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Wiring a second workshop (Advice on consumer units and armoured cable)

Discussion in 'Electrics UK' started by Dave Grant, 2 Sep 2020.

  1. Dave Grant

    Dave Grant

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    I already have a small workshop that is wired to the main consumer unit in the house. In the small workshop I have consumer unit and it runs lighting and sockets for power tools, pillar drill, table saw etc.

    Now I am building a larger workshop down the bottom of the garden and will need to power this from the small workshop near the house, I did the main wiring myself and run armoured cable to small workshop and then got a sparky to wire to consumer units and certify and issue certificate.

    So I need to know:

    1. What consumer unit do I need to replace the one in small workshop so I can run armoured cable down to large workshop.
    2. What armoured cable do I need.
    3. Can I take consumer unit from small to large workshop, or should I just buy new

    To clarify: House (consumer unit) to Small Workshop (consumer unit), Small Workshop (consumer unit) to Large workshop (consumer unit).

    Have attached photos of current consumer units.

    I will be buying form https://www.cef.co.uk/ or https://www.tlc-direct.co.uk/

    Also looking for a sparky that can wire this and issue test certificate, I'm in Cambridge (CB25 area)

    Dave

    IMG_1629.jpeg IMG_1630.jpeg IMG_1631.jpeg IMG_1632.jpeg
     
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  3. Taylortwocities

    Taylortwocities

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    What are you planning to run in you workshop(s)?

    The feed from the house is 20amp, is that going to be enough?, If not, you'll need a big rethink.
    Second comment is that there is absolutely no point in having a circuit in workshop1 fused at 32amp, that can never ever in a million years do anything. The current can never be more than 20amp!
    Third. You have an RCD in that quality Excel board, what is the point of that? the circuit in the house is already RCD-protected! A fault in any of your workshops may trip the house RCD, probably defrosting the freezer (on the kitchen sockets circuit). That also needs rethink.

    Suggest you get an electrician now to advise and sort out the basic stuff.
     
  4. RF Lighting

    RF Lighting

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    As Taylor has mentioned, your existing setup is very badly designed and really needs to be properly rethought before you go any further.

    The existing supply to your outbuilding is very small and won’t really power much more than a couple of lights and a socket. There is no point having a consumer unit at all at the moment.

    What exactly are you planning on running in your two buildings? How long are the cable runs from the house to building one, and building one to building two?

    You mentioned you have a table saw. What type of lights do you have / plan to have in your buildings?
     
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  5. chivers67

    chivers67

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    Ditto above - what size Armour is that going into the Shed at present 2.5mm ?
     
  6. Dave Grant

    Dave Grant

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    I love when I unknowingly open a can of worms.

    This was wired in by a qualified electrician and test certificate issued by him.

    I'll get back to you all on questions above in the meantime thanks for replies.
     
  7. Dave Grant

    Dave Grant

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    It was a while ago when I done this so can't remember TBH
     
  8. AndyPRK

    AndyPRK

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    It’s usually written on the outside.

    2 RCds isn’t that bad.
     
  9. oldbutnotdead

    oldbutnotdead

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    If you use the Search facility (top right) to search this forum for Outbuilding Power you'll find the topic has been done to death.
    Precis- find an electrician, tell them what you want and let them design the scheme.
    They may or may not be happy with you doing the donkey work (digging big trench, clipping T & E or running conduit).
    Put the biggest cable in you're ever going to need (SWA isn't cheap but will be a small part of total cost of job)
    Don't power the outbuilding through an RCD in the house- really boring having to traipse in to reset it
    Do run comms ducting to the outbuilding ( WiFi is good but wired is better plus you can chuck alarm cables down it)
     
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  11. Notch7

    Notch7

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    If you have a woodworking workshop, like a saw of planer thicknesser allow for a fair bit of capacity for starting load.

    I'd be guessing 10mm cable would cover you and any future capacity, or maybe 6mm.... but you need sparky to do calcs- you can play around with an online voltage loss calculator to get some idea.

    You also need to decide on the earth, whether you use the armour or cps, which will determine if you need 2 or 3 core.
     
  12. JohnW2

    JohnW2

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    Yes, but first one needs an adequate supply. Unless it is an extremely long run, there's no point in having 10mm² cable to carry the OP's present 20A supply.

    Kind Regards, John
     
  13. muffking

    muffking

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    As a quick overview of the job I would:-
    -Change the 20A Wylex MCB for a 20A RCBO fed from a non-RCD terminal in the main board.
    -I wouldn't worry about discrimination between RCD's once the garage circuit is separated from the RCD in the main board.
    -Reduce the 32A MCB in garage board to a 20A MCB (possibly EXMB20).
    -Do a cable calc for the entire run to the large workshop to include both lengths, probably to find that 2.5mm, 4mm or even 6mm SWA will do the job depending on distance.
    -Then either run another garage board with 20 & 6A breakers, or just run the SWA in to a metal clad FCU and fuse it down for the lights depending on wether or not you want the option to isolate the power to the large workshop or not.
    -Also would need to check for any extraneous metalwork in both outbuilding to make sure the earthing and any bonding that may be required is adequate.

    I'd be circa £500, done in a day. That's with clipping the armoured cable above ground, although give the option of burying the cable or the customer digging a trench to spec. Although I don't cover Cambs :D
     
  14. Taylortwocities

    Taylortwocities

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    @muffking

    Surely not for most of those!

    The very first thing is, as you say
    Do a cable calc for the entire run to the large workshop to include both lengths, probably to find that 2.5mm, 4mm or even 6mm SWA will do the job depending on distance.
    I'd leave probably out of that!
    Any changing of anything hangs on that decision. The MCB at the house end will probably need to be bigger and that may negate changes to the downstream devices as a result
    - but note that achieving discrimination between MCBs is very difficult unless there is at least a 2:1 ration between their trip characteristics. Thats why a fuse at the source end is preferred.

    re
    Change the 20A Wylex MCB for a 20A RCBO fed from a non-RCD terminal in the main board
    I agree that the CPD for the feed should not put the rest of the house at risk. But it is an MCB in a non-RCD way that is required(or better a fuse carrier in the CU).
    It is SWA all the way and does not require an RCD. There's an RCD in shed1, and that can provide protection for both outbuildings.
     
  15. AndyPRK

    AndyPRK

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  16. muffking

    muffking

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    Completely agree. It was more a general idea for the OP without seeing the job.
    The calcs will confirm what is really required as it's difficult to get a feel for the job with limited info to go on.

    Re my suggestion of feeding from the main board via an RCBO. I am more of a belt & braces kind of guy. This came in to play only last week when I got a call out to domestic garage tripping the RCD in the customers main board that it was fed from and disconnecting the power to that half of the board. It was SWA all the way from the MCB to a garage board. Testing with both ends disconnected revealed low IR between L-E, but as half of the run had been built on by an extension I can only assume that it was damaged underground. If it was on it's own RCD then the they could have had a hot dinner the night before :D
     
  17. EFLImpudence

    EFLImpudence

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    I think it is accepted that such circuits should not be fed from RCDs if possible - SWA is used for that reason.
     
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