Power to an outbuilding - advice on cable size and earth

[griffta]

Hi all,

I wonder if I could ask your advice, although I appreciate that my question may have been asked before.

I'm in the process of having a garden office/workshop built and I need to determine the size of the cable that will be required to provide power. I will be hiring a qualified electrician to terminate, fit the internal wiring and sign-off, however I'm not ready for that yet. I would like to get services (i.e. power, data water) to the location before the building phase starts.

The run from the house is approximately 35m, which consists of 20m underground, 8m clipped to external brick and the remaining distance would be indoor, clipped to brick. The likely load would not be expected to exceed 10kw, i.e. there will be no large loads such as motors or heaters. The house is serviced by a TNS supply, which I understand means that I can extend the earth to the new building using 3 core SWA without the need to mess about with providing local earth via ground rods. However to meet regs, the SWA earth must be 10mm2. The building will be made from brick, and the incoming water supply pipe will be MDPE, so no bonding required for this.From my calculations, 10mm2 would be an ample size to cater for the load.

So, before I purchase a 50m reel of 3core 10mm2 SWA, would someone be able to confirm that my thinking is sound and I have missed anything?

Many thanks in advance,

Matt.

 

[JohnW2]

... The likely load would not be expected to exceed 10kw, i.e. there will be no large loads such as motors or heaters.

10 kW is quite a lot (e.g. 5 fan heaters). What are these anticipated loads which don't involve motors or heaters that would/could add up to 10kW?

To put this into perspective, the average electricity consumption (over 24h) in my entire (very large) house is around 2 kW, and the instantaneous peaks very rarely exceed 8-9 kW.

Kind Regards, John

 

[EFLImpudence]

Unless you are positive it is still TN-S, all supplies should be considered as PME. I.e TN-C-S with Multiple Earths.

It is any Bonding Conductor that would have be 10mm² for PME supplies.
The Earthing Conductor which for your outbuilding is just a Circuit Protective Conductor would probably not need to be that big.

 

[Notch7]

Online calculators are a good guide.

The answer though is get your electrician to specify.

I buy SWA from

https://quickbit.co.uk

 

[griffta]

Thanks John,

Apologies, my figure was based upon a guesstimate rather than actual numbers - so having done some working out, this is more likely to be an absolute max of 6kw. This account for computer, printer, lighting, kettle, and electric radiator for in the winter (I know I said there would be no heating loads!)

It sounds as though, from the other replies, that i'd be wise to get this specified with an electrician first.

Matt.

 

[JohnW2]

Thanks John, Apologies, my figure was based upon a guesstimate rather than actual numbers - so having done some working out, this is more likely to be an absolute max of 6kw. This account for computer, printer, lighting, kettle, and electric radiator for in the winter (I know I said there would be no heating loads!)

OK. The computer, printer and lighting should represent pretty trivial loads, an the kettle will only ever be on for a small number of minutes at a time, so is not a major issue - leaving just your electric radiator as the only really significant load. Given that, as I said, the total load in my large house very rarely exceeds 8-9kW, I think that even 6 kW for what you describe is probably 'over-generous'.

Having said that, you would be well-advised to install cable appreciably bigger than would be needed for your present estimated demand, since it is is so common for people to 'discover' the need for a lot more electricity in their outhouses a year or three down the road. So ....

It sounds as though, from the other replies, that i'd be wise to get this specified with an electrician first.

I'm sure that's the best approach, and you can then discuss with the election the true nature of your current needs and possible provision for future ones!

Kind Regards, John

 

[ban-all-sheds]

Plus there's the fact that as Matt will be expecting the electrician to sign an EIC to say that he did the design, and to certify to Building Control that he was responsible for the work, honesty and common decency say that Matt must not ask an electrician to make those declarations if he didn't actually do those things.

 

[oldbutnotdead]

Plus there's the fact that as Matt will be expecting the electrician to sign an EIC to say that he did the design, and to certify to Building Control that he was responsible for the work, honesty and common decency say that Matt must not ask an electrician to make those declarations if he didn't actually do those things.

What he said- by all means use online guides about volt drop, CCC etc to give you a clue on the cost of materials but at the end of the day it is up to the electrician you engage to design the system and specify cable sizes and installation methods. Most electricians will be happy for you to dig the trench to their specs, make sure you take pics to show the path of the trench and what you've used for mechanical protection of the cable (a layer of bricks is good). And do oversize the cable (or rather indicate to your chosen electrician that you want to be able to run 10 kw in the shed)- on that run the extra cost of the cable will become insignificant next to the cost of digging the trench and doing all the other stuff required

 

[ban-all-sheds]

And do oversize the cable (or rather indicate to your chosen electrician that you want to be able to run 10 kw in the shed)

Or oversize it slightly (to give a bit of headroom), but install underground ducts (separate power and data) so that replacing/augmenting it if needs change significantly is fairly easy.

 

[ban-all-sheds]

Most electricians will be happy for you to dig the trench to their specs

Often said, and I'm one who has said it.

I'd be interested to know if it should really be "All electricians...." - would love to hear from anybody who is an electrician who, or has ever encountered an electrician who, loves nothing more than digging trenches.

 

[JohnW2]

True, but some electricians come with a 'mate' who is an expert at digging trenches as well as making tea and carrying heavy rolls of cable!

 

[ban-all-sheds]

True.

Plus if I were an electrician, and could get my customer to pay for it, I might enjoy playing with a mini-digger. But spadework? NFC. However yes, my position is informed by living on a great thick layer of London clay.