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?




  • Hi Mike,

    As is often the case on here, what appears to be a simple question often generates many more questions before a sensible answer can be given.  In this case, questions relevant to your situation (including some areas not directly related to your question but still important) would be:

    1. What is the maximum load you anticipate in the garden building - heating loads are potentially more significant than a couple of LED light fittings and a socket for the lawnmower to plug in to?
    2. What is the earthing system of your installation, and how will you be earthing the garden building?
    3. Are there any external services (particularly metal pipes) entering the building?
    4. What's the floor construction of the building?
    5. Are you looking to extend an existing circuit, or provide a new one?  If existing what is the rating of the circuit breaker?
    6. How will the cable be run both inside and outside the house - in particular does it run through insulation in the house?

    I'm not sure why you would change the cable size between inside and outside of the house, unless the inside is to be run through insulation.  A change of cable type can make sense for an easier cable to run and work with than SWA once you get inside the house, but it is practical to terminate an SWA onto a consumer unit if you have reasonable access space to work, and space in a sensible place on the CU to get the cable in.

    Earthing is a significant consideration here as well - hence some of the questions above.

    Also remember that if this is a new circuit provision then this is notifiable work under Part P of the Building regulations - you'll either need an electrician registered with an approved scheme to self-certify, or you'll need to apply for building regs approval and the Local Authority may well just tell you to find an electrician to certify it - approaches differ.  You may be lucky and find a local electrician to work with you and sign it off at the end if you have done some of the work but again views differ as to whether this is allowed or not.


  • Indeed more questions first - why not 4mm2 SWA all the way... Price difference is minimal over that distance.

    For the same reason folk use 3 core SW and use a core in parallel the armour for earthing to keep the touch voltages down during any faults.

    You can terminate SWA into a consumer unit if it is a metal case type and has a fair amount of free space - and you need to be able to get your spanners in..

    If not then terminate the SWA into an adaptable box beside the CU, with gland and pirahna or whatever, and then carry just the cores onwards to the CU, in trunking or conduit if required to give mechanical protection . Minimising the number of joints is good, and no joints outside at all if possible.


  • Why do you think that 2.5mm² SWA will be suitable?

    As others have stated, you'll need to consider loading, supplied earthing arrangements

  • Thanks Jason. Here are some answers to your questions.

    1. 5A equipment +13A heating + lighting (2 LED ceiling lights).

    2. The house is PME but the garden building will be a TT system utilising an earth rod

    3. The primary use will be an amateur radio shack, with coax cable from the antenna entering the building. The antenna will have a static discharge to the same earth rod, hence the need for TT

    4. The building will be constructed from structural insulated panels, inside dimensions 3.4m x 2.2m. I haven't done the thermal sums but I'm hoping 3kW heating will be sufficient

    5. This would be a new circuit from the existing consumer unit

    6. If I use SWA exclusively it will run in a 5 metre trench between the buildings then around the outside wall of the house into the attached garage wherein the CU is installed. Alternatively it would be SWA between the buildings then 4mm2 T+E around the skirting of a utility room and through a brick wall to the garage

    I'm familar with the Part P requirements. I'm planning to do the wiring myself then get a third party (council or electrician) to connect, test and certify.


  • Hi Mike, the domestic CU is plastic so your suggestion is very helpful, although I would leave this part of the build to the competent person. The garden building CU will probably be metal because it's easier than doing a risk assessment for a plastic one! I take your point about outside joints.


  • Because it's rated at 30 amps and should be adequate for my current load estimate:


    If my estimate changes between now and the build, or the experts here advise otherwise, then I will upgrade it.


  • Because it's rated at 30 amps and should be adequate for my current load estimate:

    Be careful - that's the 90-degree rating, if you're connecting to domestic style accessories CUs etc) you'll be limited to a 70 degree maximum - so will have to de-rate (or use the 70 degree tables). There's also a de-rating factor for cables buried in the ground for overload protection (0.9).

    You may well be OK for a 20A circuit still - but you really should have the calculations to prove it if you're signing off for the design as well as the installation.

       - Andy.

  • Thanks Andy, that's filled another gap in my knowledge!


  • With my G7VZY hat on, be prepared to have to push for what you want, not all electricians, private, council or other, really  understand the subtleties of the extra requirements of a radio shack, and why you may want to do seemingly odd things like add ferrite beads  on the  mains earth if you have EMC issues.

    If you are going for the TT island,  and that is a good call in this case, then 2 core SWA is fine. Gland and pirahna to connect the armour to the house earth as before but at the shack end, use an all plastic compression gland instead of a metal one,  and over sleeve the armour. You must have at least one DP RCD,  ideally in the shack but could be either end or both really.

    Now,  2.5mm will certainly be OK at 25-30A but if you do need that much it will run very hot if it has to pass through insulation at either end. I'd still consider 4mm.

    You may also want to include some RFI precautions such as an L-N mains filter and perhaps the option of 12V DC (non SMPS) lights as well, especially if you intend to operate on HF where things like LED lights tend to be sources of hash as do some of those USB charger sockets. Ifnot then at least leave the space for it.

    I tend to design (professionally) such that all cables enter a radio room or bunker through one A3 sized metal access panel, as this ensures that the RF earth, the lightning earth and the power earth all flap together at a common point, and no expensive kit ends up doing the splits between 2 grounds.  It is not essential but it is something to thing about. A poor man's version has all the cables to the operating desk passing through a common plate and screened glanded to it.

    Are you also bringing in ethernet, telephone or other ELV from the house ?

    armoured cat5  is an option there if so.

    Probably more I have not remembered.


  • Mike, the SWA would pass through the SIP wall of the garden building so your point about insulation is relevant. No other cables as the wifi and cordless phone work well at that distance. I do operate HF and have thought about the points you raised.