What is a rewirable fuse?

A rewirable fuse has a fuse carrier with a piece of fuse wire that melts when too much current flows. They were the standard protection device in UK homes until MCBs became common in the 1980s.

How do I know what fuse wire to use?

Fuse wire is colour-coded: 5A (white) for lighting, 15A (blue) for immersion heaters, 20A (yellow) for radials, and 30A (red) for ring mains. Always use the correct rating for the circuit.

Why do rewirable fuses keep blowing?

Common causes are overloaded circuits, faulty appliances, or using the wrong (too low) rated fuse wire. If a fuse blows repeatedly, there is a fault that needs professional investigation.

Can I just replace the fuses with MCBs?

Not in a rewirable fuse box — the fuse carriers are not interchangeable with MCBs. The entire consumer unit needs replacing to get MCB protection.

Caution

Updated 30 August 2026

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What's happening?

Rewirable fuses are the oldest type of overcurrent protection still found in UK homes. They consist of a fuse carrier with a piece of fuse wire stretched between two terminals. When too much current flows, the wire melts and breaks the circuit. While this basic principle works, rewirable fuses have significant drawbacks compared to modern MCBs: they can be fitted with the wrong rated wire, they blow easily on startup surges, and they do not provide the precision protection of an MCB.

What usually causes it?

Consumer unit has not been upgraded since the property was built (typically pre-1980s)
Fuses have been replaced with incorrect fuse wire ratings over the years
Board is too old to accept modern MCBs as plug-in replacements
Previous rewires have kept the old board and only replaced the cables
Property owner is unaware that upgrades are recommended

Is it dangerous?

Rewirable fuses provide basic overcurrent protection but are less safe than MCBs. The main risk is that anyone can fit the wrong rated fuse wire — for example, using 30A wire in a 5A lighting fuse carrier. This defeats the protection entirely and could allow a fault to cause a fire. Rewirable fuses also lack RCD protection, meaning there is no defence against earth faults that cause electric shock.

Can I fix it myself?

You can replace a blown fuse wire if you know the correct rating and have the proper fuse wire. However, this is a temporary measure. The broader issue — lack of modern protection — can only be resolved by upgrading the consumer unit. Never use any wire, nail, or metallic object as a substitute for proper fuse wire.

When should you call an electrician?

Contact an electrician to discuss upgrading your consumer unit. While rewirable fuses are not an emergency, they provide significantly less protection than modern MCBs and RCDs. An upgrade is one of the most cost-effective safety improvements you can make to your home.