A fused spur is a fixed connection point that puts one appliance on its own fuse, taken off a ring main. It exists so a boiler, an extractor fan or a towel rail has protection sized for it, instead of relying on the 32A breaker feeding the whole circuit.
That's the whole idea. A 32A breaker will happily let far more current through a thin appliance flex than that flex can survive. The fuse in the spur is what closes that gap.
Most people meet one when a boiler or a fan needs wiring in and the instructions say "connect via a fused connection unit". Here's what that actually means.
1. What it does, in one sentence
It taps power off a ring main and passes it on through a fuse — normally 3A, 5A or 13A — so the appliance on the other side is protected by something sized for its flex, not for the whole circuit.
You'll see the same device called a fused connection unit or an FCU. Same thing. The trade tends to say FCU, the packaging tends to say fused spur.
Trade Tip: match the fuse to the appliance, not to the socket. An extractor fan wants 3A. A boiler wants 3A or 5A depending on the manufacturer's instructions. A 13A fuse in a fan spur is technically "working" and is doing almost nothing useful — it will let the fan's flex cook long before it blows. The fuse rating is the entire point of fitting the thing.
2. Switched or unswitched — and it's not just convenience
Switched gives you local isolation. You can turn the appliance off at the wall without going to the consumer unit. For anything that ever needs servicing, cleaning or resetting — boiler, towel rail, fan, waste disposal — this is the one to fit.
Unswitched is permanently on. Correct where you specifically don't want somebody switching it off: a fridge-freezer, an alarm panel, a fire alarm.
The Knightsbridge FP6300BB is a low-profile 13A switched fused spur unit in brushed brass, IP20, 230V, certified to BS 1363-4. The FP6300NPB is the polished brass version with a neon.
Trade Tip: fit the neon version anywhere the appliance is out of sight — a boiler in a cupboard, an under-sink waste disposal, a loft fan. The neon is the only way anyone will ever know whether the spur is switched on without opening the cupboard. It costs pennies more and saves a service call that was never a fault.
3. The flex outlet, which people forget to specify
Look at the front of a spur unit and you'll see one of two things: a plain plate, or a plate with a small hole and a cable clamp.
That hole is the flex outlet, and it's where the appliance's own cable comes out of the front of the unit. If the appliance flex needs to emerge at the wall — a towel rail, a fan, a boiler — you need the flex outlet version. If the cable runs on inside the wall to a fixed point, you don't.
The Knightsbridge FP6300FPL is the flat-plate 13A switched spur with both a neon indicator and a flex outlet, in a pearl finish. The FP6300FBC is the same specification in brushed chrome.
Getting this wrong is the single most common return we see on spurs: right unit, right fuse, no hole for the cable.
4. When you genuinely need one
Any fixed appliance that can't reasonably be on a plug. In practice that means:
- Boilers and heating controls
- Extractor fans
- Heated towel rails
- Under-cabinet lighting and striplight drivers
- Waste disposal units
- Outside lights and garden sockets fed from indoors
The common thread is that each of these is wired in permanently, and each has a flex much thinner than a 32A ring main is protecting.
5. Spur, fused spur, and the one that catches people
Three things sound similar and are not the same:
An unfused spur is a branch off the ring feeding one socket, with no fuse of its own. It relies entirely on the circuit breaker.
A fused spur adds the fuse. Protection sized for the appliance.
A cooker connection unit is a different animal altogether — 45A, its own circuit, no fuse in the plate. Never substitute one for the other in either direction.
6. Where DIY stops
Swapping a like-for-like spur on an existing, correctly-isolated circuit is within reach of a competent DIYer. Isolate at the consumer unit, prove dead, photograph the terminals before you loosen anything.
Adding a new spur to a ring main is a different job. It's an alteration to a circuit, it needs testing and certifying, and if it's in a room containing a bath or shower it's a special location under Part P in England and Wales. If you can't self-certify it, that's an electrician's job — and "it works" and "it's compliant" are two different tests.
Working on the switch side of a lighting circuit instead? Our guide to changing a light switch covers that.
Fused spur quick-reference
| Appliance | Typical fuse | Switched? | Flex outlet? |
|---|---|---|---|
| Extractor fan | 3A | Yes | Usually |
| Boiler | 3A or 5A per instructions | Yes | Usually |
| Heated towel rail | 3A | Yes | Yes |
| Fridge-freezer | 13A | No — unswitched | No |
| Alarm panel | 3A | No — unswitched | Usually |
| Striplight driver | 3A | Yes | Yes |
Expert Help in the West Midlands
We've been on Villa Road in Handsworth since 1975, and the trade counter is open to everyone.
Bring the appliance instructions — they'll state the fuse rating, and that plus "does the cable need to come out of the front" is the entire specification. Two minutes at the counter beats a second trip for the version with the hole in it.
Browse the full range in Fused Spurs.
Frequently asked questions
What size fuse goes in a fused spur?
Whatever the appliance instructions specify — commonly 3A for fans, boilers and towel rails, 13A for higher-draw appliances. Match the fuse to the appliance flex, never to the socket. A 13A fuse in a 3A application protects almost nothing.
What's the difference between a fused spur and a fused connection unit?
Nothing. Same device, two names — FCU is the trade term, fused spur is what it usually says on the box.
Can I put a fused spur on a lighting circuit?
Yes, and it's common for things like extractor fans and striplight drivers. The fuse still needs to suit the appliance. Adding one is a circuit alteration, so it needs testing and certifying.