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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.

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