What is a spur (fused spur)?

Glossary · Wiring, circuits and faults

How a spur works

A spur, which NICEIC also calls a cable branch, is a length of cable taken off an existing circuit to feed something extra.^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?} On a ring final circuit, each socket normally sits in the loop with a cable on each side. A spur leaves the loop and stops at its own outlet.

A fused spur adds a fused connection unit between the circuit and the load. The unit holds its own fuse, which protects the spur cable and what it feeds. NICEIC describes spurs that feed fixed equipment, or sockets, supplied through a fused connection unit.^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?}

An unfused spur has no such unit. It relies entirely on the protective device at the consumer unit, which is the 30 A or 32 A breaker or fuse protecting the whole ring.^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?}

Why spurs are limited

The ring’s protective device is much larger than the spur cable can carry for long. NICEIC explains that the current-carrying capacity of 2.5 mm² cable is generally less than the 30 A or 32 A rating of the device at the origin of the ring, so that device cannot protect the spur conductors against overload. If spur conductors were overloaded, the insulation, sheath, surroundings or accessories could be damaged by the temperature rising above rated value.^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?}

BS 7671 gets round this by relying on the spur not being overloaded for long periods. NICEIC says the live conductors of a spur are not likely to carry overload current where the spur feeds only:^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?}

• one single or one twin socket-outlet
• one or more items of fixed equipment supplied through a fused connection unit
• one or more socket-outlets supplied through a fused connection unit.

Short-circuit and earth-fault protection still apply. NICEIC explains that any of the 30 A or 32 A devices listed for a ring will protect a 2.5 mm² spur against short-circuit current, and will protect the typically 1.5 mm² protective conductor in a flat twin and earth cable provided the earth fault loop impedance at the spur is within the maximum value required for a 0.4 second disconnection time.^{[1]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?}

The ring itself carries a related condition. Its cable must have a current-carrying capacity of at least 20 A after corrections and the load must be unlikely to exceed it for long.^{[2]NICEIC, Overcurrent protection - overload}

Example

Lena in Roundhay wants an extra socket in her utility room. Her electrician identifies the nearest ring circuit and tests it first. Because the new socket will feed only a washing machine and a spare outlet, he adds a spur to a single twin socket, tests the circuit again and issues a certificate. Lena does not carry out any of the connection work herself.

For a heavier load, the electrician would first assess whether a spur is suitable at all. That choice is a design decision, not a standard answer.

What people get wrong

Many people think a spur is a free way to add as many sockets as they like. NICEIC’s article shows the reverse: a spur is acceptable because it feeds a limited load, and the ring’s protective device cannot protect the spur cable against overload.

Another mistake is adding a spur to a ring that has not been tested. The IET warns that if ring final circuits are added to or altered incorrectly, a dangerous situation could occur with overloaded cables.^{[3]IET, Back to the Forum - The ring final circuit} Testing before and after is part of doing it properly.

The bottom line

A spur is a branch off a ring or radial circuit. A fused spur adds a fused connection unit, and an unfused spur depends on the circuit’s own protective device. Spurs are acceptable only for limited loads, and the ring or radial itself must stay within its rating. Ask a registered electrician to add or alter one.