SWA cable for outbuilding

Discussion in 'Electricians' Talk' started by AshleyL, Oct 12, 2019.

  1. AshleyL

    AshleyL New Member

    I’m about to build a new workshop in my garden and need to run an swa for power. My house is on a TT set up (earth rod) so that means my workshop will also be a TT.

    the run of cable will be 28m from my incoming main to the workshop fuse board, I’m going to use 10mm2 swa minimum but wondering is it worth going to 16mm2 ( the only problem is there is hell of a jump in price) in the workshop will be the following radials

    lights (all LED)
    Garage door
    Welding table supply (sockets)
    20 amp welder supply
    20 amp compressor supply

    Biggest individual tools I have are dewalt table saw 1800w dewalt chop saw 1675w and hopping to add band saw and pillar drill in the future but all these are on 13amp sockets and it will 90% of the time only be me there so all these will be run on their own not all at once

    so with that info I’m hoping someone can advice if 10mm is fine or if I should go to 16mm.

    next question is...... as it is a TT system obviously I only need a 2core cable and this saves money but my sensible head is telling me to spend the money and run 3 core even though I can’t use the 3rd core as a CPC now but by having it there it future profs me if my mains is ever upgraded by the supplier?? Or if that is the case is it still fine to leave it as a TT even if they do upgrade me??

    I hope all this makes sense and thankyou now for any replies and advice you give.

    P.s I’m not an electrician so if all this sounds wrong that’s why
  2. Bazza

    Bazza Screwfix Select

    A large number of things to consider. my first two are
    1. How were you planning to attach power to the house end of the armoured cable?
    2. Are you aware that the work that you describe is notifiable?Do you have the skills/knowledge/calibrated test equipment sufficient to convince LABC that you can do this as a DIY project?
  3. AshleyL

    AshleyL New Member

    I want it to be a sub main, pull mains fuse, new tails in to 2 100amp double pole switches one switch to house consumer unit and other switch for swa to the outbuilding. And by adding the 2 switches this means any mains work needed in the future on either the house or swa then it can be done without pulling the mains fuse.

    yes I’m fully aware that it’s notifiable work my plan is to do all the wiring myself in the workshop and install the consumer unit but have a part P sparky to check over my work and then do the mains connections.
  4. Bazza

    Bazza Screwfix Select

    It doesn’t work like that. Your electrician will need to certify that he/she has
    DESIGNED, INSTALLED & TESTED the installation. Also they can only notify work THEY have done. get your sparky on board first and agree what work you can do under their guidance.

    Other points. You are not allowed to pull main supply fuses. Some electricians in some regions can do this, but there are special requirements.
    plus, where is the current limit for the SWA.?
    There should be a switch fuse, at least at the supply end. And other things too, it’s a TT. are you aware of the protection and isolation requirements for that supply type??
  5. AshleyL

    AshleyL New Member

    I totally understand what your saying but me and you both know that what I’m on about happens on a day to day basis and if everything was done by the book then no one would get anywhere.

    can you please advise on the first post? Regarding cable size? And if 2 or 3 core is better?
  6. rogerk101

    rogerk101 Screwfix Select

    I am doing almost exactly as you describe, only I laid a 32m length of 2 core 25mm2 SWA.
    I opted for the more expensive 25mm2 cable so that my garage to granny flat is completely future proof, in case I choose to install a heat pump or some other futuristic electricity-based heating system. The incremental cost of oversizing your SWA cable now is negligible when compared to the cost of digging the trench again to upgrade it later. (I know this from experience as the SWA I ran about 10 years ago was perfectly OK for my garage, but not OK for my granny-flat!)
    My electrician advised me to use 2 core and an earthing rod rather than 3 core without earthing rod.
  7. AshleyL

    AshleyL New Member

    that makes sense, is your electrician doing all the work? Or are you doing some to?
  8. Tony Goddard

    Tony Goddard Well-Known Member

    Bazza is giving sound advice here, of course those of us who are professionals know this kind of thing goes on, we spend many hours putting right DIY efforts.

    From a technical standpoint 16mm will be best for what you are describing. As it is a TT supply the shed will have to be it's own TT supply with it's own electrode. Even if you get upgraded to PME you will still want to keep the outbuilding as TT

    Everything must be protected by 30mA RCDs in the house and the outbuilding

    A typical connection to the main would have the meter tails running to a Henley block (a splitter) a pair of tails from there to the house CU, another pair of tails from the Henley to a switched fuse, with a 60A cartridge fuse to protect the SWA.

    At the shed end the SWA connected to a suitable CU but the earth (armour) not connected, instead the earth goes to a stake.

    As Bazza says pulling the service fuse (cut out) is allowed by some DNO's and not others, I live and work in a region where it is allowed, but only those of us who are registered with the DNO and have the re-sealing tags should do it, we then phone in the temporary seal number - The rights and wrongs of breaking the seal apart, unless you know what you are doing you don't want to pull that fuse, older ones have a tendency to disintegrate, it is also possible to trigger a nasty arc situation - If something goes wrong and you live to tell the tail the DNO will not be happy.

    I would make contact with your chosen electrician first, run through you plans with him, make sure he is happy with what you are doing and arrange for him to inspect the job as it progresses and carry out the required tests, he will also make that final connection.
    seneca likes this.
  9. AshleyL

    AshleyL New Member

    I knew Henley blocks would be used, my fault I thought I had put that in my explanation. Also yes I agree and understand that rcds are needed, my house already has a dual rcd board and that has been checked and tested recently. I plan on having a full 18th edition RCBO board in the workshop.

    I will 100% percent be getting a electrician in on this project so please don’t think I’m trying to bodge this together myself. the reason I’m asking so much is because before he comes I still have to dig a trench and bury the cable long before any connections are made so I just want advice on the cable size.
  10. Tony Goddard

    Tony Goddard Well-Known Member

    16mm is what I have and laid on to a caretakers workshop at a school recently, an outbuilding sone 20+ meters from the school. Once you start running compressors, welders etc you want the cable belt and braces to take anything you could conceive.

    I have 50x50 galvanised trunking running all round the top of my garage walls, from this the various sockets and switches are fitted via a 20mm pvc conduit drop. This has the advantage of making modifications to the layout much easier and making the wiring much neater and more contained.
  11. peter palmer

    peter palmer Super Member

    A 60A fuse wont be doing much protecting on a TT submain.
  12. Tony Goddard

    Tony Goddard Well-Known Member

    You are quite right Peter, not thinking for a moment there!, 100mA time delayed RCD on the front end at the house would be best + MCB or fuse to protect the cable, either 30mA RCD and MCBs at the shed or a main switch and 3mA RCBo's.
  13. Bazza

    Bazza Screwfix Select

    If you want to do the hard bit first, then dig your trench, using the correct depth, base, warning tape, etc.
    Do not decide on cable size until you have consulted your electrician on site. Instead, lay twin-walled duct (aka civil Rigiduct)
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    it has a draw string, so you can pull the appropriate cable when you have proper advice.

    I’m sorry, but I am not prepared to provide detailed design consultancy on a forum.
    so, as they say in Dragons’ Den
    “I’m Out”
    Jja90 and Dam0n like this.

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