Hi.
I have had two quotes from local electricians to supply power/lights to garden shed. They are both asking for the same price but:-

the first said he would fit a 20A MCB to the non RCD protected side of the consumer unit then cable to shed connect to a 2 way RCD protected garage unit - a 16A socket circuit and a 6A lighting circuit from this.

the second said she would fit a 20A RCBO to the non RCD protected side of the consumer unit then cable to shed connect to 2 off sockets and a 3A FCU for the lights.

The shed is about 20m total cable length from the house consumer unit. and I will have 2 x 2gang sockets and 2 x fluorescent tube type lights.

I know it's difficult to advise without seeing the job but, do either of these methods offer advantages/disadvantages over the other? If so what are they?

Thanks. D.

The first way is better, as that puts the RCD protection for the shed local to it, so if it trips you don't have to schlep back to the house to reset it.

But it is flawed - I'm not sure that a 16A and a 20A MCB will properly discriminate, i.e. a fault in the shed might trip the 20A back at the house and not the 16A in the shed.

The actual cable cost will be a small %ge of the overall price, particularly if you're paying someone to dig a 20m trench, so I'd advise speccing 6mm² or even 10mm² cable. This will provide two benefits; firstly it'll give you extra capacity should you ever decide to have a pottery kiln, or hot tub, or particle accelerator in the shed, and secondly it'll allow a higher rated MCB at the house and avoid discrimination poblems.

But IMHO neither design is the best - what you've got there are, respectively, numbers 2 & 4 in the list of preferences.

If it were my shed I'd have the tails split, supplying a switchfuse at the house as well as the CU, and then run the shed cable from the switchfuse to the shed CU.

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I'm sorry - I can't come in to work today, the voices are telling me to stay at home and clean the guns.

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Thanks for that.



I will go with option 1 but mention to him that I have a dream of running my own whisky distillery in the shed and see what he suggests to future proof the design!

Would the lack of discrimination be unsafe? or a nuisance?

Cheers. D.

OK, but in the shed CU have an RCBO for the sockets, not an RCD covering sockets and lights.



You tell me.

You're in the shed after dark, using some kind of power tool, and something happens which trips the house MCB and plunges the shed into darkness.

Unsafe? Or a nuisance?

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I'm sorry - I can't come in to work today, the voices are telling me to stay at home and clean the guns.

.