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Wiring Design for Cabin

Discussion in 'Electrics UK' started by Dave Lockyer, 28 Mar 2021.

  1. Dave Lockyer

    Dave Lockyer

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    Hi,

    I'm currently designing the wiring for a garden cabin which I'm planning to self instal and employ a sparky to complete the connection to the main CU in the house and test. Here's what I'm planning, I'd be interested to hear is there are any obvious changes that might need to be made :

    From House to Cabin :
    30m XLPE 3 x 10mm armoured cable buried at a depth of 600mm and with a hazard layer 300mm deep.

    At Cabin :
    5 Way RCD consumer unit with a dedicated earth rod into ground.

    From Consumer Unit :
    1 x 6A feed to 3 x room lights via a 1 way 3 gang switch and 1mm 3 core insulated cable
    1 x 6A feed to 1 x separate internal light/extractor fan via single gand switch and 1mm 3 core insulated cable
    1 x 20A feed to 2 x double sockets (Max draw = 2KW) via 2.5mm 3 core insulated cable
    1 x 32A feed to 3 x double sockets for kitchenette appliances (Max draw = 7.5KW) via 2.5mm 3 core insulated cable
    1 x 32A feed to water heater (5.5KW) via (Fused 30A?) junction box/switch and 2.5mm 3 core insulated cable

    I'm planning on using galvanised counduit to channel the cable.

    My qustions are :
    Bearing in mind the loss over 30M will the proposed cabling carry the load OK?
    The electrician will advise as to how best to connect the SWA to the consumer box at the house. I have a (rather old) RCD consumer unit as well as an additional 100A RCD which feeds our oven and shower. I guess I may need to replace either the main panel or the additional box to provide a 100A option for the cabin?

    Thanks...
     
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  3. davelx

    davelx

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    Not unless it's a Ring Final - 2.5mm² cable is only rated to 27A max and possible less depending on how/where it is installed. If it's a radial you need at least 4mm² cable or to downsize the MCB.

    Same goes for your water heater - 32A MCB requires at least 4mm² cable - possible bigger depending on where it (the cable) is running.

    The whole job would also be better using RCBOs rather than a single RCD.

    Lots of other things need to be looked at also - you need to design your submain properly, which requires some calculations. Also numerous other factors of which you are probably not aware. This is all notifiable work under the Building Regs.

    You need to get your electrician on board now and allow him to do the design and specify the hardware needed. You and he can then agree how much of the work he is willing to let you do yourself, effectively under his supervision.
     
  4. ETAF

    ETAF

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    Any outside lights ?
     
  5. Notch7

    Notch7

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    I'm guessing you have an electrician you know well who will be happy to do this?

    What is the earthing coming into main the house?
     
  6. Dave Lockyer

    Dave Lockyer

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    Thinking of possibly using one of the switched from the 3 gang to feed an outside light under the veranda.

    I haven't found an electrician yet. Might get a quote to do the whole instal but I'm guessing it'll add significant costs to a tight budget. The SWA carries and earth with it which will connect at the house where there is an earth rod. I'm also planning to add a dedicated rod at the cabin end for extra safety.
     
  7. flameport

    flameport

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    No.
    If the existing supply is TN, earth rod not required.
    If it's TT then an earth rod should be installed, but if that's the case, the SWA should be 2 core, not 3.
    If you really want 5 circuits, a main switch consumer unit with 5x RBCOS. Earth rod not required.

    A single 6A circuit will be more than plenty for all of the lighting and extractor.

    A 20A 2.5mm² circuit for 2 sockets is ok.
    If you want a 32A circuit for just 3 sockets, it's 4mm² cable as a minimum, unless you install an entirely pointless ring in 2.5mm²

    if what's listed is all of the socket outlets (5) then a single circuit for all of them would be plenty.

    A 5.5kW water heater is presumably an instant heat on demand affair - far from ideal, a stored water unit at 2kW or 3kW would be far better.

    Not a problem with 10mm² over 30m.
     
  8. Dave Lockyer

    Dave Lockyer

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    Sorry - the 32A ring is for 3 x DOUBLE sockets so that's 5 x Double sockets in total with a max load of 9.6KW which is why I split them. Does that mean I can stick with 2.5mm or do I need to jump to 4mm?
     
  9. Dave Lockyer

    Dave Lockyer

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    Good point... I was trying to save some energy in hot water storage costs.
     
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  11. flameport

    flameport

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    Exactly what do you expect to have plugged in that is 9.6kW for any length of time?
    If you must have a 32A ring, it's one for all the sockets.

    If you are really going to use that and add substantial extra cost, you will need to use insulated singles, not T&E cables.
     
  12. Dave Lockyer

    Dave Lockyer

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    It's unlikely to ever reach that. Was including appliances for a kitchenette area (MWave/Electric Hob/Kettle/Toaster). If I go for a 32A single ring for all the sockets is 2.5mm adequate or will I need 4mm cable...
     
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  13. Dave Lockyer

    Dave Lockyer

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    I suspected there may be some extra requirement ont he cables if I use galvanised conduit. I may opt for simple plastic conduit and in the process save a bit of money. Most of the cabling will be run under the (wooden flooring) where there will be a 40mm cavity between the boarding and the (DPM isolated) slab. There will then be lengths of conduit from the floor to the sockets/switches/lights etc...
     
  14. JohnW2

    JohnW2

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    That presumably depends crucially on intended usage of the cabin, wouldn't it? Having 'stored hot water' in any of my outhouses would be a total waste of energy.

    Kind Regards, John
     
  15. Notch7

    Notch7

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    Yes I appreciate you have a budget and an electrician price will be a fair bit for doing all that.

    But before doing anything you must find an electrician. Many will not be interested in signing off your work.
    I know you are asking for advice on the specification, but when it comes to it, you need to do it to the exact specification of your electrician. He will also want to supply his own materials - or at least the consumer unit....it's because pro electricians get to like certain brands, because reliability and ease of fitting are more important than price.

    I'm sure you could find an electrician that would be happy for you to do the SWA run and maybe the first fix cable routing.
    Certainly domestic electrical work involves a lot of non technical bits - like drilling through loads of studwork and joists, pulling cables through etc. Chopping ouch opting out for back boxes etc.......but you still need to know all the safe zones.

    The responses you are getting on here are mostly pro electricians so you could always ask what they would do if requested to complete and sign off a job.
     
  16. Dave Lockyer

    Dave Lockyer

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    I've actually already run the SWA to the cabin. Would first fix consist of running the cbles beneath the floor and then leaving for the electircian to complete the installation? That's a possibility. I need to get the cabling in so I can lay the flooring but could get an electrician to quote for everything else...
     
  17. JohnW2

    JohnW2

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    That depends upon what you mean by 'specification'. It is for the customer to provide the 'functional specification' (i.e. define what he/she wants) - and, whilst an electrician may advise, that is essentially down to the customer. It's when it comes to the 'detailed technical specification (how 'best'/safely to satisfy the customer's 'functional specification') that the electrician has a role. However ...
    Again, the electrician can advise as to what brands he/she "likes" or considers more reliable and/or easier to fit, but when there are (electrically acceptable) alternatives available, it is for the customer to make the final choice (accepting that, if they choose something which is 'less easy to fit' that might result in greater labour charges). Who actually purchases ('agreed') materials is pretty irrelevant, other than that it may impact on what is paid and/or what 'mark up' goes into the electrician's pocket.

    Ultimately, of course, an electrician can refuse to do things in the requested way (even if 'electrically acceptable') and/or to use 'electrically acceptable') materials as requested by the customer - but the customer is equally free to turn to a different electrician who would not do that 'refusing'.

    Kind Regards, John
     
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