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Correct Mcbs For Garage



Hi All


First post here so please go gently.


I've recently moved to a new build house and the developer has kindly installed a single double socket and light fitting in the detached (6.5M x6.5M <1.5M from the house) double garage. - the general layout is this 

20mcb on the RCD protected side of the house CU delivering power to a CU in the garage via 10mm armoured buried cable. - The garage CU has a 16a mcb for power and 6 a for lighting and seperate RCD.


As I don't want flexes trailing everywhere I want to convenience of 2 x double sockets on the back wall and 2 x double sockets on each side wall and maybe one in the ceiling so seven in all.


Should I run them as a radial off the existing 16a mcb or run a ring back to the garage CU? If I run a ring my understanding is I would then need a 32a mcb but given the house CU has a 20a one should I plan on changing the garage one, the house one or neither?



My plan is to site the sockets myself and cable them but to get a proper sparky to terminate the cables to the garage CU and do appropriate testing (the developers are still on site so they may do this for me). Buying the right stuff in the first place will just save me time and money


Any advice would be most appreciated 


Many thanks



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6 answers to this question

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Given what you outline adding them to the garage cu make perfect sense.

You say you want to add sockets but what for - calculating load is about what is plugged in and not the amount of 13a sockets

As for buying kit, talk to a sparky first or else you could be wasting time and money

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The maximum load that you want to have in use at any one time governs the size of cables and protective devices (MCB's). It is not the number of sockets. just because you have more sockets doesn't mean they will all be in use at the same time taking maximum power. If these sockets are for general DIY power tools, battery chargers, lawn mower etc then the 16A extended as a radial should be more than adequate. If however you intend using some heavier duty workshop power tools, or machines, or heaters for regular use in the winter etc then a larger rated circuit may be needed. But you need to know the load first to ensure the correct cable and MCB sizes.


Doc H.

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You say the garage is fed from the rcd side of the house CU.


I would say that's wrong, as the garage CU has it's own rcd.


Normally in this situation you would take a feed from a non RCD protected MCB in the house so you don't have nuisance tripping in the house if you plug in a dodgy lawn mower for instance in the garage.


i suspect they have fitted a dual rcd CU in the house so can't do that. they should have fitted a high integrity cu.  I would mention this to them and see what they say.

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IMHO it'd be better if you got someone in to supply and fit.

They'll probably get the materials cheaper than you

You won't have to buy cable, and other bits and pieces, that won't be used

They will guarantee the whole job

...and it'll be designed and installed and tested properley

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Many thanks for the replies

Will talk to the developers - it's a dual RCD CU.

Only going to be using domestic power tools and a lawnmower

Arranged for sparky to come round Monday so I'm sure all will be good

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you have an rcd in the house cu and an rcd in the garage cu. They do not discriminate. ie, if a fault occurs in the garage, either rcd could trip. This could be against regs 531.2.9, 536.1 and 536.3 if, for example, the house rcd also protects a lighting circuit covering a stairway. I am thinking of what constitutes a "danger", as I am not sure whether it is required for "proper functioning of the installation"
You could have a word with the developer to possibly rectify this, free of charge, and perhaps add a couple more double sockets in the garage as a goodwill gesture.

There may be a reason for putting the garage supply cable on an rcd in the house cu, eg cable in wall at less than 50mm, and the devices just might discriminate according to the manufacturers, but get the developer to confirm this.

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