Fused Connection Units: When to Use an FCU Instead of a Socket

Every kitchen has appliances that should never be plugged into a socket, because the socket disappears the moment the appliance is fitted. A dishwasher pushed home under the worktop. An under-counter fridge wedged into a run of units. A waste disposal motor bolted under the sink. A boiling water tap tank tucked inside the sink cabinet. Building Regulations and the wiring rules say that any fixed appliance must have a means of local isolation you can reach without tools, so you can cut the power to fix it, clean it, or replace it. A plug you can no longer reach does not count. When the socket is buried, the answer is a fused connection unit. This page explains what an FCU is, when it is legally required, which fuse goes in it, and how many to provision before first fix begins.

What an FCU actually is

A fused connection unit is a wiring accessory that makes a permanent hardwired connection between a circuit and a fixed appliance. Behind the faceplate sit two sets of terminals, supply and load, with a fused carrier between them. The supply side takes the circuit cable. The load side takes the appliance flex. The fuse in the carrier is the whole point of the device.

That fuse protects the appliance flex, not the circuit. The circuit itself is already protected by the MCB at the consumer unit, which is sized for the cable feeding the whole ring or radial. The thin flex on a dishwasher or a fridge cannot carry that much current safely, so the FCU steps the protection down to match the appliance.

Fuse ratings follow the load:

3A for small-draw appliances: extractor fans, under-cabinet light strips, small pumps.
5A for mid-draw appliances: waste disposal units, some extraction motors.
13A for full-load appliances: dishwashers, electric towel rails, instant boiling water tanks.

You change the fuse by lifting the fuse carrier out of the front face of the unit. On almost every FCU this is tool-free or needs nothing more than a screwdriver to release the carrier. The fuse inside is a standard cartridge type, the same as you find in a 13A plug.

3A / 5A / 13A

The three fuse ratings you will use in a kitchen. Match the fuse to the appliance flex, not the circuit. A dishwasher needs 13A, a waste disposal 5A, an extractor fan 3A.

When an FCU is legally required, and when a socket is fine

The rule behind all of this is Regulation 537.2.2.4 of BS 7671, the IET Wiring Regulations. Every circuit must have a means of isolation that is accessible. For a fixed appliance, "accessible" means you can reach the isolation point without dismantling the kitchen. If the only isolation point is a plug behind a built-in appliance, and that plug is unreachable once the appliance is pushed in, the installation fails the rule. An FCU mounted somewhere you can actually reach is the standard fix.

These are the positions where an FCU is the right call:

Dishwasher, under the worktop, where the socket would sit behind the unit and become inaccessible.
Under-counter fridge or drinks fridge, built into a run of units.
Waste disposal unit, mounted inside the sink base cabinet.
Boiling water tap tank, sitting inside the sink cabinet with no reachable plug.
Extractor fan, ceiling or wall-mounted and well out of reach without a ladder.
Electric towel rail, hardwired with no socket position designed in.

A socket is perfectly legal where it stays accessible. A washing machine in a utility room with a reachable socket behind or beside it does not need an FCU. The point is not to hardwire everything: it is to provide local isolation only where a plug cannot do the job. Treat the FCU as the solution to inaccessibility, not as a default for every appliance.

Warning

A built-in appliance fed from a plug you cannot reach is a common reason installations get flagged at building control sign-off. The flex either needs to terminate in an accessible FCU, or the socket has to be relocated to a reachable position. Decide this at first fix, before the units go in, not when the electrician returns for second fix and finds the dishwasher already plumbed in.

Switched, unswitched, or flex-outlet

FCUs come in three common forms, and choosing the wrong one is a frequent first-fix mistake.

A switched FCU has an on/off rocker and usually a neon indicator light. This is the workhorse for under-worktop appliances. The switch gives you visible local isolation: you can power the appliance down without dragging it out of its housing, and the neon tells you at a glance whether it is live. Boiling water taps in particular benefit from a switched FCU, because the tank needs to be switched off for descaling, filter changes, and salt top-ups.

An unswitched FCU has no rocker, just the fused terminals behind a blank faceplate. Use it for appliances that should stay on permanently and have no day-to-day reason to be isolated locally, a fridge being the classic example. You can still isolate it by pulling the fuse carrier, so the regulation is satisfied, but there is no switch to knock off by accident.

A flex-outlet plate FCU is a switched or unswitched unit with a small hole in the front face for the appliance flex to exit. You fit it where the cable needs to emerge neatly through the front of the plate rather than routing back through the wall or the cabinet panel. It keeps the connection tidy where the appliance sits right in front of the unit.

Position and wiring

The FCU usually lives inside the sink base cabinet on the back panel, or on the wall behind the appliance, within roughly 1 metre of the appliance it feeds. It is wired in 2.5mm twin and earth taken from a ring main or a radial circuit. The circuit cable lands on the supply terminals, the appliance flex on the load terminals. Get those the wrong way round and the fuse no longer protects the flex.

Where the FCU sits inside a cabinet that fills up with bottles, bins, or pipework and becomes hard to reach in practice, add a separate flex-outlet position on the front of the cabinet using a flex-outlet plate. That gives you a reachable switch on the cabinet face while the connection itself stays hidden behind it.

What to buy

FCUs are cheap and widely stocked, so buy a known brand and match the finish to your sockets and switches. Order them at the same time as the rest of your wiring accessories so the faceplates match across the kitchen.

Product	Rating	Budget option	Mid-range option	Notes
Switched FCU with neon	13A	LAP (Screwfix), ~£4	MK Logic Plus, Hamilton, ~£6–10	The standard choice for dishwashers and boiling water taps. Neon shows it is live.
Unswitched FCU	13A	LAP (Screwfix), ~£3	BG Electrical, MK, ~£6–8	For always-on appliances like a fridge. Isolate by pulling the fuse.
Flex-outlet plate (switched)	13A	Toolstation own-brand, ~£5	BG Electrical, ~£8–12	Where the flex needs a neat exit port through the front of the plate.
Switched FCU	5A	Silverline, CED, ~£4	BG Electrical, ~£7–9	Less common. For waste disposal units, which draw up to around 4.5A.

For a typical kitchen extension, budget for 3 to 6 FCUs: one per under-counter appliance (dishwasher, fridge, waste disposal if fitted, boiling water tap), plus a position for the extractor fan if it is hardwired at ceiling level. All four of the products above are stocked by Screwfix, Toolstation, CEF, and Edmundson Electrical. Specify them alongside your socket and switch faceplates so every visible plate in the kitchen is the same range and finish.

Tip

Keep one or two spare cartridge fuses of each rating in the kitchen drawer. When an appliance trips its FCU fuse, you want a replacement to hand rather than a trip to the shop. A mixed pack of 3A, 5A, and 13A fuses costs almost nothing and saves a dead dishwasher mid-cycle from ruining the evening.

Common mistakes

Putting a 3A fuse in a 13A FCU for a dishwasher. A 3A fuse is for small-load appliances only. A dishwasher heating element pulls 8 to 10A on a heated cycle and will blow a 3A fuse over and over. Match the fuse to the appliance: 13A for a dishwasher, 5A for a waste disposal, 3A only for genuinely small loads.

Fitting an unswitched FCU where a switched one is needed. A boiling water tap tank has to be switchable for descaling and salt top-ups, and a switched FCU makes that a two-second job. An unswitched unit forces you to pull the fuse every time, which is awkward and easy to skip.

Feeding an FCU off an extension lead or a spur from a plug. A fixed appliance must run from a dedicated, properly protected connection. Wiring an FCU off an extension socket breaks BS 7671 for fixed appliances and is a sign-off failure.

Burying the FCU where you can never reach it. If the unit ends up behind a full bin carousel or a tangle of waste pipes, the local isolation it is supposed to provide does not exist in practice. Mount it where you can reach it with the cabinet in normal use, or add a flex-outlet position on the cabinet face.

Where you'll need this

Kitchen electrical provisions, decide how many FCU positions to provision and where they sit before first fix.
First fix electrics, 2.5mm T&E circuit routing and cable sizing to each FCU position.
Kitchen installation, where the FCUs end up relative to the final appliance positions.

Provision FCU positions on the electrical plan before any cable is run. Moving a hardwired connection after the units are fitted is far more disruptive than agreeing the layout once and sticking to it.