Underground supply to garage & barn

Discussion in 'Electricians' Talk' started by Robn, Jun 28, 2016.

  1. Robn

    Robn New Member

    Sorry for the rambling question below, any help much appreciated. I've been renovating a house for a while and am now thinking about starting work on some outbuildings.As part of this I need to sort out an electric supply for the garage, barn and a new car port. I’m hiring a digger and putting in some foundations, so it seems a good time to dig some trenches and put some electric cables in at the same time.
    Hope photo of the diagram comes out ...
    upload_2016-6-28_20-24-2.png

    So from the diagram :

    A – This is the incoming main going through a 100A fuse into the meter, and into our main house fusebox.

    B – This is an auxiliary fuse box in the annexe, fed off a 40A fuse in the main fusebox via 18m of 10mm2 cable through a 32/37Polyduct. This supplies the kitchen extension and annexe, and was done this way as the extension wasn’t built at the time of installation but we wanted power in the annexe

    C – Garage, which will be 25m away from B

    D – Car port, which will be 43m away from C

    E – Barn – which will be 50m away from C


    So I’ll be digging out runs B-C, C-E and E-D


    1. What size of cable will be advisable for the main runs? I’ve seen estimates of 16mm2 or higher for that length of run bearing in mind the consumption in the barn (which will ultimately be a conversion) and the fact that it will be almost 100m from the incoming main.

    2. Should I use SWA cable and/or ducting? Or both? I gather that the SWA doesn’t need a duct, but I’m also thinking that one day I might want to run a network or phone cable and so it might be a good idea to stick that in at the same time. Is it usual to just use SWA without a duct or more usual to use non-armoured cable in ducting, is this a personal preference ? As it seems a lot cheaper to run non-armoured in a duct, are there any downsides to this?
    2a. If ducting, polyduct or twinwall, what are the advantages?

    3. Will I need to upgrade the link A-B from 10mm2 to something better?

    4. Will I need to upgrade the connection at A to a higher rated fuse off the main wires coming out of the meter rather than through a 40A fuse in the main fusebox? The 40A fuse already supplies the kitchen extension etc and is through an RCD already.

    5. The distances I’ve quoted are to daisychain, ie up into the garage 2m and into some sort of glanded box presumably joining with a Henley block, (and out to a local fusebox with RCD) then back down and out again, I assume this is the best way to do it ?

    6. The car port (D) will have only a few lights and a socket, and will only be foundations and perhaps a slab for the near future I’m thinking that it is more sensible to run the big cable straight from C to E and then a smaller cable back from E to D. What sort of duct/cable would you think necessary ?


    Any advice or pointers very gratefully received, thanks! Rob
     
  2. leesparkykent

    leesparkykent Well-Known Member

    TBH there are to many variables for any one to give you a definitive answer. Some other things that need to considered are the design current of each out building/structure, incoming services/extraneous conductive parts in each building, discrimination/selectivity of over current protective devices, Ze and earthing arrangement. I take it your employing the services of an electrician so might be worth just running in some ducts with a rope in each between points at the recommended depth with some warning tape above.
     
  3. Robn

    Robn New Member

    Thanks Lee, sounds like it might be the best solution to just run an empty duct, at some point I'll definately be getting an electrician out but I just wanted to understand the arguments for and against different arrangements and underground kit, more than anything to get some likely idea of the expense. 100m of 16mm SWA doesnt come cheap!!
     
  4. leesparkykent

    leesparkykent Well-Known Member

    IMO depending on the load of the barn you could easily be using an SWA with a cross sectional area bigger than 16mm. To give you a rough idea a load of 24-25A will give you roughly 3% volt drop with 16mm SWA over 100 meters.
     
  5. Robn

    Robn New Member

    Yeah that's what I'm worried about. :confused:
    100m 16mmSWA ~£480
    100m 16mm T&E ~£240

    So for 16mm, half the price to not use SWA, no doubt a bigger difference if bigger cables, hence my questions about ducting ... I'd still be grateful if anyone has opinions on NOT using SWA ...
     
  6. leesparkykent

    leesparkykent Well-Known Member

    Another problem with using 16mm T&E would be the fact it has a 6mm CPC the R1+R2 value of 16mm T&E over 100meters is roughly 0.42 so depending on Ze, type and size of the overcurrent protection device it may not leave a lot of room to comply to the tabulated values. Also another thing to be considered would be the fact its a distribution circuit there will be final circuits coming off the end of it so if the Zdb is towards the higher end of things with regards to the maximum values then you may also struggle to get the final circuits to comply. If there are extraneous conductive parts that require bonding in the barn then the 6mm will more than likely not be adequate either. There are different earthing arrangements that could be implemented etc to make it comply if this was the case and so on. At than length of circuit I probably wouldn't be looking at exporting the equipotential zone anyway. Where about in the country are you Robn?
     
    FatHands likes this.
  7. Robn

    Robn New Member

    Thanks again Lee. I hadnt clocked the SWA has a larger earth wire and didnt realise that was important. I'm near Bath.
     
  8. leesparkykent

    leesparkykent Well-Known Member

    No problems. Bath is a lovely part of the word but unfortunately a bit far for me lol. Had you been closer I would of come had a look and helped design it :)
     
  9. Lectrician

    Lectrician Screwfix Select

    To comply with BS7671, you cannot use a non armoured cable in the ground, ducted or otherwise, so it's a non starter. You used to be able to, but not for sometime. If ducted, the duct must afford the same degree of protection as an armoured cable. Standard everyday duct is not deemed to provide this.

    I would directly bury the SWA, and run a 68mm twin wall duct for comms.

    The load in the barn sounds like it could be substantial, 16mm likely being too small. It would need running from the origin, and the size calculating.
     
  10. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    Have you thought about a DNO new supply as an alternative. .?

    This sort of work won't be cheap and SWA is the only way to go, you need a sparks on site to pace it and re calc as Lec says

    I would be looking at 25mm 3 core for starters, whats the loading out there. . .you can bet you will want more in 10yrs time!
     
  11. seneca

    seneca Screwfix Select

    The last similar job I done needed a 70mm SWA. The total run was about 120m, (40amp supply) I know your runs are not all that length but I think you can forget about 16mm SWA!
     
  12. philthespark

    philthespark Active Member

    Agreed,I did some pond pumps recently and ended up using a 16 mm,purely down to the length of run,load was less than 10 amps
     
  13. leesparkykent

    leesparkykent Well-Known Member

    That's not strictly true although I do agree the easiest/cheapest option would be SWA.
     
  14. Lectrician

    Lectrician Screwfix Select

    And after the bit you bolded is the reasoning why. I'm not quoting word for word, but ducted twin and earth is not compliant.

    "Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor."

    It has long been the opinion that standard ducting or conduit does not afford the necessary protection.
     
    Last edited: Jun 29, 2016
    leesparkykent likes this.
  15. Coloumb

    Coloumb Screwfix Select

    Don't sound right to me mate. What's the reg? I would have thought a bit of twin and skin 2 foot under would be fine in a bit of placky with suitable mechanical protection.
     
  16. leesparkykent

    leesparkykent Well-Known Member

    yes the bit I bolded as you said or in a duct or otherwise which isn't true. 522.8.10 "except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor. The location of buried cables shall be marked by cable covers or a suitable marker tape. Buried conduits and ducts shall be suitably identified. Buried cables, conduits and ducts shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground."

    NOTE: BS EN 61384-24 is the standard for underground conduits.

    IMO basically stating a cable has to incorporate an earthed armour or metal sheath suitable for use as a protective conductor is the only method available is wrong as the reg clearly states EXCEPT where installed in a conduit or duct which provides equivalent mechanical protection. So a cable installed in a conduit that conforms to the standard BS EN 61384-24 would comply wouldn't it?
     
  17. Lectrician

    Lectrician Screwfix Select

    Not in the eyes of the NIC. It's difficult to say some plastic duct will provide equivalent protection to steel armouring. Does the reg mention the BS the duct would need to be manufactured to as written above?

    Umm. I'm not sure now, it's always been my beliefs, led by the NIC, that conduit/duct does not provide the necessary protection.
     
    Last edited: Jun 30, 2016
  18. Coloumb

    Coloumb Screwfix Select

    I have always been under the impression that additional mechanical protection, such as a inch thick paving slab, would be ok. I'm sure it's the same for drains and sewers.
     
  19. leesparkykent

    leesparkykent Well-Known Member

    unfortunately no it doesn't give a bs number regarding plastic ducts.
     
  20. Robn

    Robn New Member

    Thanks everyone for your input. Due to the location a new supply would be even longer and I guess a lot more expensive. Looks like I'll involve the electrician to calculate the size >= 25mm before I get digging and just run SWA directly in the ground.
    Cheers
    Rob
     

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