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  1. The General
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    The General Electrician's Arms

    Morning all.

    I'm after some suggestions please - I'm at the very early stages of a new build for a customer and just planning ahead for getting cables in the ground before the foundations go down.

    There's a new PME supply cut-out which has been brought into a (very small) shed, and the plan is to run a cable underground for about 10 metres across to the C/U in the new house.

    I'm thinking that I will put a Type S 100mA RCD in a small (metal) enclosure in the shed, and then run cables buried direct in the ground. Although, if it makes life easier, the client is happy to run a length of soil pipe or something similar to run cables inside.

    So,
    Do I run a length of SWA 25mm 2-core and also a length of 16mm earth, or is it going to be easier to run some SWA 3-core 25mm?

    I'm concerned about working space for terminations inside the small enclosure at the supply end, so I was thinking that using the 2-core would allow me to run the earth straight to the MET rather than attempting to terminate a 25mm earth inside the enclosure which will probably only have 16mm holes in the earth bar.

    Or, could I even just run some 25mm tails inside the soil pipe (or does it have to be armoured because it's buried?)

    Does that make sense? Have I missed anything obvious? Would I also need to put in a 80A switch fuse at the supply end or am I covered by having the RCD?

    If I go with the 2-core and separate earth, is the earth okay direct in the ground or does it need to go inside some conduit?

    Sorry, just realised that's quite a few questions, but hopefully that all makes sense!
    Thanks for your input and advice.
     
  2. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Hi - from 522.8.10 with your proposed installation methods it needs to be SWA.
     
  3. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
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    Telectrix
    opefully that all makes sense!
    Thanks for your input and advice.
    Do I run a length of SWA 25mm 2-core and also a length of 16mm earth, or is it going to be easier to run some SWA 3-core 25mm? use 3 core. if you fit a 80A sw/fuse, 16mm 3 core will be adequate.

    I'm concerned about working space for terminations inside the small enclosure at the supply end, so I was thinking that using the 2-core would allow me to run the earth straight to the MET rather than attempting to terminate a 25mm earth inside the enclosure which will probably only have 16mm holes in the earth bar.install a metal (galv.) adaptable box under your Switch/Fuse. joint the earthing conductor to 16mm., and carry the L & N tails into the Sw/Fuse.

    Or, could I even just run some 25mm tails inside the soil pipe (or does it have to be armoured because it's buried?) you could do, but SWA is preferable.

    Does that make sense? Have I missed anything obvious? Would I also need to put in a 80A switch fuse at the supply end or am I covered by having the RCD? a switch/fuse is essential to protect the feed cable

    If I go with the 2-core and separate earth, is the earth okay direct in the ground or does it need to go inside some conduit? non-starter.

    Sorry, just realised that's quite a few questions, but h
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  4. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
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    Why are you fitting an rcd.
     
  5. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    in case he used T/E for the underground feeder. :D:D:D
     
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  6. The General
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    The General Electrician's Arms

    Also, before it reaches the new C/U I suspect that the route the cables are taking will probably mean that they're going to be buried in the fabric of the building at points, hence RCD.
     
  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    It will be swa though which "should" have an earthed sheath and anyway a 100ma rcd does not provide additional protection.
     
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  8. Ian1981
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    Ian1981 Electrician's Arms

    Location:
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    Cables buried in walls <50mm requires additional protection.
    I've never read that where cables are buried in the floor (concrete?)that additional protection is required.
     
  9. SparkyChick
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    SparkyChick Making a banana smoothy for my fave gorilla Electrician's Arms

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    Just out of interest, is the supply head going to stay in the shed indefinitely? Sounds like it. What's the shed made of? What happens if the shed falls down or future owners want to move it?

    Just playing devils advocate a bit and throwing a few thoughts out there.
     
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  10. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    trust a woman to complicate the issue. :D:D:D
     
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  11. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    I'm with sparkychick on this ........ Not that I'm taking sides!
     
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  12. The General
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    The General Electrician's Arms

    Good question though!
    Purpose built wooden frame/corrugated roof on a concrete base, specifically to house the incoming supply. It's a permanent structure.
     
  13. The General
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    The General Electrician's Arms

    Ah yes. Good point, well made.
     
  14. The General
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    The General Electrician's Arms

    So, the general consensus is that I can fit a 80A switch fuse, no RCD, terminate some 3-core 16mm SWA into a galv box and then run the cable buried direct into the ground. At the other end, terminate into a 30mA RCD if necessary to protect cables if buried in the walls <50mm deep.
    Did I miss anything?!
    Thanks for all your thoughts and comments.
     
  15. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Providing the protective device gives fault protection then yes. I would personally use an installation method that avoids the supply cables being in the wall as an overall rcd will lose your division of circuits. So either install surface or used earthed containment for buried cables.
     
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  16. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    as it's a new build, the SWA can go direct to the CU, thus eliminating the need for RCD if cable is buried in wall/s for part of the run.
     
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  17. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
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    What size main protective bonding is required because this will influence the core cpc of the swa cable.
     
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  18. Ian1981
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    Ian1981 Electrician's Arms

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    North east
    I'd imagine it's 10mm bonding conductor sizes being domestic and a maximum 100 amp supply
     
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  19. The General
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    The General Electrician's Arms

    Yes, domestic, 10mm bond to gas/water - thanks for checking though!
     
  20. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Needs to be 25.0 if your are burying it the ground on an 80A protective device.
     
  21. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    What does?

    Edit
    You mean the swa
     
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  22. Ian1981
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    Ian1981 Electrician's Arms

    Location:
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    As it's a new build I doubt you will need to provide a protective bonding conductor to the water as the incoming supply externally will be in plastic but you'll have to see
     
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  23. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    why?
     
  24. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    16mm swa buried is only 78 amps ambient temperature 20 degrees and that's without applying the 0.9 correction factor as cable is buried for ambient temperature for 30 degrees plus any other factors like depth etc
     
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  25. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
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    I always opt for the three phase rating with a three core but that is another debate but yes either way it is undersized if you use 16.0
     
  26. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Just trying understand - Table 4D4A for direct buried at 0.7m, ambient 30, ground ambient 20 and conductor 70 is 78A. So question is, why pick 30 ground temp to further derate to?
     
    Last edited: Aug 23, 2017
  27. Ian1981
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    Ian1981 Electrician's Arms

    Location:
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    0.9 is applied where overload protection is required and prevents the cable going over temperature when overloaded.
    The factor is used because cable ratings for cables in the ground are based on a ground temperature of 20 degrees compared with 30 degrees for other reference methods.
    See regulation 433.1.203
     
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  28. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    which is all wrong. rarely will the ambient temperature rise above 20dg. C. in this country. once you are down 2 ft. underground the temp. is down to around 10dg.C.
     
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  29. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    What about the part of the cable that comes out of the ground?
     
  30. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    fit a fan.:D
     
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  31. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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  32. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    as an experiment, try shoving 100A down some 16mm for an hour. then measure the temp. of the cable. if you read >40deg.C. then your temp gauge is wrong. :eek:
     
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  33. Wilko
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    Wilko Electrician's Arms

    Location:
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    Business Name:
    Wilko Electrics
    That part would be ok - 89A clipped direct or 97A in free air ?
     
  34. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Based on 30 degrees tho yes?
     
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  35. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Only if you pay my electric bill
     
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  36. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Deleted while thinking a bit more ... might be a while ...
     
  37. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    we not in the sahara . not even spain. 30deg. my arse. we'd all be like essex boys at that temp. but with real tans.
     
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  38. Ian1981
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    Ian1981 Electrician's Arms

    Location:
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    Well it errs on the side of safety but I didn't write the regs , send any complaints to jpel/64 ha
     
  39. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    haha, trouble is that the regs. are so pessimistic. written to cover arses. as a famous person once said." rules are for the strict adherence of fools, and for the guidance of wise men"
     
  40. Wilko
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    Wilko Electrician's Arms

    Location:
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    Business Name:
    Wilko Electrics
    Yes, 4D4A is for ambient air temp of 30. Still thinking ...
     
    Last edited: Aug 23, 2017
  41. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    The overall ccc of the cable needs to be the lowest rating if installed using different reference methods using the most onerous method
     
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  42. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Got it - thanks Ian :)
     
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  43. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    I would assume tests would have been performed and the appropriate ccc recorded as we see them today.
    Perhaps cables buried in walls and installed in trunking etc are more like at an ambient temperature of 30 as there's no free air?
     
  44. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Don't thank me yet wilko as someone might come along and shoot me down in flames.
    It's how I understand it and it's written in be 7671 so got to go with that don't we?
     
  45. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Ha ha, always on the cards !
    I needed to re read 433.1.203 a few times. Using an 80A fuse for overcurrent protection of a buried cable then the cable needs to be rated >=89A and clearly 16mm isn't when it's buried :)
     
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