Small outbuilding supplied with 10mm cable, using no more than 30A. Consumer unit currently has 63A main breaker. Is this too high and if so should it be changed ? I read correct size is somewhere at 125% x 30A  ?

Does the cable come from the CU?  Through an MCB?  What is it connected to in the outbuilding?  Good to use a small CU with 50-63amp RCBO main breaker.  From there you can feed to power points, lights, etc. with appropriate MCB/cable.

cable comes direct from the meter, into a small consumer unit in the outbuilding, into the 63a main breaker. small mcbs in the consumer unit look right size for lights, sockets, aircon, etc. Since the cable from the meter to the consumer unit is only 10mm I thought the main circuit breaker may be too big (i am assuming its job is to protect the 10mm cable from overload but perhaps i’m wrong ?).

 

Separate cables (16mm) run from the meter to a consumer unit in the house, and all looks ok there.

I'd be tempted to stick a 30A in the shed CU, more to protect the meter than the cable.

 

EDIT To avoid confusion, there's already a 16mm² feed to the house which you can bet has a 63A main breaker on the other end.

 

28 minutes ago, Crossy said:

I'd be tempted to stick a 30A in the shed CU, more to protect the meter than the cable.

The OP didn't say what size meter but, in any case, I have never seen you so kind to the beast previously.  ????

7 minutes ago, bankruatsteve said:

The OP didn't say what size meter but, in any case, I have never seen you so kind to the beast previously.  ????

 

I'm assuming a 15/45 coz no Thai would put 16mm² into a 5/15 meter ???? 

 

I really don't like splitting at the meter, it leads to two loads both with 63A incomers so potentially 120A+ before something opens. With no distribution network overload protection we have the potential for meters competing with Space-X.