Can a garage consumer unit have a 20 A MCB despite being connected back to a 16 A MCB in its upstream consumer unit?

Question

I'm planning to rewire my detached garage to replace an old fuse box and add several more sockets. I plan to get an electrician to swap the fuse box for a modern garage consumer unit and test my wiring but I want to do most of the actual installation myself.

The layout of the garage that I'm rewiring and the load I'm expecting (approximately 11-13 sockets; small electronics and a drill press, nothing high power) suits a radial circuit topology using 2.5 mm² cable into a 20 A MCB. However, the garage fuse box that I'm replacing is connected back to a 16 A MCB in the house, which would ultimately limit the current in this circuit to 16 A. Is there any regulation in the UK that prohibits the use of a higher rated MCB in a garage consumer unit than the total rating of the consumer unit itself as governed by its own connection? Would it in any case be a better idea to use a 16 A MCB in the garage consumer unit circuit so that it's obvious to later users what the real limit of the circuit is?

I can in principle replace the cable from my house consumer unit to the garage if needbe, but I'd rather not.

Answer 1

Here in the US, the main circuit breaker (MCB) is used to protect the installed wiring, not a downstream circuit box or devices. Therefore the MCB must be at least matched to the circuit it's connected to or a smaller (lower amp) breaker. Why you'd do that is nonsensical but it's code legal. In other words, you could connect a 10ga cable (ampacity of 30 amps) to a 20 amp breaker to a sub-panel. Maybe you'd want to do that because of a very long run and to minimize voltage drop but I still see no reason not to match the breaker to the wire size.

In your case you'd have to check with local authorities, but I see nothing wrong with feeding a 20 amp circuit with a 16 amp breaker , your mileage may vary! check with your AHJ to be sure.