Fused Connection Units and Fused Spurs — Switched, Metal Clad, and Flex Outlet FCUs for UK Electricians

A fused connection unit (FCU) — also called a fused spur or fused outlet — is a fixed, fused point of connection used to supply an appliance directly from the ring final circuit or a dedicated radial without using a plug and socket. FCUs are standard on cooker hoods, extractor fans, bathroom heaters, outdoor light circuits, and immersion heater feeds. They provide overcurrent protection via a BS 1362 cartridge fuse while giving a permanent, tidy termination point directly at the appliance. This guide covers when and why to use an FCU, the main types available, fuse selection, wiring method, and the relevant requirements under BS 7671.

When to Use an FCU Instead of a Socket

BS 7671 permits a fused spur to be taken from any point on a ring final circuit, or as the termination of a dedicated radial, provided the spur feeds only one item of fixed equipment and is protected by a fuse not exceeding 13A. An FCU is the correct choice when:

• The appliance is permanently fixed and not intended to be unplugged (cooker hoods, extract fans, towel rails, outdoor lighting)
• The supply needs to be switched at the point of connection for isolation purposes (Regulation 537.2)
• The appliance requires a lower fuse rating than the 32A OCPD protecting the circuit — for example, 3A for a cooker hood or 5A for an extractor fan
• The connection is in a location where a standard socket would be inappropriate — above a worktop near steam, inside a loft, or on an exposed outdoor surface
• A flex outlet is required — metal clad FCUs with a flex outlet allow direct cable entry without a separate flex or plug

FCUs may not be used to supply socket outlets or other FCUs downstream. Each FCU feeds one piece of fixed equipment. Where multiple appliances need supply from a single spur, a junction box or a dedicated sub-circuit is required instead.

Switched vs Unswitched FCUs

The majority of FCUs installed by UK electricians are switched. A switched FCU incorporates a double-pole (DP) switch that disconnects both line and neutral when open — providing local isolation in compliance with BS 7671 Regulation 537.2.1.1. This is essential for any appliance that the user or an engineer may need to isolate safely without going to the consumer unit.

Unswitched FCUs are occasionally specified where the appliance has its own integral switch and local isolation is controlled elsewhere. They are more common in commercial fit-outs and on permanent supplies to alarms, ventilation, and CCTV where the FCU is a fusing point only.

For domestic kitchens and bathrooms, a switched FCU is the standard choice. Many models include an illuminated rocker or neon indicator so it is clear whether the supply is live — useful on cooker hoods where the switch is often above or behind the unit.

White Plastic vs Metal Clad FCUs

White plastic FCUs are standard for internal installations: kitchens, utility rooms, airing cupboards, loft spaces, and any surface where a wiring accessory plate is appropriate. They mount on a standard single-gang back box (25mm or 35mm depth) and match existing socket and switch plates.

Metal clad FCUs are specified for:

• Garages and outbuildings where impact resistance matters
• Outdoor surface-wiring installations where a metal clad plate gives a more robust surface seal
• Industrial and commercial applications where a white plastic plate would be unsuitable
• When combined with a flex outlet — many metal clad FCUs have a bottom-entry knockout for a flex grip, allowing direct connection of a short length of heat-resistant flex to the appliance

Metal clad FCUs with a flex outlet are particularly common behind cooker hoods and bathroom shaver or heater feeds where a short run of flexible cord runs directly into the FCU terminal. The flex grip (often a supplied rubber grommet) secures the cord and prevents chafing against the metal knockouts.

Fuse Selection — 3A, 5A, or 13A?

The FCU's cartridge fuse should be rated as close to the running current of the connected appliance as possible without nuisance blowing. The three most common ratings are:

• 3A (red) — Cooker hoods (typically 70–150W), bathroom extractor fans (10–30W), garden lighting circuits, doorbells, and small fixed appliances below ~700W
• 5A (black) — Larger extractor fans, bathroom heaters, and appliances up to ~1,150W
• 13A (brown) — Immersion heaters, panel heaters, storage heaters, and other fixed high-load appliances up to 3kW

Most FCUs are supplied with a 13A fuse fitted. Always replace this with the correct rating for the appliance before commissioning. A 13A fuse in a cooker hood FCU provides virtually no additional protection — the 3A fuse is needed to protect the cord and the appliance's internal wiring. BS 1362 fuses are available as replacement packs or are often included with the FCU.

Wiring an FCU from a Ring Final Circuit

A spur from a ring final circuit is taken from a socket outlet, a junction box, or a junction in the back of an existing socket. The supply to the FCU is wired into the "FEED" or "MAINS" terminals, and the "LOAD" or "SWITCHED LIVE" terminals connect to the appliance. Standard wiring practice:

1. Confirm the ring is correctly wired before adding a spur — an end-to-end ring test (Regulation 612.2.2) confirms continuity
2. Take the spur from a socket on the ring, not from another spur or from the consumer unit (unless it is a dedicated radial)
3. Use 2.5mm² T&E for the spur cable — even if the FCU fuse is 3A, the spur cable must be rated for the protective device protecting the circuit (typically 32A MCB), not the FCU fuse
4. Connect L, N, and E to the "IN" or "SUPPLY" terminals; connect the appliance cable to the "OUT" or "LOAD" terminals
5. Fit the appropriate cartridge fuse for the appliance before closing the FCU

For FCUs with a flex outlet, the appliance flex connects to the LOAD side terminals and exits through the flex outlet knock-out with the grip tightened. The FCU then terminates directly on its own surface-mounted back box adjacent to the appliance.

FCUs in Bathrooms (Zone Considerations)

BS 7671 Section 701 restricts electrical equipment in bathroom zones. Socket outlets are not permitted in zones 0, 1, or 2. However, shaver supply units and SELV switches for extractor fans and heaters may be installed in Zone 2 (and Zone 1 with appropriate IP rating) provided the equipment has the correct IP rating and the installation meets 701.512.2.

An FCU supplying a bathroom extractor fan or heated towel rail must be located outside the bathroom zones if possible, or at minimum in Zone 2 with an IP44 or higher rating. Metal clad FCUs or appropriately rated white plastic FCUs with splash-proof ratings are available for these applications. A switched FCU outside the bathroom zone with cable routed to the extractor is the preferred approach in most domestic installations.

FCUs for Cooker Hoods — Common Installation

The standard approach for a cooker hood connection in domestic kitchens:

• Spur from ring final circuit (kitchen socket circuit) via a switched FCU above the hood location, fitted with a 3A fuse
• FCU mounted at or above worktop height, accessible without moving the hood
• A metal clad FCU with flex outlet is appropriate where the hood's own flex can connect directly at the FCU, avoiding an unsecured loop of cable
• White plastic switched FCU (surface or flush) used where a cable drops down behind the unit from an adjacent consumer unit radial

If the cooker hood is on its own dedicated radial (uncommon in domestic), the circuit breaker in the consumer unit provides the primary overcurrent protection and the FCU fuse still provides close protection for the appliance and its flex.

Product Spotlight

Multiple FCUs on One Circuit — What's Permitted?

BS 7671 Regulation 433.1 and Appendix 15 set limits on the number of spurs from a ring final circuit. Each spur may feed only one FCU (or one socket). The total number of spurs must not exceed the number of socket outlets on the ring. In practice, on a typical 32A kitchen ring, 1–2 FCUs for cooker hood and perhaps an undercounter appliance are well within limits. Always count existing spurs before adding a new one.

For multiple permanently connected appliances — for example, an extractor hood, waste disposal unit, and water softener in a fitted kitchen — consider a dedicated 20A or 32A radial circuit with individual FCUs at each appliance, supplied from the consumer unit. This keeps each fixed appliance independently protected and allows individual isolation without affecting other circuits.

Replacing an FCU — Fault-Finding Notes

An FCU fuse blows for a reason. Before fitting a replacement fuse, confirm:

• The appliance is not drawing excessive current due to a fault (check with a clamp meter if available)
• The correct fuse rating was previously fitted — a 3A fuse on a 1,000W fan heater will blow immediately
• The flex is not damaged or kinked, causing a short on the load terminals
• The supply connections on the FEED side are tight and not loose or corroded

If a 13A fuse blows in an FCU on a cooker hood, the hood's motor may have seized or there is a fault with the internal lamp circuit. Isolate at the FCU switch, inspect the appliance, and fit a correctly rated replacement fuse — 3A for standard hoods — before switching back on.

Cross-References

• Bathroom Electrical Accessories — Pull Cord Switches, Fan Isolators, and Shaver Sockets
• PVC Conduit and Oval Conduit — Surface Cable Wiring for UK Electricians
• Metal Back Boxes and Dry Lining Boxes — Flush Mounting Wiring Accessories
• WAGO Connectors and Push-Wire Terminals — Junction Boxes for UK Electricians
• Consumer Units, MCBs, and RCBOs — BS 7671 2018 Guide

Summary

An FCU is the correct termination for any permanently fixed appliance taken from a ring final circuit or dedicated radial. Choose a switched (DP) FCU for all installations where local isolation is required — which is virtually every domestic application. Select white plastic for internal wall mounting, metal clad with flex outlet for cooker hoods and garages. Always fit the correct cartridge fuse rating for the appliance — 3A for hoods and small fans, 5A for larger fans and heaters, 13A for immersion and panel heaters. Confirm the spur count before adding an FCU to a ring and refer to BS 7671 Appendix 15 for ring final circuit loading limits.