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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

    Thanks all for the responses.

    Having lifted some floors and stuck a camera in I can see the submain cable is running in the soffit space, I think sitting on the soffit board with other cables. Not ideal, but ok at the moment because the entire installation is protected by a single 30mA RCD. But as soon as I replace the 30mA RCD with a 100mA time delayed RCD it becomes a problem.

    I can route a cable so that it's well above the insulation. Looking at 25mm or 16mm swa the difference in diameter between 2 core and 3 core is less than 1mm, although there is some oval 2 core available. I wonder how that would work in round glands?

    I have an alternative to the section going through the soffit, which is through 6 inch floor joists. As I can keep the cable more than 50mm from floor and ceiling I can avoid using armoured cable and the challenging minimum bend radius of about 168mm, 6.7 inches.

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  • 0

    Thanks all for the responses.

    Having lifted some floors and stuck a camera in I can see the submain cable is running in the soffit space, I think sitting on the soffit board with other cables. Not ideal, but ok at the moment because the entire installation is protected by a single 30mA RCD. But as soon as I replace the 30mA RCD with a 100mA time delayed RCD it becomes a problem.

    I can route a cable so that it's well above the insulation. Looking at 25mm or 16mm swa the difference in diameter between 2 core and 3 core is less than 1mm, although there is some oval 2 core available. I wonder how that would work in round glands?

    I have an alternative to the section going through the soffit, which is through 6 inch floor joists. As I can keep the cable more than 50mm from floor and ceiling I can avoid using armoured cable and the challenging minimum bend radius of about 168mm, 6.7 inches.

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