A fused spur, more accurately called a fused connection unit or FCU, is a fixed electrical accessory containing a replaceable fuse that can supply and protect connected equipment. Correct installation is not simply a matter of matching wire colours: the existing circuit, cable arrangement, earthing, protective devices, connected load and test results all need to be considered.
The practical point:
a typical FCU has a supply side and a load side. The incoming circuit supplies the FCU, its fuse limits the current available to the downstream load, and a switched version also provides local switching. The physical terminals and connection arrangement must always follow the accessory manufacturer's markings rather than a generic internet diagram.
Key Takeaways
- A fused spur and a normal 13 A socket-outlet are different electrical accessories.
- Modern fused connection units are covered by BS 1363-4 and may be switched or unswitched.
- The fuse fitted in an FCU should suit the connected equipment and cable arrangement; it should not automatically be treated as a 13 A load.
- Adding an FCU to an existing circuit still requires the existing circuit and protective measures to be suitable for the alteration.
- Electrical work should be inspected and tested after installation, with the appropriate certification for the scope of work completed.
What Is a Fused Spur?
“Fused spur” is the common UK name for a fused connection unit. It is a fixed accessory containing a BS 1362 fuse and is commonly used where fixed equipment needs a locally fused electrical supply.
The current British Standard for these accessories is BS 1363-4:2023. BSI describes it as covering 13 A fused fixed connection units for household, commercial and light-industrial use, including both switched and unswitched versions.
An FCU can therefore perform two related jobs: it provides a connection point for the downstream equipment and limits the current through that connection by means of its replaceable fuse. A switched FCU can additionally provide convenient local switching.
How Does Fused Spur Wiring Work?
At a conceptual level, fused spur wiring has three stages: the existing electrical circuit reaches the supply side of the FCU, electricity passes through the fused connection unit, and the load side supplies the connected equipment.
Existing Circuit
Incoming supply from the correctly assessed circuit
→
Fused Connection Unit
Fuse protection and, where fitted, local switching
→
Downstream Load
Fixed equipment or another correctly designed downstream connection
Many FCUs identify separate supply and load terminals for line and neutral conductors, together with an earth terminal. Terminal positions are not something to assume from appearance alone. Different accessories can have different layouts, so the markings and manufacturer's instructions for the actual unit must be followed.
This is a functional diagram, not a live-wiring instruction.
Before conductors are altered, the circuit needs safe isolation and the existing arrangement must be identified correctly. The completed work also requires electrical testing; visual inspection alone cannot confirm that an alteration is safe.
Switched FCU, Unswitched FCU or Normal Socket?
The accessories can look similar on a wall, but they serve different purposes. Choosing between them depends on what is being supplied and whether local switching or fused protection is required.
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Where Does a Fused Spur Connect to the Existing Circuit?
An FCU may form part of an alteration to an existing final circuit, but the correct arrangement depends on the circuit that is already present. A ring final circuit, a radial circuit and an existing spur do not automatically have the same design options.
Before adding an FCU, the existing circuit should be identified rather than guessed from the number of cables behind a socket. The installer also needs to consider conductor sizes, the circuit protective device, continuity, earthing, existing loading and the condition of the installation.
A particularly common question is whether another spur can be taken from an existing fused spur. That narrower circuit-arrangement question is covered separately in
How to Fit a Fused Spur from a Fused Spur.
It should not be treated as equivalent to adding an ordinary unfused branch to a circuit.
Can a Fused Spur Supply a Socket?
A fused connection unit can be used as part of a correctly designed supply to downstream electrical accessories or equipment, but the 13 A fuse in the FCU places a limit on the downstream supply. The fact that a socket can physically be connected downstream does not by itself prove that the proposed arrangement is suitable.
The intended use, circuit arrangement, cable size, protective measures and expected load all matter. Where the actual objective is simply to add or reposition socket outlets, that is better treated as planned
electrical installation work
rather than assuming an FCU is automatically required.
What Fuse Should Be Fitted in a Fused Spur?
The front of an FCU may be rated for 13 A, but that does not mean every connected item should automatically receive a 13 A fuse. The fuse selected needs to be appropriate for the connected equipment, its flexible cable where relevant, and the manufacturer's requirements.
The purpose of the fuse is protection, not simply allowing the largest possible load. Fitting a higher-rated fuse without checking what it is protecting can defeat the reason for using the fused connection unit in the first place.
Does a Fused Spur Need RCD Protection?
An FCU does not create RCD protection by itself. Whether additional RCD protection is required depends on the circuit, cable route, location, connected equipment and the requirements applicable to the installation.
This is another reason the existing circuit should be assessed before an FCU is added. The protective arrangements already present at the consumer unit and the way new wiring is installed can affect the final design.
What Regulations Apply to Fused Spur Installation?
Fixed electrical installation work should be designed and installed in accordance with the applicable electrical-safety requirements. BS 7671 is the UK national standard for electrical installations, and the IET has now published BS 7671:2018+A4:2026.
There is currently a transition period. The IET states that BS 7671:2018+A3:2024 remains a valid standard until 15 October 2026, so installers may work to either the Amendment 3 or Amendment 4 position during that period.
Part P in England
Building Regulations requirements are jurisdiction-specific. For dwellings in England, Part P applies to fixed electrical installation work. Not all work is notifiable, however.
Current IET guidance for England explains that installing a new circuit is notifiable, as is adding to or altering an existing circuit in a special location. Other additions and alterations to existing installations outside special locations are generally non-notifiable.
That means adding an FCU to an existing circuit is not automatically a notifiable job merely because an FCU is being fitted. The actual scope and location of the work determine the notification requirement.
Non-notifiable does not mean unregulated.
Electrical work still needs to meet the applicable safety requirements even where prior Building Control notification is not required. Rules outside England differ, so the England Part P position should not be assumed to apply unchanged across Scotland, Wales or Northern Ireland.
What Should Be Tested After a Fused Spur Is Installed?
Completing the physical connections is only part of the job. The alteration should be inspected and electrically tested so that the installer can verify the protective measures and confirm that the modified circuit remains suitable for continued use.
The exact tests depend on the work carried out and the existing circuit. They may include checks relevant to conductor continuity, protective conductors, insulation, polarity, fault protection and operation of relevant protective devices.
Certification also depends on scope. The IET's current BS 7671 model forms state that a Minor Electrical Installation Works Certificate is intended for additions and alterations that do not include a new circuit. Where a new circuit is installed, more extensive installation certification is appropriate.
Can You Install a Fused Spur Yourself?
Understanding how an FCU works is different from being able to verify that a circuit alteration is safe. Safe isolation, identification of the existing circuit, correct termination and electrical testing all matter, and mistakes can leave conductors incorrectly protected even when the accessory appears to work.
If you are deciding whether the work is appropriate as DIY, the separate guide
Can I Install a Fused Spur Myself?
covers that decision specifically. This article instead focuses on how the accessory and its wiring arrangement function.
When Should an Existing Fused Spur Be Checked?
An FCU should be investigated if there are signs of overheating, discolouration, a loose faceplate, crackling, repeated fuse failure, unreliable switching or loss of power to the connected equipment. Replacing the fuse repeatedly without establishing why it has operated can hide the underlying problem rather than solve it.
If the issue appears to involve the circuit rather than a straightforward accessory replacement,
have the electrical fault investigated
before assuming the FCU itself is defective.
References & Standards
Technical and regulatory information in this article was checked against current official guidance available on 21 August 2026.
Need a Fused Spur or New Electrical Point Installed?
If an FCU, socket, switch or fixed electrical supply needs adding or altering, the existing circuit can be assessed first so the installation method, protection and testing match the work actually required.
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