Connecting Workshop with SWA Steel Armoured Cable


Postby EtheAv8r » Wed Jun 02, 2010 1:27 pm

I need to run a new power supply to an outbuilding that I will be using for a hobby workshop.

The existing Consumer unit for the house is full.

I was planning to use 10.0mm 3 core SWA XLPE insulated Steel Armoured Cable and the run from the incoming supply in the garage to the terminating position in the workshop is 31 meters of SWA.

One electrician has advised that I only need 2 core SWA as I will need to have an earth rod installed at the workshop end, but the other two, although also stating the earth rod requirement, specified 3 core SWA, so I am unsure on this point.

I plan to us a small RCD consumer unit for lights and ring mains to terminate the SWA in the workshop, but do not know what it is I need to make the connection from the incoming mains supply to the start of the SWA cable run.

Guidance please, on what it is I should be buying/budgeting for this connection.

I will employ an electrician to advise/supervise and make the actual connections, but feel I am fully able to undertake the actual installation to an approved design myself, and therefore am compiling the shopping list, which I can also use to compare with what an electrician says I need. I have had 3 wildly different quotes for this job and need to be able to judge them with a degree of confidence.
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Postby ericmark » Thu Jun 03, 2010 10:52 am

The statement “run from the incoming supply in the garage” makes me wonder if you intend to protect the supply end? It would need protecting as main DNO fuse is likely between 60A and 100A and you cable is rated more like 45A.

The method of supply would alter what would be needed where it terminates. I have in the past feed a new consumer unit from a large MCB in existing consumer unit so DNO do not have to fit an isolator. Remember under Part P the LABC has to sanction all you do so no cutting corners or DNO seals.

How much power do you need? There is a huge difference in what is needed as one goes from 13A to 32A and then again above 32A and your question without that knowledge can't really be answered.

I would agree if using TT then two core is enough however the IET (Was IEE) do a series of papers called wiring matters Google "theiet.org/publishing/wiring-regulations/mag/index.cfm?" In Issue 16 Autumn 2005 Electrical installations outdoors: a supply to a detached outbuilding gave very good instructions on pros and cons.

This is of course outdated (Before BS7671:2008 and RCD requirements) but still very useful.

If you can give more information as to what you want then we may be able to advise further. Unless in Scotland Part P needs considering it can cost more to DIY than to have work done by a scheme member electrician.
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Postby EtheAv8r » Thu Jun 03, 2010 9:47 pm

Ericmark, thank you for your response. I will attempt to answer your questions:

Yes I did intend to protect the supply end (as this will protect the SWA cable whereas the consumer unit will protect the Workshop circuits I believe), sorry I thought this was a mandatory requirement (and if not just plain common sense to have one) and so did not specify it, but seek guidance on just what ‘box’ this connection should be.

There is no space in the House Consumer Unit so there is no option to feed from a large MCB it that, it will have to be a new connection from the two wire feed from the main RCCB between the meter and the existing Consumer Unit (Crabtree 80A RCCB 30mA trip).

How much power do I need? Sorry I meant to include this in my original post but the fingers forgot that paragraph! My biggest power tools are rated at 2000w, and I would not normally expect to be running more than one at a time. Maximum power consumption is not expected to exceed 5Kw, therefore this has been doubled to 10Kw which according to the online SWA Cable calculators will require 6mm SWA, so I added another safety factor of going up one more cable size to arrive at the 10mm SWA cable specification.

The actual measured SWA cable total run is 29m but I have added another 3m (10%) as contingency to make the 32 meters, but it will in fact be a little less than the full 32m.

The Consumer Unit should allow for at least 3 power and 2 lighting circuits, with hopefully a spare slot or two to allow potential future expansion. I do appreciate that I could ‘get away’ with just a single Power and Lighting pair of circuits (i.e. a typical “Garage Consumer Unit”), but prefer to spend a bit more for greater flexibility.

If it is indeed cheaper to have all the work done by a scheme member electrician, then that is surprising as I thought it would take a lot less time to inspect and test, than to actually install and test, and I know my installation will be neater and more carefully routed. (I fly a two seat light aircraft than I completely built myself – a lot more complex that inter-connecting simple 3 wire cable to make the workshop circuits – and highly safety critical!). However this is something for me to discuss with my contractor, when I find one, and if it is cheaper to have him do it all, then I will!

I am not in Scotland, I am in Cambridgeshire. I hope this provides all the detail you need?

Many thanks for your help and patience.
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Postby ericmark » Fri Jun 04, 2010 1:33 am

With the power going through the supply cables one would not normally double up on cables in holes and a connector block 11497 3-Way 100A SP Service giving screwfix references as easy to work around that site. To fit one of these means you need the DNO either to fit an isolator or at least remove and refit main fuse which will cost. Then you need tails 95832 to a second consumer unit something like this 61591 so you have metal to gland SWA in to.

Now if you had a little larger unit like this 26018 then you could move some items in original consumer unit to new one therefore leaving a free way in original consumer unit and removing the need for connector block and also need for DNO involvement.
Same type of consumer unit other end would also help in glanding cable.

As to LABC charges, I am lead to understand there have been some changes, but it was £100 for each £2000 of work plus vat. So at least in Wales we had a £100 bill to council just to comply with Part P, which I agree is daft, and we all hope the new government does something about these rules, but not holding my breath.

So if you knock the £100 of the electrician’s bill it becomes hardly worth the effort to DIY.

Depending on make of consumer unit 45A to 50A is likely biggest MCB you can fit giving around the 10Kw you want however not sure about idea of going TT there are problems both ways and I would make a risk assessment with consideration of other buildings with electric supplies. To fit an earth rod is all very well but you would need to measure the rod and give council readings before they should allow you to switch on which is a real pain. Electricians will normally cheat and connect up supply then use an earth loop impedance meter to test rod which is easier than using a proper earth rod meter plus the earth rod meter needs 30 meters to lay out test probes.

It’s not just that the LABC test all you do but they want to check it at each stage of the work so you just can’t get on with the job. Testing was all part of the LABC job although they did not have to give you the results but now they can charge for all tests and likely they will employ an electrician to do tests then charge you.

Then they wonder why people break the law. However selling a house without the completion certificates is also a problem so unless you intend staying put until you die then not really an option.

I am an ex-electrician due to an industrial accident and even I have problems with LABC and they wanted proof I knew what I was doing and would not accept my sons C&G2391 which was the best qualification but would accept my degree in electrical and electronic engineering which was really silly.

One way is to make it into a disabled loo for you Mother first then you don’t have to pay if to help the disabled then change to what you want!

Being fair some councils are very helpful but others see it as a pain in the neck and seem to do anything to stop you DIY’ing.

OK I am lucky I have a full 17th Edition test set in my garage cost my son around £750 but even testing the RCD to ensure it trips within 40ms is impossible without the expensive meters.

Best option is to get an electrician who will allow you to do donkey work. Dig trench etc. He has to be in control so you do what he says and he decides what you do and what he does but lets face it electricians don’t like digging holes so very likely you can come to an arrangement but do it first before you touch anything.
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Postby EtheAv8r » Fri Jun 04, 2010 9:30 am

ericmark - thank you so very much for your detailed explaination, very helpful indeed. You have provided excellent insite so that at least I understand, and can talk to an electrician at a sensible level.

In particular "Now if you had a little larger unit like this 26018 then you could move some items in original consumer unit to new one therefore leaving a free way in original consumer unit and removing the need for connector block and also need for DNO involvement.
Same type of consumer unit other end would also help in glanding cable." seems like an excellent way forward. (I don't know what DNO stands for, but is sounds expensive!).

Seems like the budget for this 'little' project just got bust again!

Thank you once again. 01:30 AM - don't you sleep?
EtheAv8r
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Postby ericmark » Fri Jun 04, 2010 5:17 pm

DNO is the supplier can't remember what letters stand for but it separates the supplier from the billing agent. So where I live the DNO is Scottish Power even if I pay my bill with British Gas.

To reduce bill you could go for a 13A supply. Using a fuse connection unit (FCU) metal clad so you can gland you could take a spur from ring main even inside the consumer unit. At other end into double socket box (gives more room) again metal clad and then radial to other sockets and use a switched FCU with 3A fuse for lights.

If consumer unit does not have RCD then use a RCD FCU.

Dinner so must go
ericmark
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Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.


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