Main Earth on a 90 metres sub-main

Discussion in 'Electricians' Talk' started by unphased, Sep 29, 2010.

  1. unphased

    unphased Screwfix Select

    I am quoting for a job which includes a sub-,ain run of some 90 metres to a building at the end of a long garden. The buried part of the sub-main is around 65 metres. I don't know what to do about earthing and wondered if you lot can help. :)

    The design load I am using is 26A. It is simply based on the mcb sizes for simplicity (20A power, 6A lighting) so I have chosem a 10mm2 3-core SWA which is principally for VD as it eats the load (60A buried in ground). But I can't decide whether to just use the 10mm2 third core as the main earth from the origin to the new building or whether to addan earth rod as well at the new building due to the distance involved.

    So, am I okay to just use the third 10mm2 core (and the armour csa) for the main earth alone or should I also bang in a rod at the new building end?

    Ta muchly.
     
  2. spark&half

    spark&half Active Member

    1 Option is to use 2core swa and stake it, stick in a 2way db 16A/6A mcb and a main rcb.

    swa earthed at main end only.

    ;)
     
  3. spark&half

    spark&half Active Member

    16A or 20A as mentioned

    ;)
     
  4. unphased

    unphased Screwfix Select

    Thanks s&H

    My thoughts were to use an RCD at the origin so the sub-main is also protected, and use a bog std 2-way fuseboard at the new building end. However, using your suggestion, I could keep costs down by using a 2-core but would I rod at the new building end only or connect armour to earth at origin?
     
  5. Lectrician

    Lectrician Screwfix Select

    You are only just in on volt drop on that run, leaving little, if nothing for final circuits.

    If you want to TT the outbuilding, you would use a 32 breaker in the house, gland and earth the SWA a usual at the origin.

    In the outbuilding, gland the SWA into a plastic enclosure (or DB). The earthing on the armour would stop here and not be used for anything, nor connected to anything. Then connect an earth rod to the earth bar in the DB.

    If you want to use the earth from the origin you would need to measure the Ze to ensure your cable is sized accordingly to ensure a suitable Zdb and Zs in the outbuilding.
     
  6. unphased

    unphased Screwfix Select

    Thanks Lec

    The house end is old wylex rewireable and I want to avoid a fuseboard change due to costs.

    My Plan was to split the tails in to a henly block then go through a 30mA RCD then in to the SWA sub-main which will climb up and over the house (terraced) through the loft down in to the garden then 65 metres to the new single story. I have over estimated loads for design purposes and the VD is 4.5%.

    Here are my calcs:
    Cable Size

    Length of cable 90 metres

    Load on cable:

    3 no. light points at 100W per point = 300W/230V = 1.3A
    1 no. floodlight at 500W 500W/230V = 2.2A
    20A radial circuit assume fully loaded = 20A

    Total design current say 26A (total rating of mcbs)

    Try 10mm2 3-core SWA cable max permissible load 60A Table 4D4A direct in ground

    Voltage drop = 4.4mV/A/m

    26 x 4.4 x 90/1000=10.3V

    Max allowed VD is 5%

    10.3/230 x 100 = 4.5% therefore satisfactory.

    I am working on the quote right now so would you agree with my choice or would you omit the RCD at the origin, earth up like you said, a switched fuse instead of RCD, then RCD at the new build end on a stand alone rod?

    Thanks mate
     
  7. seneca2

    seneca2 New Member

    I suppose the cable's already in now UP but I think i'd have gone for 16.00mm over that length, just to keep a little bit in reserve?
     
  8. seneca2

    seneca2 New Member

    omit the RCD at the origin, earth up like you said, a switched fuse instead of RCD, then RCD at the new build end on a stand alone rod?
    ----------------------------
    That's the way i'd do it UP, as long as the origin isn't TT of course in which case you would need an rcd at the house end.
     
  9. unphased

    unphased Screwfix Select

    No sen I haven't installed the SWA yet, I am pricing up the job. I did tell the builder to assuem 16mm2 anyway but I thought I could save a bit if I went for 10mm2. Will do 2-core though. :)
     
  10. seneca2

    seneca2 New Member

    I'm sure you'd be telling me to use 16.00mm UP?
     
  11. spark&half

    spark&half Active Member

    10mm 2core is good for a load of 26A upto 100m perhaps 16mm would be a selected choice?

    have a look a the price difference of 2core 10mm and 2core 16mm and see if it's worth it.

    ;)
     
  12. J.P.

    J.P. New Member

    I suppose you would buy a 100 meter drum..

    TLC Pricing..(Ex Vat)

    10mm 2 Core....£245

    16mm 2 Core....£333

    Difference of £88 (EX Vat)

    If it was my job at home would go for the 16mm most definitely. If it was for a customer then would explain fully to the customer the possible downfalls of using 10mm. So for the difference maybe its worth paying extra, especially with all the work laying it etc (trenching out and stuff)
     
  13. J.P.

    J.P. New Member

    If your laying it to spec UN..why use an RCD at origin? If it does trip at the house end then its a long walk to re-set the RCD.
     
  14. spark&half

    spark&half Active Member

    Does anyone actually use TLC? there prices are an absolute rip, would you ever pay that price for that swa? go to your local wholesale, it will be much cheaper.

    ;)
     
  15. Lectrician

    Lectrician Screwfix Select

    5% volt drop? What about the lights?
     
  16. spark&half

    spark&half Active Member

    I was waiting for someone to say that.....

    ;)
     
  17. Risteard

    Risteard Screwfix Select

    "Max allowed VD is 5%

    10.3/230 x 100 = 4.5% therefore satisfactory."

    Definitely not satisfactory. You have a lighting circuit which should not exceed 3% volt drop from the origin of the installation if using the suggested figures for voltage drop in BS 7671.
     
  18. Risteard

    Risteard Screwfix Select

    Look like I was typing the same thing at the same time as someone else!
     
  19. spark&half

    spark&half Active Member

    16mm it is then...
    ;)
     
  20. J.P.

    J.P. New Member

    Of course it is S and H..it wouldn't be anything else within the proposed deployment parameters Un has stated..;)

    I still don't understand why a RCD would be fitted at origin though..surely EEBADS is satisfied? of course I ask that question on the preempt that Un is running the SWA according to recognized practices.
     

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