

Remember: the cable from the circuit to the FCU (the supply side) must be the same size as the circuit cable. Only the cable from the FCU to the load (the load side) can be reduced because the fuse provides independent protection for that section. Always check voltage drop for the total cable length from the distribution board through the circuit cable and spur cable to the load.

The rules for spurs from ring circuits come from Regulation 433.1.204 and Appendix 15 (Figure 15A) of BS 7671:2018+A4:2026, with further practical guidance in the IET On-Site Guide. The key rules are:

Before adding a spur to a ring circuit, you should verify that the circuit is a correctly wired ring (not a radial mistakenly identified as a ring) and that the socket you are spurring from is part of the ring, not already on a spur. A ring circuit continuity test will confirm this.

Free to use, no sign-up and no email required. Enter your three end-to-end readings (r1, rn and r2), the ring cable size and the approximate ring length, and the calculator returns the expected cross-connected reading at each socket and the R1+R2 figure for the Schedule of Test Results.

That expected figure is what identifies a spur. On the live-to-neutral cross-connection the conductors are the same cross-sectional area, so every socket wired directly into the ring reads substantially the same — (r1 + rn) ÷ 4. A socket on a non-fused spur sits at the end of an out-and-back branch, so it reads noticeably higher than the rest. Compare each measured value against the calculated one before you decide which socket to spur from, and before you record R1+R2 and Zs on the certificate.

On the live-to-CPC cross-connection the readings will not be identical — twin and earth has a smaller CPC than the line conductor, so the values rise gently to a peak part way round the ring and fall again. That gradual pattern is normal. A sudden step at one socket is the one to investigate.

Spurs from radial circuits follow the same principles as spurs from ring circuits. The key considerations are:

In practice, electricians adding spurs to radial circuits often prefer fused spurs because they provide clear overcurrent protection for the spur cable and allow the use of smaller cable sizes for the branch run. This is especially common when feeding fixed appliances such as outside lights, garage equipment, or garden installations from an existing socket circuit.

These mistakes are regularly found during EICR inspections and result in observation codes that make the report Unsatisfactory. Avoid them by following the rules above and always testing the completed work before issuing a certificate.

Every spur addition must be tested and certified. The type of certificate depends on the scope of the work:

Elec-Mate generates both Minor Works Certificates and EICs on your phone. Enter the test results, describe the work carried out, and send the certificate to the customer as a professional PDF — all from site.

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