20 metre SWA run to a shed

Discussion in 'Electricians' Talk' started by PS1964, Jul 11, 2020.

  1. PS1964

    PS1964 New Member

    Greetings, my wife would like a double gang socket in the potting shed for her heat mat for the plants and other very low voltage items with no lighting required - just a double gang socket.
    So my plan was to add a fused spur to a ring main, run some domestic 2.5mm T&E through an external wall straight into an IP rated junction box and make a connection to a 20 metre run of SWA cable.
    There are no other forms of power in the shed, and no metal pipes etc.
    What sort of size does the SWA need to be? I've read sizes of 6mm and 10mm for this kind of run but if it's only a double gang socket with low voltage items is that sort of size necessary or does the distance form part of the calculation?
    Should the fused spur have an RCD or is it better to have a RCD double gang socket at the shed?
    And in terms of earthing, the fused spur is in a dry lining plastic junction box on the ring main, the external IP rated junction box is plastic, and the cable will likely terminate at another plastic junction box or isolator switch in the shed and join to domestic T&E for ease of use so what is the best method of earthing either a dedicated earth wire or the armour?
    The cable will run alongside a wall and it will be tricky to bury so if it gets tacked to the wall does it have to be in trunking or conduit even though it seems safer in view rather than buried 240mm down?
    And in terms of the part P certification and the BC etc, is it true that it is only needed if a brand new circuit is attached to the CU, and not for a spur such as this?
    Sorry lots of questions but hopefully valid ones.
    Thank you in advance for your help.
  2. Bazza

    Bazza Screwfix Select

    Is the source circuit already RCD protected? If so you don’t need another one.

    for that length I would use 4mm minimum.
    3-core, separate earth not required.

    look and learn time! John Ward’s tutorial on how to terminate SWA cable is below. It shows you how SWA glands work. Don’t forget the armour must be earthed.

    If it’s a brick wall, then the SWA can be cleated to it. But cleat it a few inches above the ground level so it is clear of enthusiastic gardeners. Burying not necessary.

    Ungortunately outside electrics is no longer a notifiable activity unless it’s a new circuit. So this extension is not notifiable. But Part P (electrics safety) applies to ALL electrical work.
    Last edited: Jul 11, 2020
  3. Bazza

    Bazza Screwfix Select

    Plus. You’ll need an external box that is suitable to take a 20mm SWA gland. Sometimes it’s easiest to fit an external weatherproof FCU. Then you don’t have to chop your inside walls.

    of course, your shed socket* needs to be suitable for SWA glands too. Suggest you use an external weatherproof socket. There may be a lot if water sprayed around!

    *do you need an RCD one? See post #2
    Last edited: Jul 11, 2020
  4. Peterdevon

    Peterdevon Screwfix Select

    1.5mm looks good to me with 20x13x27= 7V drop that's only 3%
  5. PS1964

    PS1964 New Member

    I think maybe my measurements were a bit over zealous! It's more like a 25 to 30 metre run. I added the parameters to the Doncaster Cables website and it seems like I can get away with 2.5mm cable which is nice because I've already got it!
  6. Bazza

    Bazza Screwfix Select

    I’d go for larger than 1.5mm. There may be heating mats there. And a bit of future proofing. Her potting shed might be his man cave in a year or two.

    edit 2.5mm ok, as U already have it!
  7. PS1964

    PS1964 New Member

    Thank you for taking the time to reply Bazza.
    Interesting your choice of hardware, I was considering an internal FCU on the skirting board mainly for security reasons I suppose. Is there an unthought of advantage to going with an external FCU? Also I do like the IP rated external sockets, I fitted in the garden to power a fish pond. As there will be water present in the intended environment it might be a good idea. Funnily enough I had viewed John Ward's videos before posting and wanted to take the SWA gland route but can't see the route to earth at either end? The heat mats for plants are very low power and if my calculations are right in terms of their wattage and the mains voltage of 230/240, they barely make a dent in 1 amp. It will never be my man cave, that's the understairs cupboard!
  8. MGW

    MGW Screwfix Select

    There is a difference between design current and the fuse rating, so my beer heat mat is just 150 watt. But 1.5 mm cable at 13A is good for around 32 meters but that would be direct and your coming from a ring final so you would need to work out the volt drop to the point where you tap into the ring final and add both volt drops together, however at 20 meters with items which volt drop hardly matters anyway, I see no problem with 1.5 mm SWA, the junction box would want to cover the hole which the twin and earth comes out of as the sun can damage twin and earth, at least in theory, but in practice seen it used many times, depends on how much sun light it gets, but it is normally OK, there was a batch of twin and earth which caused problems in Australia to their Woolworths because the plasticiser leached out, but a lot hotter in Australia.

    There is a special nut which allows you to earth the SWA gland, but I am happy with the bango washer as long as the back nut is brass not steel on inside of box, if steel nuts then outside of box so bango washer contacts brass, and drill through box with 6 mm brass nut and bolt. I will normally use 3 core and use one core for earth as well, in case at some time the outer of the SWA is damaged and the SWA rusts through. I know it should be tested each time an EICR is done, but only with street lights have I seem the SWA tested to show if there is a damaged outer, and that was mainly so the council could charge contractors if they nicked the cable outer.
  9. PS1964

    PS1964 New Member

    I used this calculator and added values or 25 metres, 230 volts, 2.5mm² 3 Core PVC SWA XLPE Armoured Cable, non-lighting circuit, clipped direct to a wall, number of circuits = 1.
    Adding a current of 6.5 kw returns a cable size of 2.5mm and a voltage drop of 11.3 which is 4.9 per cent. The current load is 28.3 amps and the max cable load is 31 amps.
    Then upping the current from 6.5 to 7 returns a cable size of 4mm and a voltage drop of 7.61 which is 3.3 per cent. The current load is 30.4 amps and the max cable load is 42 amps.
    Seems like quite a jump and now I'm thinking maybe 4mm cable might be the way to go after all, especially as I'll sell the house one day and leave someone else with a circuit which might not be fit for their purposes.
    Last edited: Jul 11, 2020
  10. That is absolutely pointless, the most you can draw is 13amp because you are coming from a fused spur. You would get away with 1.5mm but I would run 2.5.
    If as you say you are mounting the spur on the skirting, fit a metal clad one and run the swa straight to that.
  11. PS1964

    PS1964 New Member

    The 2.5 SWA isn't that flexible and when it comes out of the wall it needs to do an immediate 90 degree turn. So are you suggesting earthing the SWA to the metal box behind the FCU? Would that satisfy the earthing requirements of the circuit? What about when it reaches the shed? Also should the FCU be a RCD or is it better to have a RCD socket at the shed? The ring main that the FCU is coming out of goes to a circuit breaker on a modern CU.
  12. unphased

    unphased Screwfix Select

    You don't drill holes horizontally and square to the wall for SWA cables. Drill 45 degrees downwards so the SWA has a nice easy curve when it enters and exits the wall. No 90 degree bends then.
  13. Bazza

    Bazza Screwfix Select

    That will be hellish to do. To gland to a metal back box, sunk in a wall is tres difficile. If you come out the back then you'll have a big loop of SWA before it can come flush to the wall again (SWA is not very bendy, ). If you come out of the bottom of the FCU back box, then your SWA has to go vertically and then out the wall, again, there will be a loop as it emerges, this time at the bottom of the wall, you'll need to drill a suitable hole down through the two layers of brick that form the cavity.

    The better method is to come through the wall horizontally and into a weatherproof box. The SWA then leaves vertically, or horizontally flush with the wall.
    Something like this

    The pic is a bit small, but the centre of the SWA (bedding) and the inner cores have been stripped long from the SWA. Then threaded through the wall into the box inside. That way, you do not need a junction inside the outer box, or a short bit of T&E to join inside and outside. NOTE the additional earth conductor running through to inside. This is needed to earth the SWA. (see below***)
    You can choose to have the 13A FCU inside,or (my prefernce) have it outside, come through the back of the socket box into the external FCU and gland the SWA on to the bottom of the FCU box.

    OK. I'm going to be a bit harsh here. With SWA you havent got a choice about taking the "SWA gland route". There isnt an option. Thats how you terminate SWA!
    You obviously didn't view JW's video properly:mad:!! In there he explains that for a metal box the gland's mounting on to the metal box provides the earth. Because metal boxes are always earthed aren't they?
    ***He also says, for PLASTIC boxes etc (and this may be where you go) you use the banjo connector that is included in the gland set. This goes under the securing nut and you'll need an earth fly lead and a nut and bolt to connect the banjo (and therefore the armour) to earth.

    Here endeth the second lesson.. Amen
    Last edited: Jul 11, 2020
  14. PS1964

    PS1964 New Member

    It hadn't occurred to me do drill at 45 degrees, it doesn't sound like the done thing. I was going to terminate a domestic T&E in a black adaptable IP rated box and join to the SWA from there, although I suppose the least joins the better.
  15. unphased

    unphased Screwfix Select

    Tres difficile, mon ami. :)

    Yes drilling is one of my specialities. I can make holes line up in the most awkward of locations and judge entry and exits to extraordinary precision. Trust me a 45 degree downward hole inside to out is a magnificent way of doing it. I would avoid weatherproof accessories outside and get the jointing done inside.
  16. Bazza

    Bazza Screwfix Select

  17. PS1964

    PS1964 New Member

    Is that a galvanised round box with a back entry photo in your photo? I like those round boxes and they seemed a perfect choice for exiting the house and a route to earth but I couldn't find one with a back entry.
  18. unphased

    unphased Screwfix Select

    You drill your own back entry hole with a 20mm hole saw!
    PS1964 likes this.
  19. PS1964

    PS1964 New Member

    I like the idea of using a circular galvanised junction box and partly stripping the SWA to achieve the bend radius, and then earthing via a gland kit and strapping to the circular galvanised box. So having a RCD at the FCU at the house is preferable to having a RCD socket at the shed?
  20. Are your sockets not rcd protected already?
    That is another way to do it with a box outside and run the core into the spur inside.

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