What Is a Fused Spur and When Do You Need One?

A fused spur is a fused connection unit (FCU): a wall plate with its own cartridge fuse that takes power from a socket circuit and feeds one fixed appliance, such as a boiler, extractor fan or cooker hood. The fuse, usually 3 A or 13 A, protects the appliance's smaller cable, which the circuit's 32 A or 20 A breaker cannot do on its own. Electricians say "FCU"; everyone else says "fused spur".

Where the name comes from

A spur is any branch cable taken off a socket circuit to feed something new. When that branch goes through a fuse first, it is a fused spur. Over time the name stuck to the fitting itself, so "fit a fused spur for the boiler" now means "fit an FCU and wire the boiler to it".

The fitting looks like a blank socket plate with a small fuse carrier. Some have a switch and a neon, some have a hole for a flex to come straight out, and some do both. Connection units for fixed wiring are made to BS 1363-4, and the fuses that go in them to BS 1362, the same fuses you find in a plug.

Switched vs unswitched fused spur

The difference is simply whether there is an on/off switch on the front.

• Switched fused spur: a double-pole switch lets you isolate the appliance locally for cleaning or maintenance. Common for boilers, cooker hoods, under-cabinet lighting and heated towel rails fed from outside the bathroom.

• Unswitched fused spur: no switch, so nobody can turn it off by accident. Used where the appliance should always stay on, such as an alarm panel or a fridge freezer in an integrated kitchen, or where isolation is provided somewhere else.

Neither is safer than the other. The choice depends on whether you want someone to be able to switch the appliance off at that point.

What fused spurs are used for

Typical jobs include gas boilers and their controls, extractor fans, cooker hoods, integrated appliances whose sockets would be hidden behind cabinets, under-cabinet and plinth lighting, garage door motors and alarm panels. Electrical Safety First's regulations guidance also confirms that a 13 A fused connection unit can supply a detached shed or garage from a ring or radial circuit, provided all the design checks are met.

One thing a fused spur should not feed is a standard 3 kW immersion heater. IET guidance says a 3 kW immersion heater at 230 V is unsuitable for a protective device rated below 15 A, and recommends an appropriately rated double-pole switch (for example 20 A) instead. The same IET article limits long-running appliance loads through plugs and FCUs to 2.7 kW.

If you are planning several of these in a new kitchen or utility, our sockets and power installations work covers the whole layout, fused spurs included, so appliances are not fighting over one overloaded circuit.

Which fuse goes in a fused spur?

Match the fuse to the appliance and its cable, following the maker's instructions first. As a guide, Electrical Safety First says appliances rated from 700 to 3,000 watts should have a 13 A fuse, and lower-powered items such as lamps and TVs use 3 A. Boilers and extractor fans often ask for a lower rating, and the installation manual will state it.

Never fit a bigger fuse because the small one keeps blowing. A fuse that blows is telling you about a fault or an overload.

Fused vs unfused spurs: the rule that catches people out

An unfused spur is a branch of the same cable size taken straight off a socket, with nothing but the circuit breaker at the board to protect it. On a ring, that single branch cable is not sharing its load two ways like the ring itself, and nothing limits what is plugged into it. That is why wiring guidance limits each unfused spur to one single or one twin socket (or one fused connection unit), and advises against having more unfused spurs than there are sockets on the ring itself.

A fused spur gets round this. The FCU caps everything after it at 13 A or less, so the cable beyond it can be smaller and the rest of the ring is protected. That is also why "spur socket" questions often end with an FCU: if a socket has to come off a point that already has an unfused spur, feeding it through a fused spur is the usual fix. How rings and radials behave is covered in our post on ring vs radial circuits.

Fused spur wiring: supply and load

An FCU has two sets of terminals, marked supply (or "mains") and load. The cable from the socket circuit goes into supply; the cable or flex to the appliance goes into load. Brown to L, blue to N and green/yellow to E on both sides, and the flex grip must clamp the outer sheath. If the conductors in the box are the older red and black, see our guide to live wire colours before touching anything.

Go by the markings on the back of the unit, not by which side looks tidier. With supply and load reversed the appliance still works, so the mistake can sit unnoticed until the unit is inspected.

Is fitting a fused spur a DIY job?

Replacing an existing fused spur like for like is not notifiable under Part P. Adding a new fused spur to an existing circuit outside a bathroom is not notifiable either, according to Approved Document P and the IET's Part P guidance, but it still has to meet BS 7671. That includes the buried-cable rules: a cable less than 50 mm deep in a wall must run in a safe zone and have 30 mA RCD protection unless it is mechanically protected.

It becomes notifiable if it needs a new circuit from the consumer unit, or if it is within the zone around a bath or shower. In either case it has to be done by a registered electrician who can self-certify it, or notified to building control before work starts, and it is not one to take on yourself.

Whether you need an electrician in Newport, Shifnal or Telford, SM Electrical fits and replaces fused spurs across Wolverhampton and Shropshire, and tests every one before we leave.

Fused spur questions, answered

Can a fused spur feed a socket?

Yes. A socket fed through an FCU is a common way to add a socket without breaking the unfused spur rule. Everything plugged into that socket shares the FCU's fuse, so it suits light loads.

Why does my fused spur keep blowing its fuse?

Either the appliance draws more than the fuse allows, or there is a fault in the appliance or its cable. Moisture in an extractor fan or a failing boiler component are common causes. Switch it off and have the appliance checked.

Where should a fused spur for a boiler go?

Near the boiler so it can be isolated for servicing, and where the engineer can reach it. Check the boiler installation manual, as makers state the fuse rating they want.

Does a fused spur need its own circuit?

Not usually. Most are taken off an existing ring or radial. A heavy load that runs for long periods is better on its own circuit, and a consumer unit upgrade may be needed if the board has no spare way.