What fuse should I use in an FCU?

Use a 3A fuse for motors and loads below 700W, such as extractor fans, boiler timers, central heating controls and low-wattage lighting. Use a 13A fuse for electric towel rails, panel heaters and similar loads above 700W. Never fit a 13A fuse where a 3A was specified just because 13A is what you have to hand.

What is the difference between Line In and Line Out on an FCU?

Line In (also marked Mains or IN) is the supply side: the cable from your ring main or spur. Line Out (also marked Load or OUT) is the appliance side: the cable going to the device you are powering. The fuse sits between these two sets of terminals, so connecting them the wrong way round leaves the appliance on the unprotected side of the fuse.

Do I need Part P permission to fit an FCU?

Replacing an existing FCU like-for-like on an existing circuit is generally minor electrical work and does not require Part P notification. Fitting a new FCU where one did not exist before, or extending a circuit to supply one, counts as new work and should be notified under Part P. If in doubt, a qualified electrician can do the work and certify it in one visit.

How to Wire a Fused Connection Unit (FCU) in the UK

Helpful video reference. The GSH Electrical Training tutorial "Connections in 3 Switch Fuse Connection Units (Switch Fuse Spurs) Layout of the Terminals Explained" is used as the reference here. GSH Electrical Training produces UK-specific content for electricians in training, so the terminal layouts and wiring methods shown are consistent with BS 7671 and UK practice.

Before you start. Turn the ring or radial circuit off at the consumer unit and confirm dead with an approved voltage tester before touching anything. If the existing wiring has rubber sheathing, cloth covering, or cable colours you do not recognise, stop and call a qualified electrician.

1. What an FCU is and when to use one

An FCU, or fused connection unit, is a socket without a plug. It connects a permanently installed appliance directly to the circuit wiring, with a cartridge fuse built into the unit to protect the appliance's flex or the cable between the FCU and the appliance. Common uses include:

Electric towel rails and panel heaters
Extractor fans (cooker hood, bathroom, utility)
Boiler power supplies and central heating timers
Under-floor heating thermostats
Outdoor lighting supplies taken through the wall

You would not use an FCU for an appliance that gets plugged and unplugged regularly. Those need a socket outlet.

2. Choose the right FCU type

Three common types:

Switched FCU — has a double-pole rocker switch that isolates both live and neutral. Use this where you want a local isolation point for the appliance.
Unswitched FCU — no switch, just the fuse. Used where isolation is not needed at the FCU itself (for example, behind a boiler where the boiler has its own switch).
FCU with flex outlet — has a cord grip and strain relief built in, for appliances with a fixed flex that connects directly into the unit rather than to a separate back box.

3. Choose the correct fuse rating

This is the most common mistake made with FCUs. The fuse should be sized to protect the cable or flex between the FCU and the appliance, not the circuit the FCU is connected to.

3A fuse — for motors, fans, boiler controls and anything drawing less than 700W
13A fuse — for electric towel rails, panel heaters and resistive loads above 700W

If the existing FCU had a 3A fuse, replace it with a 3A. Do not substitute a 13A just because that is what is in the drawer.

4. Isolate the circuit and confirm dead

Switch off the relevant ring main or radial circuit at the consumer unit. Test the supply point with a voltage tester before touching a single conductor. If you are not sure which circuit feeds the supply point, turn everything off and work methodically back to the correct breaker.

5. Identify Line In and Line Out terminals

Open the old FCU and photograph the terminal connections before removing anything. FCU terminals are typically arranged in two groups:

Line In / Mains / IN — the supply from the ring main or spur (this side carries mains voltage even when the FCU switch is off)
Line Out / Load / OUT — the cable going to the appliance (this side is isolated when the FCU switch is off)

The fuse bridges these two sets of terminals, so the appliance side is protected. Getting IN and OUT the wrong way round means the fuse sits on the wrong side of the switch, leaving live conductors on the appliance side after you switch off.

6. Connect the supply side (IN)

On the new FCU, connect the supply conductors to the IN terminals:

Brown (live) to the Line In / L terminal
Blue (neutral) to the Neutral In / N terminal
Green/yellow (earth) to the earth terminal

Tighten each screw firmly enough that the conductor cannot be pulled free by hand.

7. Connect the appliance side (OUT)

Connect the cable going to the appliance to the OUT or LOAD terminals, matching brown to live, blue to neutral, and green/yellow to earth. If the FCU has a flex outlet, follow the manufacturer's fitting instructions for securing the flex in the cord grip and making off the cable ends.

8. Fit the fuse, refit and test

Insert the correct fuse cartridge. Fold the cables neatly into the back box, taking care not to trap any insulation under the faceplate. Fit the faceplate screws evenly. Restore the circuit at the consumer unit and test the appliance.

Stop and call an electrician if: the existing wiring has no earth, the cable is in poor condition, the FCU is feeding an appliance above 3kW (such as a shower or larger heater), you want to add a completely new FCU where one did not exist before, or the circuit has any unusual characteristics you cannot explain. New work requires Part P notification and certification.

When to call us

Replacing a like-for-like FCU on an existing spur is tidy DIY territory. Running new cable to create a new FCU position, or wiring an FCU for a high-load appliance, is work that needs certifying. Richard covers small electrical jobs in Sandwich at £10 per 10 minutes.

Need an FCU fitted in Sandwich?

From a single fused spur to a full kitchen wiring run, Richard quotes fixed prices before any work starts.

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How to wire a fused connection unit (fused spur)