Advice on specifying an outbuilding supply
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Advice on specifying an outbuilding supply

by Skillster » Mon Aug 10, 2009 5:36 pm

Can someone offer a bit of advice please?

I am currently fitting out a small outbuilding to use as a microbrewery and workshop. I need to get an electrician in to install a power supply, but would like to understand what sort of cabling and hardware should be used so that I can ask the right questions and understand the quotes as far as possible.

I will need to run 2 10A water heaters / boilers at the same time plus power ancilliary sockets (radio, desk lights etc). I will also need a lighting circuit.

The outbuilding is about 3 metres from the rear corner of the house, adjacent to the boiler room that houses the house consumer unit. Cabling could be run through the house wall, along the boundary wall linking house to outbuilding (no need for undergound run).

I assume a new circuit will need to be created from the consumer unit with a secondary box mounted in the outbuilding; separate circuits for sockets and lights would be run from that.

Is my understanding, such as it is, anywhere near? All thoughts, comments or advice would be gratefully received.


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by kbrownie » Tue Aug 11, 2009 12:50 pm

Hi Skillster
Just about answered all your questions yourself.
Your electrician will work out the output for your outbuilding, they will need to assume the correct current that will be needed to supply your power and lighting circuits, once they have done that the will calculate the the protective device that can deliver this current and cables size taking in to account any voltage drop.
The selection will be made so that the cable can carry this current safely between house and outbuilding.
A small CU will be need within the outbuilding.
The circuits will need RCD protection and the install will come under part p of building regulations. So work will need to be notified to building controls useful links are:
and google competent persons scheme.
You will need this work to be signed off and certificates issued.

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by moggy1968 » Tue Aug 11, 2009 10:32 pm

The run to the outbuilding will probably be done with SWA, steel wire armoured cable. strictly speaking the outbuilding should have it's own independent earth through the use of an earth rod.
if it was me (bearing in mind I haven't seen the installation) I wouldn't take a run from your house consumer unit. I would take the meter tails into a thing called a henley block. this would have two outlets (2 live, 2 neutrals) one to your consumer unit for the house, the other would run to a main switch of the appropriate amp rating. this would also usitlise a time delay RCD. this then runs, using the SWA to a consumer unit in your outbuilding feeding the required circuits via MCBs mounted on an small split load board, say an 8 way (although a 12 way may actually be better as they are often cheaper, but correspondingly more bulky). This provides discrimnination on your outbuilding circuits so that if you trip the RCD the ones in the outbuilding will trip first, saving you having to troop inside if they ping. IF you come off your house consumer unit using the 30ma RCDs there and a further 30ma rcd in the outbuilding all that will happen is they will both trip, which is a bit of a nuisance.
I would run each boiler of a seperate side of the consumer unit so if there is a problem only one is affected, likewise the lighting should be on the opposite side to your sockets.
This isn't the cheapest option (time delay RCDs are quite pricey, but in the great scheme of things, a fraction of the total cost), but IMHO the most useable.

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