wiring up a shed?

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  • wiring up a shed?
  • can anyone talk me through wiring up a shed please.

    i’ll be running the power from my garage, and in the shed will need a couple of strip lights and maybe two double sockets.

    Premier Icon wwaswas
    Subscriber

    You’ll probably need an electrician to do it and get buildign regs.

    Easiest is to run an armoured cable down to the shed from the consumer unit, I think, as it doesn’t need to be buried. Wasn;t that expensive when I had mine done.

    Premier Icon nickjb
    Subscriber

    It is building control notifiable work so you need to inform them before you start work and pay them a handsome fee. You also need to be ‘competant’ but that is loosely defined. The other option is to get an electrician who can self certify which might work out a similar price but much less work for yourself.

    … Or just run some armoured cable and don’t tell anyone or just run the whole thing from an extension lead. If you do go for one of these slightly dodgy solutions them make sure the whole set up is RCD protected.

    wrightyson
    Member

    Building control 😯
    Screwfix do a nice line in mini consumer units for sheds/garages. Connect a 6mm armoured if not going miles into your existing unit. Probably stick it just on a 16a breaker as that’ll be plenty unless you’re firing up all manner of stuff.

    mattrgee
    Member

    I’ll be keeping an eye of this thread as I need to do something similar.

    Does it need to be a single run of armoured cable from the consumer unit in the house all the way to the shed? What determines the size of cable that needs to be used?

    Premier Icon nickjb
    Subscriber

    Cable only needs to be armoured where it is buried. Generally you run the armoured all the way as it saves having joins, or run the armoured up the house then use normal t&e internally.

    Size will be determined by the load, length and the way it is mounted. Basically the voltage can’t drop by more than a certain amount.

    Premier Icon Sandwich
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    Is the house on PME (earth bonded to neutral)? If so advice used to be don’t connect the earth on the cable from the garage. The shed should be earthed via a stake connected by 6mm earth cable to the shed consumer unit.

    jamiea
    Member

    A mate, ex boss actually, who a was a qualified spark (a few editions of the regs ago), did mine and had the bright idea of using 2-pin&earth outdoor sockets between the house and the shed so it wasn’t technically permanently wired. Dunno if it’d get around building regs but I saw the logic! Buried the cable in a length of hose pipe a couple of feet down. RCD in the shed and roberts-yer-mother’s-brother.

    EDIT, it’s actually a mini consumer unit in the shed and standard cable, none of that expensive armored stuff 😉

    Cheers,
    Jamie

    giant_scum
    Member

    @ Sandwich, not connecting the earth at the shed end is still in the current regs! My understanding is you still need to earth the cable at the supply end and controlled through a RCCD.

    rwamartin
    Member

    Ok, wiring a shed requires notification to building control. If the work is done by an electrician who is a member of a self certification scheme (NAPIT, NICEIC +others) he will do this once the work is completed. If you want to do the work yourself, which you are fully entitled to do, you need to notify building control BEFORE you start, pay the fee (I think it’s £75 here in Pembrokeshire) and they will send someone out to inspect/test the work on completion. Alternatively you could just do it yourself and not tell building control. I’ve never heard of any homeowner being prosecuted for doing it although your insurance company may have an issue if there was a fire.
    If you do have an electrician do it make sure that they give you an electrical installation certificate for the work. You should also get notification through to say it’s been notified.

    If you’re doing the work yourself there are a number of things to consider, some of which have been mentioned already.

    In the shed I’d use a “garage” type consumer unit. This will normally have a 30mA RCD plus a 16A and a 6A MCB in. Wire the sockets in 2.5FTE onto the 16A MCB and lights in 1.0FTE to the 6A MCB. If you’re likely to use high loads you might need a different cable size for the sockets but 2.5FTE takes 27Amps if clipped to the walls.

    If there is no water/gas services into the shed that bring in earth potential you’re good to go. If there is you’ll need to bond those back to the garage unit using 10mm2 yellow/green earth cable. There may also be a need to put in an earth rod for earthing. (See later comments).

    From the house to the shed you need to run a cable that is suitable for the load and the conditions it’s being run in. Armour is usually a good bet but it could be FTE in conduit to provide protection.

    The sheath of the armour should be properly glanded off and earthed. This should definitely be done at the supply end but at the garage end will depend on whether you’ve got an earth rod or not there.

    At the consumer unit in the main house use an MCB that is rated lower than the current carrying capacity of the cable that’s been used to supply the shed. I would prefer it not to be on an RCD as there is an RCD in the shed which provides the necessary protection. However it’s ok to be on an RCD in the main board but there is no guarantee the one in the shed will trip first.

    Earthing systems will have some impact on what you do. If you have an earth rod at the house, you’ve no big issues but the MCB at the main board should have at least a 100mA RCD to ensure disconnection in a fault situation.
    If it’s TN-S (a third wire into the house from your supplier, or the sheath of the incoming cable is used), things are fine.
    If it’s TN-C-S (ie PME) it is recommended that the shed be earthed using a rod. This isn’t strictly necessary if there is no water/gas/structural steel in the shed. However, if you don’t want to put a rod in I would ensure that the feed to the shed is at least 10mm2 including the earth so that the earth can act as a bond too.

    It sounds a bit complicated and obviously it depends on each installation as to how it would actually be done but easily in the capability of a good DIYer with a bit of guidance.

    Any questions, don’t hesitate to ask and I’ll do what I can to help. If you’re in West Wales we’ll come and quote for the job!

    Rich.

    Premier Icon AlexSimon
    Subscriber

    Ace info in this thread! Bookmarking for future reference.

    andyl
    Member

    also need to bookmark. Will be doing this in the new year.

    Premier Icon footflaps
    Subscriber

    I ran a 10mm^2 mains cable 50m down the garden to the workshop in steel conduit.

    I’ve still not got round to actually wiring it up though and been using a 50m extension lead hung over the fence posts all year!


    Kitchen end by brf, on Flickr

    cheers guys, i’ll have a good read through later.

    interesting though that rcd boxes are mentioned. i used a large local electrical company when i got the house rewired and had exatly what i’m asking here done in the garage and the outside workshop…. neither is rcd protected. just a cable in and a junction box. unless they’ve done something in the house main box?

    to be honest building though regs is the least of my worries, the house is listed and i can’t do diddly without permission so a 8 x 14 workshop!!!

    Premier Icon footflaps
    Subscriber

    If you have an RCD in the house, you don’t need one in the workshop, although I’m going to fit one as the longer the run, the harder it will be for the RCD, in the house, to detect a short to earth.

    alanl
    Member

    Footflaps, have a think again about what you wrote there.
    How fast do the electrons flow down the cable?
    How long for them to travel the 20 metres or so to a shed?
    Basically, there will be no difference between having an RCD in the shed, or the house.
    The main point of having one in the shed is to stop the one in the house tripping. But to achieve that, you must run the cable to the shed from a non-rcd protected outlet in the CU.

    Having 2 RCD’s in series can mean them both tripping, the inside, or the outside one tripping, it is pot luck until tested. There is no discrimination between the 2 unless the cable is very long, so only fit one RCD.

    Premier Icon footflaps
    Subscriber

    Footflaps, have a think again about what you wrote there.

    You are right I need to do the maths. The speed of electrons in copper (c/3) isn’t what I was thinking about, it was whether the resistive / inductive losses of the long wire to the workshop might mean you need a bigger / longer current miss-match to trigger the RCD.

    trail_rat
    Member

    “Having 2 RCD’s in series can mean them both tripping, the inside, or the outside one tripping, it is pot luck until tested. There is no discrimination between the 2 unless the cable is very long, so only fit one RCD. “

    Is this a huge issue ? as long as one of the bleeders trips.

    FWIW i have a twin RCDs on the board in the house then separate MCBs set up on mini consumer units the garage and shed – which is handier than you think.

    Means i can flick the plugs or the lights off from the garage and work on the circuit without going inside and upstairs repeatedly.

    Have the garage on a big RCD protected circuit- with an big amp MCB (cant remember value off top of head possibly) and monster cable and then break this down into 3 smaller circuits in the garage with 16amp industrial plugs for the welder , standard double plugs and a lighting circuit.

    previous owner had the whole garage wired into the 6amp lighting circuit it transpired when i was striping it out. – and it never tripped once even when i was welding 4mm plate ! – the cable had stretched somewhat though – old style wire fuses are scary – this had been signed off as well – so that gives me loads of faith in their system.

    Premier Icon footflaps
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    According to the IEE it’s capacitance of long runs that is the issue

    3. Incorrect application. An RCD must
    be correctly selected and erected for
    the particular application. For
    example, protecting an entire
    installation using a single high-
    sensitivity RCD can, in many cases,
    lead to unwanted tripping,
    particularly in industrial
    environments where inductive loads
    will cause greater transient
    overvoltages and where longer cable
    runs will result in larger values of
    capacitance to earth.

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