Can I add a spur to a ring circuit without notifying Building Control?

Adding a spur to an existing ring circuit is generally classed as minor electrical work and does not require Building Control notification under Part P of the Building Regulations, provided the work is not in a kitchen or a special location (such as a bathroom or within 3 metres of a swimming pool) and does not involve work on the consumer unit. A kitchen is treated the same as a special location for the purposes of this rule — work in a kitchen (adding socket outlets or fused spurs to an existing ring or radial circuit) is notifiable, even though the kitchen itself is not a special location under BS 7671. A Minor Works Certificate should always be issued for the completed work, regardless of whether Building Control notification is required. If you are registered with a competent person scheme (NICEIC, NAPIT, ELECSA), you can self-certify the work. If you are not registered, and the work is in a kitchen or a special location, you must either use a registered electrician or notify Building Control before starting the work.

How many spurs can I have on a ring circuit?

BS 7671 does not set a specific maximum number of spurs on a ring circuit. The On-Site Guide states that the total number of non-fused spurs should not exceed the total number of socket outlets and items of stationary equipment connected directly in the ring. In practice, this means you can have a reasonable number of spurs without issue, but each non-fused spur is limited to supplying one single or one twin socket outlet, or one item of fixed equipment. Fused spurs are more flexible — each fused spur can supply multiple outlets because the fuse in the FCU limits the current. If you find yourself adding many spurs to a ring circuit, it may be more appropriate to install an additional circuit from the distribution board.

What is the difference between a fused spur and an unfused spur?

An unfused spur is a direct cable connection from the ring or radial circuit to an additional outlet, with no separate fuse protecting the spur cable. The spur cable must be the same size as the circuit cable because it relies on the main circuit protective device (MCB) for overcurrent protection. An unfused spur from a ring circuit can only supply one single or one twin socket outlet, or one item of fixed equipment. A fused spur uses a fused connection unit (FCU) — a device containing a BS 1362 cartridge fuse (typically 3A or 13A) — at the point where the spur connects to the circuit. The fuse protects the spur cable independently, so the cable after the FCU can be smaller than the circuit cable. A fused spur can supply multiple outlets or items of equipment, up to the rating of the fuse.

Can I spur off a spur?

No. BS 7671 and the IET On-Site Guide do not permit a non-fused spur to be taken from another non-fused spur (a "spur off a spur" or "double spur"). This is because the cable on the original spur was sized to supply only its intended outlet, and adding a further spur increases the load beyond what the cable was designed to carry. However, you can take a fused spur from any point on a circuit, including from a socket that is itself on a non-fused spur. The fuse in the FCU provides independent overcurrent protection for the new spur cable. If you need to add outlets and have already spurred from the nearest socket, the correct approach is either to use a fused connection unit or to run a new spur from a socket or junction box that is connected directly in the ring or radial circuit.

What cable size should I use for a spur?

For an unfused spur from a ring circuit or radial circuit, the spur cable must be the same cross-sectional area as the circuit cable — typically 2.5mm² twin and earth for a 32A ring circuit or 20A radial. For a fused spur, the cable after the fused connection unit can be reduced in size because the fuse provides independent overcurrent protection. A 13A fused spur can use 2.5mm² cable. A 3A fused spur (for small fixed loads like LED drivers, extractor fans, or clock points) can use 1.0mm² or 1.5mm² cable. In all cases, you must also check that the voltage drop at the furthest point on the spur does not exceed the BS 7671 limit and that the earth fault loop impedance allows disconnection within the required time.

How do I identify if a socket is on a spur or on the ring?

During testing, you can identify whether a socket is on a spur or part of the ring by performing a ring circuit continuity test. The three-step test (measuring end-to-end resistance of each conductor, then cross-connecting and measuring at each socket) gives a clear diagnostic: sockets directly in the ring should all show approximately the same resistance reading during the cross-connected step (approximately half the total ring resistance). A spur socket will show a noticeably higher resistance reading at that step than the ring sockets, because the current path to a spur has to travel further — out and back along the spur cable — rather than dividing around the ring. Misidentifying a spur socket as a ring outlet during commissioning is a known source of incorrect R1+R2 and Zs entries on the Schedule of Test Results. A socket on a non-fused spur will also have only one cable entering the back of the socket. A socket on the ring will have two cables — one arriving from each leg of the ring. A socket on a spur fed via a junction box will also have one cable, but the junction box itself will be in the ring. Visual inspection during an EICR should note the circuit arrangement and identify any spurs.

Is a fused connection unit the same as a switched fused spur?

The terms are often used interchangeably, but technically a fused connection unit (FCU) is the device itself — a plate containing a BS 1362 cartridge fuse. A switched fused spur (or switched FCU) is an FCU that also includes a double-pole switch, allowing the connected equipment to be isolated without removing the fuse. Switched FCUs are preferred for most fixed appliances because they provide a visible and accessible means of isolation. Unswitched FCUs are used where the appliance has its own integral switch or where a separate isolator is provided nearby. Both types are available with or without a flex outlet and a neon indicator.

INSTALLATION GUIDE

Spur Socket Regulations: Fused and Unfused Spur Guide UK

Spurs are the fastest way to add socket outlets and fixed equipment connections to existing circuits. But the rules for fused and unfused spurs are different, and getting them wrong is one of the most common EICR defects. This guide explains when to use each type, correct cable sizes, connection methods, and the mistakes that cost electricians time and money.

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11 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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What are the rules for spurs off a ring final circuit?

An unfused spur may supply only one single or one double socket-outlet, or one item of fixed equipment, connected at a socket on the ring, at a junction box, or at the origin (IET On-Site Guide, ring final arrangements in Appendix 15). A fused spur runs from a fused connection unit (BS 1363-4, normally fused at 13 A) and can supply any number of outlets because the fuse protects the thinner spur cable. The ring itself is wired in 2.5 mm² copper and protected by a 30 A or 32 A device. Missing or oversized unfused spurs are one of the most common EICR coding defects.

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Key Takeaways

1A spur is a branch cable that extends from an existing circuit to supply one or more additional outlets — it can be fused (via a fused connection unit) or unfused (direct connection).
2An unfused spur from a ring circuit must supply no more than one single or one twin socket outlet, or one item of permanently connected equipment.
3A fused spur can supply multiple outlets because the fuse in the FCU limits the current independently of the main circuit protection.
4The cable from a fused connection unit to the load can be reduced in size (e.g., 1.0mm² or 1.5mm²) because the 3A or 13A fuse in the FCU provides overload protection for the smaller cable.
5Elec-Mate's cable sizing calculator confirms the correct cable size for spurs and checks that the protective device coordination is correct under BS 7671.

01 · Installation Guide

What Is a Spur in Electrical Wiring?

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.

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02 · Installation Guide

Fused vs Unfused Spurs: The Key Differences

Unfused Spur

Direct cable connection — no separate fuse
Cable must be same size as circuit cable
From ring circuit: max 1 single or 1 twin socket, or 1 fixed item
Cannot spur from another unfused spur
Protected by the main circuit MCB only
Simplest and fastest to install

Fused Spur

Connected via a fused connection unit (FCU)
Cable after FCU can be smaller than circuit cable
Can supply multiple outlets or equipment
Can be taken from any point on the circuit
Fuse in FCU provides independent protection
3A or 13A BS 1362 cartridge fuse

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.

03 · Installation Guide

When to Use a Fused Spur vs an Unfused Spur

Use an unfused spur when you need to add a single socket outlet (single or twin) to an existing ring or radial circuit and the cable route from the existing socket to the new outlet is short and straightforward. This is the simplest and cheapest option. The existing socket you spur from must be on the ring (not itself on a spur).
Use a fused spur when you need to supply a fixed appliance (extractor fan, heated towel rail, waste disposal unit, boiler, LED driver, outdoor lighting) or when you need to supply multiple outlets from a single connection point. Also use a fused spur when the cable after the connection point needs to be a smaller size than the circuit cable — for example, 1.0mm² flex to a wall-mounted appliance.
Always use a fused spur when connecting to an appliance that uses flex rather than cable, when the appliance is permanently connected (no plug and socket), when you need to provide a local means of isolation for the appliance, or when you are feeding a circuit in an outbuilding from a domestic ring or radial.

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.

04 · Installation Guide

Connection Methods for Spurs

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

At an existing socket outlet. The spur cable connects to the terminals of an existing socket on the ring or radial circuit. The socket must have spare terminal capacity — most modern socket outlets have terminals that accept two cables. The existing socket must be on the ring (not itself on an unfused spur) if you are adding an unfused spur.
At a junction box in the circuit cable. A 30A-rated junction box is inserted into the ring or radial cable, and the spur cable connects to the spare terminal. The junction box must be accessible for inspection after installation (not buried behind plasterboard without an access panel). Maintenance-free junction boxes may be used if BS 7671 Regulation 526.3 is met.
At a fused connection unit. An FCU is installed at the connection point (either replacing an existing socket or installed alongside one) and the spur cable runs from the load side of the FCU to the new outlet or appliance. This method provides independent overcurrent protection for the spur.

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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05 · Installation Guide

Cable Sizes for Spurs

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

Spur Cable Size Reference

Unfused spur from 32A ring circuit — 2.5mm² twin and earth (same as ring cable). Must match the ring cable cross-sectional area.
Unfused spur from 20A radial — 2.5mm² twin and earth (same as radial cable).
13A fused spur — 2.5mm² twin and earth from FCU to load. Suitable for socket outlets and higher-power fixed appliances.
3A fused spur — 1.0mm² or 1.5mm² twin and earth or flex from FCU to load. Suitable for low-power fixed appliances: extractor fans, LED drivers, clocks, shavers, towel rails up to 700W.
Unfused spur from 32A radial — 4mm² twin and earth (same as radial cable).

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.

30mA RCD protection required for the new socket outlet

BS 7671:2018+A4:2026 Regulation 411.3.3 requires that all socket outlets rated up to and including 32A are provided with additional protection by a 30mA RCD. This applies to the new spur socket outlet as well as to the existing circuit. In a dwelling, there is no exception — the RCD requirement is mandatory. In non-domestic installations, omission is only permitted where a documented risk assessment determines it is not necessary. Ensure the new spur is fed from a circuit that is already RCD-protected, or add RCD protection for the new outlet.

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06 · Installation Guide

Spurs from Ring Final Circuits

The rules for spurs from ring circuits are set out in the IET On-Site Guide and Guidance Note 1. The key rules are:

Non-fused spurs — each non-fused spur must be connected to the ring at a socket outlet or junction box that is part of the ring. An unfused spur can supply one single or one twin socket outlet, or one item of permanently connected equipment. You cannot take a non-fused spur from another non-fused spur.
Number of non-fused spurs — the total number should not exceed the total number of socket outlets and fixed items connected directly in the ring.
Fused spurs — a fused spur can be connected at any socket outlet, junction box, or point in the ring. The FCU can supply multiple outlets or items of equipment. There is no limit on the number of fused spurs, provided the total circuit load remains within the rating of the ring circuit protective device.
Cable size — the cable from the ring to the spur (non-fused) must be 2.5mm² minimum for a 32A ring. The cable from a fused spur depends on the fuse rating as described above.

High-integrity CPC connections required on ring spurs

Where a ring final circuit has spurs, each spur must have a high-integrity protective conductor connection complying with BS 7671 Regulation 543.7.1. The CPC connection on the spur must be of equal integrity to the ring CPC — it cannot rely on the ring conductor alone (OSG 9th Ed Reg 7.5.3; BS 7671:2018+A4:2026 Reg 543.7.2.201). This is commonly missed: verify CPC continuity on every spur with a low-resistance continuity tester and record the result on the certificate.

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.

07 · Installation Guide

Spurs from Radial Circuits

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

Unfused spurs — must use the same cable size as the radial circuit cable. The spur should supply no more than one single or one twin socket outlet, or one item of fixed equipment — the same rule as for ring circuits.
Fused spurs — can be connected at any point on the radial circuit via an FCU. The cable after the FCU can be reduced in size according to the fuse rating. Fused spurs are particularly useful on radial circuits for supplying fixed appliances.
Total load — when adding spurs to a radial circuit, consider the total load on the circuit including the spur. A 20A radial on 2.5mm² cable has less spare capacity than a 32A ring circuit, so additional loads must be carefully assessed.

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.

08 · Installation Guide

Common Spur Wiring Mistakes

Spur off a spur (double spur). Taking a non-fused spur from a socket that is already on a non-fused spur. This is the single most common spur wiring error found during EICR inspections and would be coded C2 (Potentially Dangerous) or C3 (Improvement Recommended) depending on the cable sizes involved.
Undersized cable on unfused spur. Using 1.5mm² cable for an unfused spur from a 32A ring circuit. Without a fuse to protect the smaller cable, the 32A MCB will not disconnect before the cable overheats in an overload condition.
Wrong fuse rating in FCU. Using a 13A fuse in an FCU that feeds a 1.0mm² cable. A 13A fuse does not protect 1.0mm² cable from overload — a 3A fuse is the correct rating for this cable size.
Multiple sockets on unfused spur. Feeding two or more double sockets from a single unfused spur. An unfused spur from a ring circuit should supply no more than one single or one twin socket outlet.
Inaccessible junction boxes. Burying junction boxes behind plasterboard or under floorboards without an access panel. Junction boxes must remain accessible for inspection and maintenance unless they are maintenance-free connectors complying with Regulation 526.3.

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.

09 · Installation Guide

Certification Requirements for Spur Additions

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

Minor Works Certificate — the correct certificate for adding a spur to an existing circuit. It covers the new work only and requires completion of the relevant tests (continuity, insulation resistance, polarity, earth fault loop impedance, and RCD operation if applicable).
Electrical Installation Certificate (EIC) — required if the work involves a new circuit from the distribution board rather than a spur from an existing circuit.

Record pre-existing defects on the Minor Works Certificate

BS 7671 Regulation 644.1.2 requires the installer to record any defects found in the existing installation on the Minor Works Certificate (or EIC) so far as is reasonably practicable. GN3 9th Ed Reg 1.4 confirms: any defects observed during the course of the works that may give rise to danger must be noted on the certificate, even if they do not affect the safety of the new spur itself. Failure to do so is a legal duty omission. Any defect that does affect the safety of the new work must be corrected before the certificate is issued.

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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Frequently Asked Questions About Spur Socket Regulations

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Spur Socket Regulations: Fused and Unfused Spur Guide UK

What are the rules for spurs off a ring final circuit?

Key Takeaways

What Is a Spur in Electrical Wiring?

Get the BS 7671 A4:2026 Cheat Sheet — free

Fused vs Unfused Spurs: The Key Differences

Unfused Spur

Fused Spur

When to Use a Fused Spur vs an Unfused Spur

Connection Methods for Spurs

Issue a Minor Works Certificate on site

Cable Sizes for Spurs

Spur Cable Size Reference

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Spurs from Ring Final Circuits

Spurs from Radial Circuits

Common Spur Wiring Mistakes

Certification Requirements for Spur Additions

Certify spur additions in minutes

Frequently Asked Questions About Spur Socket Regulations

Can I add a spur to a ring circuit without notifying Building Control?

How many spurs can I have on a ring circuit?

What is the difference between a fused spur and an unfused spur?

Can I spur off a spur?

What cable size should I use for a spur?

How do I identify if a socket is on a spur or on the ring?

Is a fused connection unit the same as a switched fused spur?

Continue reading

Cable Sizing Calculator

Radial Circuit Explained

Ring Circuit Calculator

Minor Works Certificate

Part P Building Regulations

Distribution Board Wiring

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