A complete practical guide to fused connection units (FCUs): types, correct fuse ratings, when to use a spur instead of a socket, BS 7671 requirements, and step-by-step wiring instructions.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Run a 2.5 mm² twin-and-earth spur from a socket on the ring final circuit (or a junction box in the ring cable) to the supply/IN terminals of the FCU. Connect the appliance flex — line, neutral and green/yellow earth — to the load/OUT terminals. Fit a BS 1362 cartridge fuse rated to the appliance flex (3 A, 5 A or 13 A), not 13 A by default. Then test polarity, earth continuity and insulation resistance before energising.
A switched FCU gives local double-pole isolation of all live conductors (Regulation 462.2). Where the spur supplies socket-outlets up to 32 A, the circuit needs 30 mA RCD protection (Regulation 411.3.3).
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A fused connection unit (FCU) — commonly called a fused spur — is a wiring accessory that provides a permanent, fused connection to a fixed electrical appliance. Unlike a standard socket outlet, the FCU does not have a removable plug — the appliance is hardwired to the load side of the FCU or connected via a short flex to a flex outlet plate on the FCU face.
The FCU contains a cartridge fuse (BS 1362 type, identical to those used in 13A plugs) sized to protect the appliance flex rather than the circuit cable. This means the fuse provides an additional level of overcurrent protection downstream of the ring main protection — protecting the flex and the appliance rather than the supply cable.
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FCUs are available in several configurations. Choosing the right type for the application is important for compliance, safety, and usability.
Incorporates a double-pole rocker switch that disconnects both line and neutral simultaneously, giving a local means of isolation (Regulation 462.2). The default choice for dishwashers, washing machines, extractor fans, heated towel rails and most kitchen appliances.
Fuse only, no switch. Used where the appliance has its own integral switch or where isolation is provided by a separate DP switch. Less common in practice since switched FCUs cost little more.
Includes an LED or neon lamp that illuminates when the switch is on. Useful for appliances in concealed positions where it matters that you can see the supply is live (e.g. immersion heater, frost-protection heater).
Has a small aperture in the face plate through which the appliance flex emerges, connected to the load terminals inside the back box. Common for cooker-hood extractors and where the flex routing is tight.
For higher-current fixed loads such as immersion heaters and some fixed heaters. Not a standard FCU — it has no cartridge fuse — but a double-pole switch for fixed equipment. Circuit protection is provided by the MCB at the consumer unit, so the load is run as its own dedicated radial rather than a spur.
The decision between a socket outlet and an FCU is not always obvious. These are the situations where an FCU is the appropriate choice:
Not moved in normal use. An FCU inside the adjacent cupboard gives an accessible isolation point without pulling the appliance out to reach a socket. A 13 A fuse matches the appliance flex rating.
Wired as a 3 A or 5 A fused spur from the lighting circuit or ring final circuit. The FCU protects the fan flex and provides isolation without going to the consumer unit.
Panel heaters and heated towel rails installed as fixed equipment need a means of isolation. A switched FCU provides local double-pole isolation plus flex protection.
Where a weatherproof garden socket is spurred from an indoor ring final circuit, an FCU gives the fused connection point. The garden socket (IP44 minimum) connects as the load and must have 30 mA RCD protection.
One of the most common mistakes with FCUs is fitting the wrong fuse rating. The fuse in an FCU protects the appliance flex — it should be rated as close as possible to (but above) the rated current of the appliance, not automatically set to 13A because that is the maximum.
Approximate wattage limits at 230 V. Always confirm against the appliance rating plate — the fuse protects the flex, not the ring.
| Fuse | Max load (approx.) | Typical appliances |
|---|---|---|
| 3 A (red) | up to 720 W | Extractor fans, cooker hoods, lamps, clock radios, doorbell transformers, most low-wattage electronics |
| 5 A | up to 1,150 W | Larger fans with integral lighting, small fridges, low-wattage panel heaters |
| 13 A (brown) | up to 3,000 W | Dishwashers, washing machines, tumble dryers, electric heaters, microwave ovens |
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Fused connection units are covered by BS 7671:2018+A4:2026 and the relevant British Standard for the accessory itself (BS 1363-4 for FCUs to BS 1363). Key regulatory requirements include:
| Regulation | Requirement and what it means for an FCU |
|---|---|
| 462.2 | Every circuit must be provided with isolation means for all live conductors. The double-pole switch of a switched FCU disconnects both line and neutral, providing local isolation adjacent to the appliance. |
| 464.1 | Means for switching off must be provided where mechanical maintenance may involve a risk of physical injury — relevant for fans, pumps and motorised appliances. A switched FCU provides this off-switch with disconnection of all live conductors. |
| 411.3.3 | Socket-outlets rated up to 32 A require 30 mA RCD additional protection (subject to the permitted exceptions). An FCU spur taken from a ring final circuit must be RCD-protected at the consumer unit or by an inline 30 mA RCD. |
| 433.1.204 | Ring final circuit rule: accessories to BS 1363 may be supplied through a ring final circuit (with or without unfused spurs) protected by a 30 A or 32 A device, wired in copper with line and neutral of at least 2.5 mm² (1.5 mm² for two-core MICC). |
| App 15 | Ring and radial arrangements (informative). An unfused spur should feed only one single or one twin socket-outlet; the number of socket-outlets fed from an FCU depends on the load, having taken diversity into account. |
| 514.4.2 | The green-and-yellow combination is used exclusively for protective conductors. Earth cores must be sleeved green/yellow at terminations; old red/black cable reused in a modification must have line and neutral re-identified with brown and blue. |
See the for common FCU-related C2 findings that arise during inspection.
Adding an FCU as a spur from an existing ring main requires a Minor Works Certificate. If new cabling is installed that creates a new circuit, an Electrical Installation Certificate is required. Use Elec-Mate to generate compliant certificates on-site.
Follow these steps to install a switched fused connection unit as a spur from a ring main circuit.
Switch off the circuit at the consumer unit and prove dead with an approved voltage indicator. Identify a suitable socket on the ring main (confirm it is on the ring, not already a spur) or install a 30A junction box in the ring cable at a convenient point. Do not use a non-standard junction — use a properly enclosed JB.
Use 2.5mm² twin and earth cable for standard FCU spurs. Route concealed cable in accordance with BS 7671 Section 522 and Table 52.1 — within a prescribed zone (within 150mm of the top of the wall or of an angle between walls, or running horizontally/vertically from the accessory), or provide additional protection by a 30mA RCD, or use a cable with an earthed metallic covering. Leave sufficient tail length at each end.
At the spur origin (socket or junction box), connect the spur cable using the appropriate terminals. At the FCU back box, connect line (brown) to the "Line In" or "Supply L" terminal, neutral (blue) to "Neutral In" or "Supply N", and earth (green/yellow) to the earth terminal. Ensure cores are sleeved correctly — earth in green/yellow.
Connect the appliance flex or a short length of flex (for a flex outlet plate) to the "Load" or "Out" terminals: line to "Load L", neutral to "Load N", earth to the earth terminal or the metal earth clip. Verify that the flex current rating is adequate for the appliance load.
Insert the correct BS 1362 cartridge fuse in the FCU fuse carrier: 3A (red) for loads up to 720W, 5A for loads up to 1,150W, 13A for loads up to 3,000W. Confirm the fuse rating against the appliance rating plate. Replace the fuse carrier and secure the FCU face plate.
With the circuit off, test insulation resistance between line/neutral/earth, verify polarity, and measure earth continuity from the FCU earth terminal back to the main earth terminal. Reconnect the circuit and verify correct operation of the switched FCU and the appliance. Issue a Minor Works Certificate (or EIC if the circuit is new) using the Elec-Mate app.
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