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What derating table do I use for multicore armoured thermosetting cable in insulation.

Alan B

Taking submain through loft of a house, I think it only needs to be rated at 60 amps, 80 amps would be nice. But ideally don't want to be fighting with 25mm swa cable, I have found out the hard way that it makes life very difficult, getting it through the house and also routing inside a fused switch.

At the moment thinking that buried in insulation a 16mm 3 core cable would probably have a capacity of about 60 amps. Looks like I will have to clip to batons screwed on to roof beams to keep the cable clear of insulation, still easier than fighting with 25mm cable.


A 16mm 70 deg C thermoplastic cable is rated at 57 amps with reference method 100. I have seen plenty of domestic consumer units supplied with 16mm twin and earth and never seen any heat issues.

 Therefore thinking 16mm swa would be good enough, your thoughts.

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  • 0

    Other thought, is rather than use 16mm² 3-core, use 25mm² 2-core (it's about the same diameter) and if you don't trust the armour as a PE for downstream, add a 16mm² G/Y along side (all permitted these days). The 90 degree version with a method C rating of 146A should be good for 63A under almost any conditions.

       - Andy.

  • 0 in reply to AJJewsbury

    I had considered 25mm two core. How does the oval shape work with standard swa glands, do I need a special gland. The widest diameter of the cable is the same as 3 core, do I gain a lot in flexibility. It's  a TT earth, so as long as I have some sort of earth path it will be enough for the 100mA time delayed RCD I will have up front anyway.

  • 0 in reply to Alan B
    Alan B said:
    I had considered 25mm two core. How does the oval shape work with standard swa glands,

    A 2-core SWA is round, not oval.

  • 0 in reply to Alan B
    Alan B said:
    It's  a TT earth, so as long as I have some sort of earth path it will be enough for the 100mA time delayed RCD I will have up front anyway.

    RCDs can't provide protection against overcurrent .. for protection against overcurrent, you'd need to take into account the maximum prospective fault current, which is not the same as the minimum fault current which is used for ADS.

    Just worth a check there are no parallel paths that might increase prospective fault current.

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  • 0 in reply to Alan B
    Alan B said:
    It's  a TT earth, so as long as I have some sort of earth path it will be enough for the 100mA time delayed RCD I will have up front anyway.

    RCDs can't provide protection against overcurrent .. for protection against overcurrent, you'd need to take into account the maximum prospective fault current, which is not the same as the minimum fault current which is used for ADS.

    Just worth a check there are no parallel paths that might increase prospective fault current.

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