Shed Electrics

23 Apr 2021
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United Kingdom
I have had an electrian out to quote me for running power to my shed and greenhouse but am looking for a second opinion please.

Shed and greenhouse to each have sockets (16 or 20 amp radial) and lighting circuit in shed.

The shed is approx 100m away from the house and the greenhouse a further 5m away. So he has suggested using 25mm SWA.

The main consumer unit feeds a 100 amp CU in a granny flat above the garage using a 63 amp MCB. I was wanting to take a feed from this CU to the shed since it is easier to run cable through the garage.

But he says the feed from the main CU to the granny flat is only using 10mm T&E and 25mm SWA cannot be used here. So has suggested feed for shed is taken from main CU instead.

Is this correct? Any other thoughts or ideas welcome.

Many thanks
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Personally, I think if you are looking for a second or third opinion, you should get an electrician out to see the job and quote rather than taking advice from a stranger on an Internet forum. I mean, I know sod all about electrics so if I said "Yeah, that seems fine to me", would you be happy with that?
He's more likely to be correct than anyone on here who hasn't seen the granny flat cu and whatever is installed there...
25mm on a 100m run isn't massively over the top given your power requirements at the end of it. If the granny flat is currently a 100m run of 10mm to the main cu then volt drop without any additions is probably too high, never mind any other considerations.
EDIT Just reread original post & realised granny flat isn't at the end of 100m run. From design point of view it'll be much better to run from main cu- less chance of a cascade of failures due to a fault in the outside circuits.
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The electricians plan certainly sounds sensible. Although he refers to taking a supply from the main CU it would be better to supply the shed from a switch fuse (also non RCD) as this would provide discrimination from the protective devices presumably to be installed in the shed.

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