Wiring a garden building

I will be building a garden office shortly. It will be 5 metres from the house and I'd like advice on the supply cable please.

I think I have two options:

Run 15 metres of 2.5mm2 SWA from the consumer unit in the house to the CU in the garden building

Run 5 metres 2.5mm2 SWA from the garden building to a Wiska box on the house wall then 10 metres of 4mm2 T+E to the house CU

Are there any strong reasons for choosing one or the other? Is it practical to connect SWA to a consumer unit?




  • the garden building will be a TT system utilising an earth rod


    The garden building CU will probably be metal

    Be aware that great care is needed on a TT system to avoid earth faults before the first RCD - get it wrong and a single fault can result in the entire TT earthing system being permanently at mains voltage. Usually you need to ensure that the incoming live conductors are protected by double or reinforced insulation all the way up to the first RCD terminals. Also exposed conductive-parts of one system aren't simultaneously accessible with those of the other.

    In this particular case, an RCD upstream (in the house) would physically provide reasonable protection although there is some debate whether relying on such an arrangement is entirely conforms to the letter of the wiring regs (specifically the TT requirement that all exposed-conductive-parts protected by a single device be connected to a common earth).

       - Andy.

  • Andy, I was presuming an RCD in a spare slot in the house CU and another in the garden building.

    Just working my way through the On Site Guide. I have 3 months to figure this all out ...


  • Thanks for all the advice, just one more question for now please. Can I run the SWA up from the trench in the cavity between the SIP wall and the external timber cladding? Any physical protection needed (e.g. steel conduit?).


  • so long as the SWA is connected in such a way that penetration by screws, nails, garden implements etc would operate some automatic disconnection - in practice this means earthing the armour at the supply end and verifying the resistance is low enough to pretty much instant trip the MCB or fuse supplying, no further precaution is required by regs.

    Non-armoured cables do however require protection except in a route that is obvious by the nature of being a line of least surprise to a fitting, or on the surface.

    Probably freedom of route is another reason to run the SWA indoors as far as possible.


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