DIY Doctor

Electrical Supply to 2 Outdoor Buildings

Postby Argos69 » Tue Mar 20, 2012 5:33 pm

I hope someone can advise on this one.

I have an existing garage which has a 4mm SWA cable supplying a 2 way RCD board (6A & 16A ways) from a 20A MCB in the house consumer unit.

I have built a shed which has very little requirements in terms of electricity, but I'd like to feed it from the the garage which is only 6 feet away. Feeding from the house would entail LOTS more work and distance.

Which option is best, assuming I'll be fitting a 2 way RCD board in the new shed, identical to the one in the garage :-

1. Loop the shed mains from the incoming mains feeding the garage board (so the 20A way in the house is feeding both garage and shed).
2. Feed the shed from the 16A way in the garage board.

I plan to use 2.5mm SWA for the garage to the shed (10 feet in total)
I've checked volt drop at the garage and it's fine.

Thanks in advance!
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Simply Build It

Postby ericmark » Wed Mar 21, 2012 12:01 am

Clearly it must go through the 16A MCB as you are changing cable size. Personally I would not use a consumer unit for shed feed fuse connection units (FCU) are ample using a switched one for lights.

Be aware of Part P and the need for inspection and testing. Earth loop impedance is important with out buildings and since location in respect of other buildings can alter requirements this is something which needs a site visit.

I would think cheaper to have a scheme member do this work than DIY.
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Postby Argos69 » Wed Mar 21, 2012 10:54 am

Hi ericmark & thanks.

Since posting, I have now borrowed a copy of the regs and found buried in 433.3.1 (i), a bit which says I can drop the cable size, so long as the protection back at the house is suitable for the smallest cable size in circuit, which it is. That means I can keep the 16A and 6A in the garage for the socket and light situated in the garage.
So I'm kinda sorted really. The friend who's regs I borrowed is now part p, so I might turn apprentice for the day and help him do the job! :-)
Thanks again.
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