Armoured cable to shed - advise please

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Hi, Apologies for an often asked question but I ve seen nothing specific so I thought I'd ask again.

I want to run in a trench armoured cable to a fairly large shed that is 25 metres from house I will put it in marked ducting. It will supply 2 strip lights and maybe 6 sockets. I dont forsee it running anything heavier than a kettle, possibly a pressure washer, hand power tools.

I will have an electrician connect and wire bothe ends but i want to get the cable in before they are in.

Thanks in anticpation
 
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If you are going to have an electrician involved then I would strongly advise talking to them upfront. There can be differences of opinion and it is the electrician who will have to sign off the work.

Having said that personally I think it's a false economy to put in anything smaller than 10mm 3-core.

Why 10mm? because that is the minimum size for main bonding conductors in a domestic-sized PME installation. So if you have (now or later) services that need bonding in the outbuilding then having a 10mm earth conductor means you can just bond them and be done with it. If your cable doesn't have a 10mm earth conductor then you may be faced with the prospect of having to create an independent TT earthing system in the outbuilding.

It's also sufficiently large that you can run a heater in your outbuilding in winter at the same time as running a fairly large power tool without having to worry about total loading.

Yes putting in a bigger cable does cost a bit more but compared to the overall cost of the project (digging a trench, getting an electrician in to connect and certify, etc) the extra cost is small. Better to go a bit overkill now than to be cursing in a few years time that you put in a cable at was too small.
 
I will have an electrician connect and wire bothe ends but i want to get the cable in before they are in.
Then you need to ask her what size cable to use, since it will she who signs the EIC to say she designed, installed and tested it, and she who notifies Building Control to say that she did all of the work and that it complied with the Building Regulations.

You also need to get her to tell you how deep the trench should be, whether any bedding material is needed, and if you intend to backfill it before she attends, where to put the marker tape and what she will want to see in the way of photos to show it was all done to spec.

I would advise thinking about "what ifs" for the shed - it may only be a small load now, but things might change, and a bigger cable now could save digging later on - often the extra cost of a larger cable is negligible compared to the overall cost of the job.

There is a case to be made for installing a pair of ducts - one for the power cable, one for phone/data, so that future changes can be done by pulling a new or additional cable through.

And talking of costs, your electrician will be able to get the cable cheaper than you.
 
I am literally in the process of doing just this, as in, I'm halfway through digging the trench!
I spent some time reading about this...... and it appears there actually isn't any particular regs on the depth etc etc.
From most that I read a depth of something like 60cm would be fine. Lay sand at the bottom of the trench, then the cable in it, then sand over the top with the warning tape. Then backfill the trench.
10mm was the size I was looking at for the reasons Plugwash gave, bonding in future.
 
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Thank you Plugwash that was exactly the information and advice that I was looking for. Sensible and non judgmental.
I will proceed with the 10mm cable - thanks again.
 
If you don't have your electrician on board yet then get them involved for the reasons b-a-s mentioned. Your method is sound, ideally you'd put some solid protection above the sand layer (bricks, concrete blocks, anything that'll reduce the odds of a random pickaxe or similar going through the thing in the future). And a big agree with the ducting for comms/data/alarm/whatever, cost is buttons compared to the labour of digging the trench.

Take a load of photos of the trench, do some measurements so you know where the cable is, maybe put some marker slabs at ground level above the ends/any bends in the trench again for future reference.

If your house is decorated etc then be aware that SWA isn't easy to thread around the place. One option is to terminate the SWA outside the house (on the outside wall in a proper enclosure) and use 10mm T & E to get from the CU to the SWA.

Another thing to consider is how to power the shed. Ideally you don't want the garage feed sharing an RCD with the house circuits- have a chat with your electrician about Henley blocks and standalone MCB at the house end and 'garage' CU at the shed end ideally with RCBOs. Bonus with this- if you do something daft in the shed it won't drop the RCD in the house & annoy rest of the family, bonus 2 (with RCBOs) if you trip the power circuit in the shed you'll still have light up there.
 
use 10mm T & E to get from the CU to the SWA.
although that would immediately mess up the PME bonding argument given further up the thread.
I'd have thought the most likely extraneous part in a shed would be the floor.
 
Leave it alone on the grounds that wood is actually not an exposed-conductive-part?
 
although that would immediately mess up the PME bonding argument given further up the thread.
I'd have thought the most likely extraneous part in a shed would be the floor.
Ooooh, good point- I'd forgotten about the E in T & E being smaller than the main conductors. You're quite right- either have to use 16mm T & E or run a 10mm E to the box. Electrician (who suggested the 10mm T & E to SWA option for my garage) didn't mention that issue at all
 
Ooooh, good point- I'd forgotten about the E in T & E being smaller than the main conductors. You're quite right- either have to use 16mm T & E or run a 10mm E to the box. Electrician (who suggested the 10mm T & E to SWA option for my garage) didn't mention that issue at all
Have we actually established whether this shed has any "extraneous-conductive-parts" (e.g. incoming metal water pipes, structural metal which is in contact with the ground etc.) that would need to be bonded?

[ Whilst some might argue that certain types of shed floor, if wet, could represent an "extraneous-conductive-part", I have never heard of anyone even thinking of finding a way of bonding the floor of a domestic shed!]

Kind Regards, John
 
I usually imagine the house is a lot bigger and the location of the shed is actually just outside the back door and wonder "what would be done then?".
Are you suggesting that the floor of the house would, by a similar argument, also 'need' bonding? If so, I suppose the difference it that house floors are rarely wet and in close contact with the ground.

However, there are situations in which one 'wonders'. One of my daughters lives in a Georgian cottage, whose kitchen floor consists of quarry tiles, with just sand between and below them, straight onto soil. After long wet periods, when the water table rises, waters sometimes actually rises up between the tiles. Particularly given that the house has a TN-C-S supply, it can be very easily argued that that floor should be bonded to it. Given the practicalities, I have tried not to think about that, and have gained a little comfort from the fact that the (bonded) metal water supply pipe enters through that floor, so that the soil in that area is to some extent 'bonded'.

Kind Regards, John
 

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