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Advise - Future Proofing Outside Building

Luke1985

Hi All,

Upfront, I am not an electrician, I'm just looking for advise on future proofing.

I recently bought a house and long term (years from now) i intend on adding a outbuilding at the bottom of my garden. this will be an office/workshop so will need power running from my house to the new building. obviously when the time comes i will need to engage an electrician to add the circuit, put in a subordinate consumer unit etc. etc. 

however, right now i have my ceilings down (for unrelated renovations) so it feels like a good time to at least run some internal cable....

I was thinking i could run some (unconnected) cable from near the consumer unit to a box of some kind at the back of the house, that way the eventual sparky doesn't need to smash up my walls/ceilings when the time comes. Consumer unit to back of house will be around 10m of cable, back of the hose to new building will be around 20m.



question is, can a sparky join SWA onto T&E (if thats what i run) and if i do run T&E is 10mm sufficent? or should i be looking at 16mm?

Option B is a I could leave a pipe of some kind running from the back of the house to the consumer unit, but not sure how easy drawing a T&E/SWA down 10meter of pipe would be!

  • +1

    Ducting for any cable is only useful if it is about the size of a drain pipe and has a pull-rope already in it that moves freely.  An SWA can be joined to almost anything if enough slack is left. The usual trick is to gland the SWA into an adaptable box and then have some terminals in there to whatever you need, TnE, flex, singles in conduit...
    The same sort of thing can be done in less space with care using a double socket back-box and a blanking plate to cover it.
    Generally you want all joints  indoors and easily accessible and inspectable if at all possible.
    Cable size depends on the expected maximum load and perhaps the distance if your grounds are extensive.
    Over a short distance 4mm2 will do 32A which is enough for a ring of sockets and some lights.  6mm is probably overkill unless you need a cooker and shower, 16mm2 is enough for a complete new build flat and more than the meter tails on some older properties.
    Mike.

  • +1

    Tricky - the future often hard to see.

    A lot depends on the exact details of the proposed building and how it's likely to be used - might it supply a EV charge point? or have solar panels connected to it? Or be of steel frame construction or have metallic services connected. Or house a heat pump to warm the main house? Many factors can affect the choice of cable size - and indeed type of cable (e.g. 10mm² T&E only has a 4mm² c.p.c. so is inadequate to use as a main bonding conductor).

    Installing ducts rather and deferring the choice of actual cable can be a good move - but two 90-degree bends or equivalent is about the limit for drawing a cable through - so not really something to have snaking all over the place above ceilings and through joists, and probably a bit thick for concealing in walls too. If you've got a void under the floors, a bit of 50/63mm twinwall going to just below the house CU could work well.

    If you're digging (or even planning) a trench consider any other services - once trench with ducts for mains electric, water and even data (for Ethernet - WiFi signals often aren't good to remote buildings) all suitably separated, is a lot more satisfying than doing one and then thinking about the next...

       - Andy.

  • 0 in reply to AJJewsbury

    Thank you Mike & Andy,

    totally understand that i'm asking a question with a lot of unknowns, future proofing is never easy, im just putting a lot of effort into renovating my house at the moment and would hate to rip chunks out of it in a few years time!!

    the routing for the ducting would have, at minimum, 3 X 90degree bends and would have to be in the ceiling through some joists (its a concrete house with solid floors!!) so it sounds like the ducting option is out!

    i figured the outbuilding would have a lighting circuit (say 6amp MCBO) with a small ring main (32amp MCBO) running power tools/office equip/maybe small heater for office. so i was guesstimating needing a 40amp connection at the main consumer unit to the subordinate consumer unit. (there wont be any EV chargers or anything big on the new outhouse, my house is terraced with no rear access so anything big is out by default!)

    in total the cable run will be about 10m of cable in the house and 20 meters outside the house, I wont pretend to understand the calculations for cable size but used an online calc and it said 10mm cable for 30meter run at 40amp. i cant imagine ever drawing 40 amps, but figured going big was the best way to play it safe and make sure the eventual sparky is happy to use the cable? 

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