How to Wire a Fused Connection Unit (FCU): Switched, Unswitched and Spur Rules

A Fused Connection Unit (FCU) is the standard way to connect a permanently wired appliance in UK domestic installations. Unlike a socket outlet where you plug and unplug, an FCU provides a fused terminal block inside a wall-mounted plate — the appliance connects once and stays connected, protected by a fuse that will blow before the cable is damaged.

This guide explains how to wire switched and unswitched FCUs, when to use a 3 A vs 13 A fuse, and how FCUs fit into ring circuit spur arrangements.

What Is a Fused Connection Unit?

An FCU is a wall-mounted accessory that combines:

• A terminal block for connecting the fixed appliance cable
• A fuse (3 A or 13 A) to protect the outgoing cable and appliance
• A switch (in switched types) for local isolation
• An indicator neon (on many switched types) showing power is present

The fuse is BS 1362 type — the same cartridge fuse used in a standard 13 A plug. This allows the fuse rating to be matched to the appliance, not fixed at 13 A like a socket.

FCUs are used for:

• Electric heaters (wall-mounted panel heaters, towel rails)
• Extractor fans (bathroom, kitchen, utility)
• Dishwashers and washing machines (where no socket is provided)
• Boilers (fused connection to the heating system)
• Under-cupboard lighting or display lighting
• Cooker control units (often incorporate an FCU-style fused connection)

Switched vs Unswitched FCU

The double-pole switch in a switched FCU disconnects both live and neutral simultaneously, ensuring full isolation for maintenance.

Fuse Ratings: 3 A vs 13 A

The fuse protects the outgoing cable to the appliance, not the supply cable feeding the FCU. Choose the fuse based on the appliance rating:

Using a 13 A fuse on a 3 A-rated fan cable means the cable could overheat and catch fire before the fuse blows. Always match the fuse to the smallest rating: either the appliance rating or the cable’s current-carrying capacity, whichever is lower.

Wiring an FCU from a Ring Circuit (Fused Spur)

The most common FCU installation is as a fused spur from a ring final circuit. This allows you to add a fixed appliance without extending the ring itself.

The spur rule

BS 7671 allows one fused spur per point on a ring circuit, or unlimited fused spurs if the total number of spurs does not exceed the total number of socket outlets on the ring. In practice, most domestic installations keep fused spurs to a reasonable number (2–4 per ring).

Wiring sequence

Ring circuit cable (2.5 mm² T&E)
         |
    FCU (fused connection unit)
         |
    Appliance cable (appropriate size)
         |
    Appliance (heater, fan, etc.)

At the FCU terminals:

• L (Line) — brown from ring circuit → brown to appliance (via fuse)
• N (Neutral) — blue from ring circuit → blue to appliance
• E (Earth) — green/yellow from ring circuit → green/yellow to appliance

The fuse sits in the line path only. Neutral and earth are continuous.

Wiring an FCU from a Radial Circuit

For high-power appliances (heaters approaching 3 kW), an FCU can be fed from a 20 A radial circuit rather than a 32 A ring. The wiring is identical; the radial simply provides a dedicated supply rather than a spur from a ring.

Cable Sizes for FCU Circuits

Supply cable to FCU

• From ring circuit: 2.5 mm² twin and earth (existing ring cable)
• From radial circuit: 2.5 mm² twin and earth (standard radial)

Outgoing cable from FCU to appliance

Important: The outgoing cable from the FCU to the appliance must be rated for the fuse size in the FCU, not just the appliance rating. A 13 A fuse requires cable capable of carrying 13 A (1.5 mm² minimum, 2.5 mm² preferred).

FCU in Bathroom Zones

FCUs are not permitted in Zone 2 or Zone 1 of a bathroom. They are allowed outside the zones (general bathroom area) but the appliance they supply must be appropriate for the zone.

For bathroom extractor fans:

• The FCU must be outside Zone 2 (or be a pull-cord type)
• The fan must be rated for the zone it occupies (Zone 2 = IPX4 minimum)
• The circuit must have 30 mA RCD protection

Related: How to Wire a Bathroom: Complete Zone-by-Zone UK Guide

FCU for Cookers (Cooker Control Units)

A Cooker Control Unit is essentially a large FCU rated at 45 A with a double-pole switch. It provides:

• Fused connection (often with a 13 A socket on the faceplate for kettle/toaster)
• Double-pole isolation for the cooker
• High-current rating for oven/hob loads

The wiring principles are identical to a standard FCU, but with 6 mm² or 10 mm² cable and a 45 A rating.

Related: How to Wire a Cooker or Electric Oven: UK Circuit Guide

Part P and FCU Installation

Adding an FCU as a new spur from an existing ring or radial circuit is notifiable work under Part P if:

• It is a new connection point in a kitchen or bathroom (special locations)
• It involves new cabling that is not like-for-like replacement

In practice, most FCU installations outside special locations are done by competent DIYers or electricians, but kitchens and bathrooms always trigger Part P notification.

Common Mistakes

Simulating FCU Protection in ElectraSim

An FCU’s fuse behaves like a protection device that blows at a specific current threshold. In ElectraSim:

1. Build a circuit with an MCB representing the ring circuit protection (32 A)
2. Add a load representing the appliance
3. Set the load current above the FCU fuse rating but below the MCB rating
4. Observe that without an FCU fuse, the ring MCB does not trip — the cable could overheat
5. The FCU fuse (represented by appropriate load limiting or fault injection) prevents this scenario

This demonstrates why fused spurs are required from ring circuits — the ring MCB is too high a rating to protect a single spur cable.

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Key Points

• An FCU provides fused protection for permanently connected appliances
• Switched FCUs allow local isolation; unswitched FCUs rely on consumer unit isolation
• Match the fuse rating (3 A or 13 A) to the appliance and cable — never exceed the cable rating
• Fused spurs from ring circuits are the standard way to add fixed appliances
• Cable from FCU to appliance: 1.5 mm² minimum for 13 A fuse, 2.5 mm² preferred
• Bathroom FCUs must be outside Zone 2; appliances supplied must match their zone IP rating
• Kitchen and bathroom FCU installations are notifiable under Part P