Electric supply to a shed

[Brett Jackson]

Hi,

I'm planning on building myself a large shed at the bottom of the garden. I need to understand how I provide an electricity supply within regs.

I've broken my query down in to a few categories. I've check on the FAQ and googled previous posts and not found quite what I'm after. I guess I'm after some expert reassurance that I'm thinking along the right lines too.

My consumer unit is to the front of my house, and I've just got a new driveway - so I need to avoid any digging to the side of the house, meaning a section of the run will be internal, the rest will be 3 core SWA.

RCD: I have a few options here, and I need to understand which is best.. a) Use existing RCD in consumer unit, put some breakers in shed b) Install new RCD inside house via 30a breaker in existing consumer unit, again breakers in shed c) RCD and small consumer unit in shed with connection to a 30a breaker in existing consumer unit

Cable: As I need to run a section inside the house, I need to understand a) if this is OK, or whether the SWA should run all the way to the fuse board, and b) what cable size (as I want 30a supply over 30m I understand 6mm T&E is required)?

Assuming I can do this, then can I run the T&E in to one of these

mounted on the outside wall to meet the SWA? Can I add a spur with an external socket at this point?

Where the SWA leaves the house I understand it needs to be protected. What's the best way to do this?

I also understand that the depth at which the cable is run is fairly discresionary. I'm thinking 20-30cm under the patio (patio yet to be laid), then 50cm under the lawn.

Paperwork: I've not done any electrical work since December 03, so I've no experience with the new Part P regs. What testing and paperwork do I need to get done so it's all legit? Are there any indicative/ball park costs?

Many thanks for your help... Brett

[John Rumm]

Have you read through:

there are still bits unanswered then post about them here, and we can update the document to add clarifications where required.

Do you need 3 core? (two plus armour is usually enough)

See wiki article on this...

You need to look at voltage drop for you maximum design current for the whole length of the circuit - including allowing enough for the wiring in the outbuilding.

Yes. A short section on conduit would probably be easiest and an external socket. Note however doing this may limit your RCD positioning choices. See the wiki section on design. You could also use a external socket with integrated RCD see

Where the SWA leaves the house I understand it needs to be protected.

Protected in what way?

Under a patio there is little to no chance that it will be dug over. Hence it can be quite shallow. Much the same probably applies under a lawn etc. You need to use common sense and cable warning tape!

You would have to speak to your building control department and ask. You may need to argue your case since they ought not charge any more than their normal building notice fee based on the commercial value of the work being done. They should provide a completion certificate when done.

[Dave]

Brett Jackson wrote: snip

I am no expert on this, but I would go for option c). That way, you can keep the shed lighting on after a fault, if you wire them up before the RCD. My house is wired up so that the lights go out when the RCD trips. It's a PITA.

Dave

[Brett Jackson]

No, but I will do. I had just seen the old FAQ before, the wiki is new to me.

I've read that requirements may vary in different parts of the country! I thought it best to be safe and go for 3 core. If 2 core is definately sufficient then I'll use that.

I've measured the voltage in my house a few times before, it's consistently been 248 volts (there's a substation 25m from my house). I calculated the voltage drop on the TLC-direct web site and I'm well within the 230v +/-

10%.

Sorry, I wasn't very clear. I was referring to the bit of SWA between the junction box and the ground. I'm assuming it may need some extra protection above ground?

Yes, I was looking at the way it's charged and it seems to depend on the cost of the job as done by a professional. This would vary wildly based on the amount of work falling outside the regs I did myself (e.g. dig trench, clear route/drill holes for cabling internally etc). I imagine I could do enough myself to get the cost of a professional down below £200, if not less.

Thanks for your help. Brett

[John Rumm]

There is a fair bit of electrical stuff on there now. Much of it has been peer reviewed by the more electrically oriented folks on the group, so should be reasonably reliable by now - but if in doubt its worth checking on the group.

Two core should be fine for any size you are likely to need in a domestic environment.

The actual voltage at your house is not really relevant in this case. You need to designed to keep your voltage *drop* at 4% of nominal supply or less. That gives you 9.2V to play with.

So you mentioned 30m and a design load of 30A. So say you used 6mm^2 cable, that would be a drop of 7.9 mV/A/m = 0.0079 * 30 * 30 = 7.11V, which is OK and leave a little for the wiring in the outbuilding.

The required limits on the supplier are actually 230V +10%/-4%

Not really, unless it is a in a place where it is likely to get hit repeatedly. SWA is robust and not affected by UV exposure.

Sounds a bit on the low side - you will probably spend a good proportion of that on materials even if you DIY the whole job.

[Jim Alexander]

Not explicitly mentioned in your OP or in any reply though it is in the wiki, is earthing, which is a big topic in itself. The 2 or 3 core debate has run before, 2 core with the armour as an exported earth or 3 core using a core as an exported earth. Not getting into that argument right now but when I ran my shed cable I did as you suggested and used a junction box, possibly the very same TLC box to transition from T/E to SWA. If I did the job again I would run SWA all the way to the consumer unit to eliminate an unecessary outdoors connection. Yes as the wiki points out the SWA is a bit unweildy but less so stripped back.

If you particularly want an outside socket at the junction box you could run an exterior RCD socket as a spur from a ring.

Jim A

[David Hansen]

On Sun, 14 Oct 2007 19:24:53 +0100 someone who may be "Brett Jackson" wrote this:-

Take a look at the electricity supply to buildings you know. If they are in towns then it is highly likely that it will consist of a cable with one or two cores and armour (providing the protective conductor function). If there is only one core then the neutral and protective conductor are combined, which is not something you should experiment with inside a building (it is something for those who know what they are doing and there are a number of strict regulations for installing such wiring inside buildings).

I don't suggest that you (or anyone else) dismantle the supply cable termination in such buildings, but that is what you would find if you did. If you looked in a library you could probably find a book with drawings/photographs of what is inside. There may even be something on this new-fangled Interweb thingy.

[Brett Jackson]

Thanks all (esp. John) for your help.

Just one remaining question now...

I need to run some T&E under the stairs between the hall and kitchen. The difficulty is that it's an external cupboard (old coal storage I think).

Do I need to run the T&E through trunking inside the cupboard, or is it sufficient to use cable clips?

I appreciate I could use SWA all the way, but I have some fairly intricate bends in the kitchen that I'm going to struggle getting the 6mm T&E through.

Brett

[John Rumm]

Well ask yourself if it needs extra protection.... If it is not going to be exposed to sunlight, and its not going to be physically knocked or otherwise abused then it ought to be ok clipped to the surface.

[jim]

in addition to other posters' advice, take a look at

is from issue 16/summer 2005 of Wiring Matters, an advice leaflet issued by the IEE as was - now the IET - every few months. They are all easily readable, or perhaps I mean that if you find they don't mean a thing to you then you might do infinitely better asking someone else to do the job for you. But your post doesn't read like that ;-) bash on!

It is quite possible you will pick up on several other relevant aspects by looking at articles in other issues.

HTH

[Richard Perkin]

"Jim Alexander" wrote in news:FrIQi.351504$ [e-mail]:

Indeed. That's what the contractor who did the electrical work for me on my (almost finished, honest!) last renovation, ie our current house. The sub-main to the shed was taken in SWA cable from the existing consumer unit just inside the front door, through the external wall by the front door, a short distance along the front of the house (under the front step, which was being replaced in stone at the time), back into the house and under a suspended floor, then into a channel cut in the concrete floor at the rear of the house, out by the back door, then (simplifying a little...) down the garden to a 4 way consumer unit in the shed.

Apart from the well-known unpleasantness of using an angle grinder indoors when cutting the channel in the concrete floor, generally it was pretty straightforward, if hard work.

Kind regards

[Brett Jackson]

> This is from issue 16/summer 2005 of Wiring Matters, an advice leaflet

That looks like a useful source of info, thanks.

I think I have just about everything I need now. Last remaining uncertainty is how I run the cable behind the skirting in my hall - looks like inside steel conduit is the only acceptable way...

Brett

[John Rumm]

Yup, behind skirting is not a recognised route. You could opt for deep burial >50mm from either side, or you could find an alternative route like up a corner and along just below ceiling level etc.

If there is an outside wall, another option is to drill through to the outside and run SWA clipped direct to the outside.

Channel in the floor is another option (solid floor), or under if its a suspended one.