The wrong size breaker in your consumer unit: why it matters

Why the breaker and the cable have to match

A circuit breaker's job is to cut power before the cable behind it gets hot enough to be a fire risk. Every cable has a maximum current it can carry continuously without overheating, depending on its cross-section, the material, and how it is installed. The breaker rating has to sit at or below that limit.

If the breaker is rated higher than the cable can take, the breaker will happily let more current through than the cable is built for. The cable heats up, the insulation degrades faster, and in a fault or a sustained heavy load the breaker simply will not intervene in time. The wire is the weak point, and nothing is watching it.

This is different to a breaker that is too small, which trips often and is merely annoying. An oversized breaker on thin cable gives no warning at all. That is what makes it the more serious of the two mismatches.

How this mismatch usually happens

The most common cause is a partial rewire or an added circuit done without checking the existing consumer unit properly. Someone extends a lighting circuit or adds a socket run using whatever cable was to hand, then fits it under a breaker rated for the rest of the board rather than for the new cable.

It also turns up after a breaker fails and gets replaced with whatever was in the toolbox, not what the circuit calls for. In older Tallinn flats with a mix of Soviet-era wiring and later additions, you sometimes find several generations of cable on one consumer unit, each added at a different time by a different hand.

A less obvious version is a consumer unit upgrade where the electrician replaces old fuses with modern breakers but keeps the original fuse ratings without rechecking them against the cables installed decades earlier, when standards and typical cable sizes were different.

What to check yourself before calling an electrician

You can gather useful information without opening anything live. Start at the consumer unit and note down what is written on each breaker, then see what you can find out about the cable on that circuit, particularly at accessible points such as socket backs or the loft.

None of this tells you for certain whether a circuit is correctly matched, but it flags the ones worth asking an electrician to check first, which makes any call-out faster and usually cheaper.

• Read the amperage printed on each breaker (typically 6, 10, 16, 20 or 32 A).
• Note which circuit each breaker feeds — check any labelling in the consumer unit.
• Where a socket or light fitting can be opened safely with the power off, look at the cable's printed size (for example 1.5 mm² or 2.5 mm²).
• Be suspicious of a 32 A or 20 A breaker feeding anything other than an oven, hob, or a dedicated heavy-load circuit.
• Note if any circuit was clearly added later — different cable colour, different clips, a visibly newer run.
• Write down what you find rather than relying on memory when you call for a quote.

When this is a real danger and when it can wait

A mismatch is more urgent the higher the breaker rating and the thinner the cable, and the heavier the load that circuit actually carries. A 32 A breaker on cable meant for lighting, feeding a kitchen with a kettle and a washing machine, is a genuine fire risk and should be looked at without delay.

A smaller mismatch on a lightly loaded circuit, such as a 10 A breaker on cable rated for 6 A on a circuit that only ever runs a lamp, is far less pressing, though still worth correcting when convenient.

Warning signs that push a suspected mismatch from 'worth checking' to 'stop using that circuit now' include warm socket faceplates, a persistent smell of hot plastic, browning around switches, or a breaker that feels warm to the touch.

What you can do yourself and what to leave alone

Identifying and documenting a possible mismatch is safe and useful. Reducing your own load on a circuit you suspect is undersized — unplugging a heater, spreading appliances across sockets on different circuits — is also sensible while you wait for someone to check it properly.

Changing a breaker yourself is not something to do, even though the part itself is inexpensive and looks simple to swap. Getting the rating wrong in the other direction, or missing a deeper fault the mismatch was masking, turns a manageable job into a real hazard. This is work for a qualified electrician.

Our electrical work starts from 49 € per hour and covers exactly this kind of check and correction. If the consumer unit turns out to have several mismatches, old fuses, or general wear, a full consumer unit replacement, from 350 € per job, is often the more sensible fix than patching one circuit at a time.