What Is a Fused Spur? FCUs, Fuse Sizes and When You Need One

A fused spur, properly called a fused connection unit or FCU, is a wall-mounted electrical unit with its own fuse inside. It takes a feed from a circuit and supplies a single fixed appliance, such as a boiler, an extractor fan or a heated towel rail, with the fuse chosen to protect that appliance and its cable rather than the whole circuit.

You'll know it as a flat plate on the wall, about the size of a single light switch. Sometimes it has a small rocker switch and a red neon, sometimes just a little fuse drawer and nothing else. If you have a gas boiler, you almost certainly have at least one, and most people only notice theirs on the day something stops working.

What a fused spur actually does

Your socket circuits are protected at the consumer unit for the cable in the walls, not for whatever gets plugged or wired in at the end. A ring final circuit is typically protected at 32A, which is fine for the 2.5mm cable buried in your plaster, but far more than a boiler control board or a fan motor ever wants to see.

The fused spur closes that gap. Inside the unit is a standard BS 1362 cartridge fuse, the same type that sits in a 13A plug, rated at 13A or below. The supply comes in on the circuit side, passes through that fuse, and feeds the appliance on the load side. If the appliance develops a fault, the small fuse goes first, quickly, instead of the fault current running up to whatever the circuit breaker allows.

That fusing down has a useful side effect. Because the fuse caps the current, the cable on the load side of the FCU can be smaller than the circuit cable, often 1.0mm or 1.5mm instead of 2.5mm. That's what makes it the neat way to hard-wire one appliance into a circuit designed for sockets. All of this sits within BS 7671, the wiring regulations published as the national standard by the IET.

Switched and unswitched versions

Both do the same protective job. The difference is whether there's a switch on the front.

• Switched fused spur: has a rocker that cuts the supply to the appliance, often with a neon to show when it's live. This is the usual choice for boilers, washing machines and anything an engineer will want to isolate locally for servicing, without a trip to the consumer unit.
• Unswitched fused spur: a plain plate, sometimes with just a fuse drawer. Used where the appliance stays permanently live or has its own controls, like a fridge-freezer behind a kitchen unit or a feed to fixed lighting.

On switched units, the better ones are double pole, meaning the switch breaks the live and the neutral together. That's the version we fit for boilers and fixed heaters, because single pole switching leaves the neutral connected and that catches people out during repairs.

Where you'll find them in a typical home

Once you know what you're looking at, you'll spot them everywhere:

• Boilers and heating controls. Usually a switched FCU with a 3A fuse, sitting next to the boiler or in the airing cupboard.
• Extractor fans and cooker hoods. Often on a 3A fuse, sometimes switched so the fan can be isolated for cleaning.
• Heated towel rails. A 3A or 5A fuse depending on the rail's wattage, with the unit sited outside the bathroom zones.
• Kitchen appliances fixed behind units. Dishwashers, washing machines and fridge-freezers are commonly wired through a 13A FCU, with the switch part fitted above the worktop so you can kill the power without dragging the machine out.
• Fixed electric heaters and plinth heaters. Hard-wired rather than plugged, fused to suit.

The kitchen setup confuses people most. The appliance looks like it has no plug because the connection happens behind the cabinet, and the only visible sign is a switch with a neon above the worktop. When we handle an appliance installation, that switch position is one of the first things we sort, because burying the only isolator behind a 60kg washing machine is asking for a miserable afternoon later.

What fuse for a fused spur

The fuse is a BS 1362 cartridge, and the common ratings are 3A, 5A and 13A. Choosing between them is simple arithmetic: divide the appliance's wattage by 230 to get the amps it draws, then fit the next fuse size up.

Electrical Safety First's guidance puts it plainly: "plugs for appliances rated from 700-3000 watts" should take a 13A fuse, while 3A suits loads like table lamps, televisions and computers, and "5A fuses are still used in some older equipment". In practice that means 3A for a boiler, fan or towel rail, and 13A for a dishwasher, washing machine or fridge-freezer.

Two rules worth remembering. Boiler manufacturers usually state the fuse rating in the installation manual, and it's nearly always 3A, so check the book rather than guessing. And if a fuse keeps blowing, the answer is never a bigger fuse. A 13A fuse in place of a 3A removes the protection the whole unit exists to provide. Replace like for like, and if it blows again, the appliance or the wiring has a fault that needs finding.

Fused spur vs socket, and can you swap one for the other?

A socket feeds whatever you choose to plug into it, up to 13A through the plug's own fuse. A fused spur hard-wires one appliance permanently, no plug involved. They look similar on the wall but they answer different questions.

The honest answer on swapping a fused spur for a socket: sometimes yes, but not automatically. Three things have to be checked first. The cable feeding the FCU may only be 1.0mm or 1.5mm, sized for that one appliance on a small fuse, and not safe to hang a general-purpose socket on. The circuit needs proper RCD protection. And if the spur feeds your boiler, removing it removes the boiler's local isolator, which the manufacturer's instructions often specifically require. We've seen boilers "fixed" by well-meaning handymen who swapped the FCU for a socket and left the boiler plugged in with no means of isolation. It works, until someone needs to service it safely.

The everyday direction is the other way round. People want a fixed appliance hard-wired where there's currently a socket, and converting that point to a fused spur is a routine job. If what you actually need is simply more places to plug things in, that's a different conversation, and our guide to the cost of adding extra sockets covers it properly. Bigger fixed loads like electric cookers don't go on spurs at all; they need their own circuit, which we explain in does an electric cooker need its own circuit.

Unfused spurs off a ring, and their limits

Not every spur carries a fuse, and this is where DIY electrics regularly goes wrong. An unfused spur taken from a ring final circuit has no fuse of its own, so the wiring regulations limit what it can feed: one single socket, one twin socket, or one fixed appliance. That's it. The cable has to be the full circuit size, 2.5mm, because the only protection it has is the 32A breaker back at the board.

Two more limits follow from that. You can't take a spur off a spur, because then the cable is feeding far more than its protection allows. And the number of unfused spurs on a ring shouldn't exceed the number of points wired directly into the ring itself. The "one more socket off that one, then another off that" chain you find under the stairs in older London houses is exactly what these rules exist to stop.

A fused spur escapes those limits because its 13A fuse caps whatever hangs off it, which is why it's the correct tool for branching a fixed appliance off a ring.

Does fitting a fused spur need building control sign-off?

Usually not, but it depends on the job. Part P of the Building Regulations is the law covering electrical safety in homes in England, and the government's Approved Document P sets out when work has to be notified to building control. Replacing an existing FCU like for like, or adding a spur to an existing circuit in an ordinary room, is non-notifiable work.

What is notifiable: installing a new circuit, and most work in special locations, which in practice means bathrooms and anywhere near a bath or shower. So the fused spur for a new heated towel rail in the bathroom sits in different territory from the one for your dishwasher. Our post on Part P building regulations goes through the full list.

The simple route around the paperwork is using an electrician registered with a competent person scheme, such as NICEIC. They self-certify the work and handle the notification for you, and you get the certificate for your records without touching a form.

Signs a fused spur is overheating

Fused spurs fail quietly for years, then not quietly at all. The usual cause is a terminal that has worked loose under load, and the warning signs are worth knowing:

• The faceplate feels warm or hot when the appliance is running.
• Brown or yellow discolouration on the plastic, especially around the switch or fuse drawer.
• A fishy or burning smell near the unit. That smell is overheating plastic, and it never gets better on its own.
• Crackling or buzzing when the appliance is on.
• The fuse blowing repeatedly, or the appliance cutting out and coming back.

Any of those, switch the unit off and leave it off. The same advice we give for a burning smell from a socket applies here: the smell means heat, and heat in a loose connection means a fire risk building inside the wall box. Replacing a burnt FCU is a quick, cheap job. The alternative isn't.

Frequently asked questions

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

Yes, same thing. Fused connection unit, or FCU, is the proper name and what you'll see written on quotes and certificates. Fused spur is what everyone calls it on site, because it usually spurs off a socket circuit.

What fuse should be in a fused spur for a boiler?

Almost always 3A, and the boiler's installation manual will say so explicitly. If you open the fuse drawer and find a 13A in there, it's worth swapping down to the rating the manufacturer specifies.

Can I replace a fused spur myself?

A like-for-like swap is non-notifiable work and within reach of a genuinely competent DIYer, but it means isolating the circuit at the consumer unit and proving it dead before touching a terminal. If those two steps aren't second nature to you, it's a quick, inexpensive job for an electrician and not worth the gamble.

Can a fused spur feed more than one appliance?

It can, because the 13A fuse caps the total load on whatever is wired after it, so you'll sometimes see a small group of under-unit lights or a boiler plus controls fed from one FCU. In most homes, though, one spur feeds one fixed appliance, which is what it's designed for.

Can I fit a fused spur in a bathroom?

The FCU itself has to sit outside the bathroom zones, and work near a bath or shower is on the government's list of jobs that need building regulations approval, so it's notifiable unless done by a registered electrician. The heated towel rail on the wall is fine; the spur feeding it just needs the right home.

Why has the neon on my boiler's fused spur gone off?

Either the neon itself has failed, which is common and harmless, or there's no supply reaching the unit. Check the fuse in the drawer first, then the breaker at the consumer unit. If the fuse has blown, replace it once with the same rating. If it blows again, stop, because the boiler or the wiring has a fault and refitting fuses won't fix it.

Got a fused spur that's running hot, a boiler with no means of isolation, or a fixed appliance that needs wiring in properly? See our sockets and switches service or call us on 020 3653 2600 for a straight answer and a fixed quote.