Fused Connection Units (FCUs) — Switched, Unswitched, and Metal Clad for UK Electricians

What Is a Fused Connection Unit?

A fused connection unit (FCU) — sometimes called a fused spur — is a wiring accessory that provides a permanently connected, individually fused supply to a fixed appliance. Unlike a standard socket, the appliance is wired directly into the FCU rather than plugged in. This gives the circuit its own dedicated fuse (typically 3A or 13A) and, on switched versions, an isolation switch that can cut power to the appliance without affecting the rest of the ring circuit.

FCUs are specified in BS 7671 (IET Wiring Regulations) for appliances that are permanently installed and do not need to be unplugged for routine use — boilers, immersion heaters, extractor fans, cooker hoods, under-floor heating, and fridge-freezers are all common applications. Using an FCU rather than a plug and socket removes the risk of accidental disconnection and provides a more controlled, professional installation.

Switched vs Unswitched FCUs

The most important choice when selecting an FCU is whether you need a switched or unswitched unit.

Switched FCUs include an integral on/off switch, which may be single pole (SP) or double pole (DP). A single pole switch breaks the live conductor only — acceptable for most domestic appliances. A double pole switch breaks both live and neutral simultaneously, providing full isolation without needing to isolate at the consumer unit. DP switching is recommended for heating appliances and any installation where a higher degree of safety is required, and is increasingly used as standard in new builds and refurbishments.

Unswitched FCUs provide a fused connection with no integral isolation — the appliance relies on another means of isolation such as a local fuse or the consumer unit MCB. These are sometimes used in situations where the FCU will be concealed or where switching at the FCU is not required by the installation design.

FCU with Flex Outlet vs Standard FCU

Some FCUs include a flex outlet — a knockout or moulded entry point that allows the appliance cable to exit neatly through the front of the plate. This is common for fridge-freezers, kitchen appliances, and other appliances where the cable will be visible and needs routing tidily. Standard FCUs without a flex outlet are preferred for concealed wiring where the cable runs inside the wall or conduit behind the back box.

Fuse Ratings

FCUs accept standard BS 1362 cartridge fuses, the same as those used in 13A plugs. The correct fuse rating depends on the connected appliance:

• 3A fuse — for appliances rated up to approximately 700W: extractor fans, LED lighting circuits, electric shavers, doorbell transformers
• 5A fuse — for appliances rated between 700W and 1,200W
• 13A fuse — for high-wattage appliances: immersion heaters, electric panel heaters, small electric boilers

Always select a fuse rating as close to the appliance's rated current as practical to provide the best overload protection.

Metal Clad FCUs for Surface Wiring

Metal clad FCUs are used where the wiring is surface mounted in steel conduit or where greater mechanical robustness is required — garages, utility rooms, sheds, light industrial premises, and any installation where a white plastic plate would be unsuitable. Metal clad units mount on galvanised steel or zinc alloy back boxes and offer the same fused protection as their white plastic equivalents but in a durable steel finish. They are compatible with surface conduit and mineral insulated cable (MICC) installations and are preferred by many commercial electricians as standard.

Installation Notes

FCUs are wired as a spur from the ring final circuit, with the feed (in) conductors connecting to the line side and the load (out) conductors to the load side. The integral fuse protects the appliance cable and the appliance itself — the ring circuit MCB or RCBO at the consumer unit provides upstream protection. Always verify that the spur from which the FCU is fed is not already overloaded and that the total connected load on the ring circuit remains within BS 7671 limits. For boiler connections, a double pole switched FCU is required to comply with Part P and current best practice.

Featured FCUs from APM Electricals

Knightsbridge 13A Switched Fused Spur Unit with Neon

Curved edge profile, white moulded, neon indicator, 13A BS 1362 fuse. Suitable for ring final spur. Fits standard 1G back box.

View Product

MK 13A 1-Gang Switched Fused Connection Unit with Neon

MK quality, double pole switch, neon indicator, white moulded, BS 7671 compliant. Popular choice for boiler and immersion heater connections.

View Product

Click Curva 13A FCU with Neon & Flex Outlet

Switched, neon indicator, flex outlet entry, white. Ideal for fridge-freezers and kitchen appliances where cable exits the front of the plate.

View Product

Knightsbridge Metal Clad 13A Switched Fused Spur Unit with Flex Outlet

Steel back, switched, flex outlet, for surface conduit and industrial/utility applications. Mounts on metal back box. 13A BS 1362 fuse.

View Product

Which FCU Should You Choose?

For most domestic boiler, immersion, and extractor fan connections, a white double pole switched FCU with neon indicator is the correct specification — the neon confirms the unit is live and the DP switch provides full isolation without returning to the consumer unit. The MK and Knightsbridge units are both widely accepted by NICEIC/NAPIT inspectors and are stocked at APM for next-day dispatch.

For surface wiring — particularly in kitchens, utility rooms, and outbuildings — a metal clad switched FCU provides the robustness required. Pair with a galvanised steel surface back box and trunking or conduit for a compliant installation.

For appliances such as fridge-freezers, washing machines, or under-counter appliances where the cable will exit from the face of the plate, select an FCU with a flex outlet to achieve a tidy, professional finish without cable loops or exposed conductors.

All FCUs at APM Electricals are stocked for trade account and online order, with same-day dispatch available on qualifying orders. Browse the full wiring accessories range.

Browse all wiring accessories at APM, including our full range of fused connection units and fused spurs for domestic and commercial installations.

APM Electricals — Electrical and Plumbing Trade Supplies
24 Western Avenue, Acton, London W3 7TZ
Tel: 020 8702 8080
Same-day collection available. Trade accounts welcome. www.apmi.uk