Garage Wiring Query

Question

Hi, I have a detached double garage to my new build which has power running to it from a 20A MCB (protected by an RCD) in the house Consumer Unit, which connects to a second CU in the garage. The garage CU contains a 6A MCB for the garage light and a 16A MCB for the double socket. Would I be able to uprate the 16A MCB to a 20A MCB to enable me to run an additional double socket via a radial circuit? Obviously the whole garage is protected by a 20A MCB in the house and RCD anyway. The wiring for the sockets in the garage is 2.5mm 2 . 
 Many thanks for any help!

broadgage · Accepted Answer

Almost certainly yes.

mapj1 · Answer

Well actually you can safely add as many sockets as you like downstream of the 16a MCB using some more 2.5mm even without any further changes if you like. What you cannot do of course is draw much more than 16A total for any long period without the risk of the breaker doing its thing. However, the difference between a 20A and a 16A breaker is not that great (25% more load) while in fact going the other way, a 16A breaker and a 13A fuse are pretty similar too. 
 IF you really want a lot more power in the garage, then you will need a larger supply - but if you are just plugging in a load of small stuff, or a few larger loads that are never all on at once, then it will be fine. Can you tell us the wattages or currents of the loads you are adding, and ideally the worst case combination ? 
 M

mapj1 · Answer

In your shoes, I'd add the sockets on the 16A branch for now, you will soon know if tripping is an issue, and then, only if it is, then you can consider if it is safe to up-size either or both of the breakers. 
 A twin and earth 2.5mm cable well cooled - i.e, in fresh air, is good for 27A, however if it is enclosed in certain types of insulation, that useful rating can drop to less than 20A, so you cannot up-rate things by too much without checking the cable routes first. 
 Come back for more advice about that aspect if you need it. 
 Mike

Garage Wiring Query

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