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Discuss need to fully understand ... and im just not getting it in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. andysparkfree
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    andysparkfree Regular EF Member

    Location:
    london
    Hi guys

    i need an idiots guide to the following and i know you guys know your stuff

    swa armouring as a cpc only

    now i think i know the following , but how to put it altogether and where 8.8 comes from is just confusing me , so if i start again and you good people

    so
    4 core 35mm swa has csa of 78mm
    and swa for a cpc needs to be at least 46 to be adequate

    as per attachments

    how do i actually find out the actual copper equivalent of the swa so i can say 35mm swa = 9.2mm earth

    really need to understand this as seems to be cropping up on a weekly basis
    and the more i read the more confused im getting lol

    thanks guys Screen Shot 2017-12-06 at 19.49.03.png Screen Shot 2017-12-06 at 19.49.24.png Screen Shot 2017-12-06 at 19.49.03.png
     
    Last edited: Dec 6, 2017
  2. polo1
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    polo1 Electrician's Arms

    Location:
    Glasgow
    Business Name:
    GW Electrical & Security
    Deleted.
     
  3. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Guidance note 8 suggests a figure of 8.5 copper equivalent ratio to steel and 1.68 for aluminium to copper ratio.
    So if you had 85mm steel then 85/8.5= 10mm copper equivalent.
    35mm steel divide by 8.5 would be 4.11mm copper
     
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  4. andysparkfree
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    andysparkfree Regular EF Member

    Location:
    london
    Ian thank you !
     
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  5. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    No problem
     
  6. Pete999
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    Pete999 Trusted Advisor

    Top Poster Of Month

    Location:
    Northampton
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    Agree Polo, not sure but it may be the OP is trying to find out the csa of the cpc for the cable he is specifying i, e, for example is the tabulated CU size is suitable for the 35mm2 he is speaking of, mind you that's how I look at it I may be wrong, but not disputing your answer.
     
  7. andysparkfree
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    andysparkfree Regular EF Member

    Location:
    london
    so 35mm 4 core swa with steel at 78mm / 8.5 = 9.17mm copper conductor
     
  8. andysparkfree
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    andysparkfree Regular EF Member

    Location:
    london
    Pete , is different job , as was pulled up last inspection for leaving cpc blank when steel armouring being used as earth , just trying to get things right
     
  9. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Yeah really if your considering using the steel armour as a protective bonding conductor at domestic level then to comply with the minimum CSA of 10mm copper conductors needed then 85mm is the minimum steel CSA you can use of copper equivalent
     
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  10. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    That table is a at best a rough guide, and at worst just plain wrong.

    There isn’t a direct ‘copper equivalent’ for use as a CPC as there is more to the calculation than just comparing conductivity.
    The simple answer is to use the equations given in BS7671 to calculate it as and when required, either the adiabatic equation or the simplified method. Actual steel CSA of the armour should be in the manufacturers data for the cable you are using.

    If you are considering the steel armour for use as a bonding conductor then it can be simplified to equivalent conductivity. Steel has roughly 9 times less conductivity than copper, so you can multiply the required copper csa by 9 to get the required steel csa. Using this method for a cpc would result in a ridiculously oversized steel csa!
     
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  11. andysparkfree
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    andysparkfree Regular EF Member

    Location:
    london
    yes , this one is office with no bonding conductors
     
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