Derating cable passing across insulated cavity

If I pass a cable across (in one side out the other, not running inside) a cavity wall filled with pumped in cavity wall insulation, do I have to derate the cable as though its entire length were surrounded by such insulation (i.e. reference method 103)?

Does it make any difference if I pass the cable through inside a short piece of conduit.

Thanks,

Alex

Reply to
Alexander Lamaison
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I wouldn't think it would make a bit of difference for the 50mm of it's entire length is in insulation - the estate of 500 houses where I live were rewired by the council 15 years ago, almost all the cables are in the cavity, three weeks later the houses were insulated with blown fibreglass wool - the council must have known the order of works taking place and also the specs for the rewiring contract. I've not heard of anyone having any problems with anything overheating etc

Reply to
Phil L

Regardless of derating issues, the cable should be inside conduit anyway (which would take care of any derating issues anyway).

Reply to
Lobster

On Friday 01 February 2013 14:45 Alexander Lamaison wrote in uk.d-i-y:

I would not derate for that - the cable will conduct any hotspots away for that sort of length.

However, you should use conduit in case there's any polystyrene insulation in the cavity that will attack the cable.

Reply to
Tim Watts

So would you ignore the

Reply to
Alexander Lamaison

I don't have regs on me, but IIRC, derating only applies when more than 500mm of cable is inside insulation, a figure which always seemed to me to be rather high, but you are probably talking of no more than

50mm through cavity wall insulation.

In theory, if you use copper or aluminium pipe as conduit through thermal insulation with the pipe extending beyond the end of the insulation (into the brickwork, say), it should conduct the heat out of the insulation, although it might form a bit of a cold bridge itself.

Reply to
Andrew Gabriel

Table 52.2 has factors for 50, 100, 200, 400, and >500mm. There is a copy here:

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(scroll down a bit)

Reply to
John Rumm

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> (scroll down a bit)

Thanks. Nice article.

Reply to
Alexander Lamaison

On Friday 01 February 2013 16:15 Alexander Lamaison wrote in uk.d-i-y:

Yes. If you think about it, it's 15cm from the centre to either side and we are not talking about balancing red heat in the middle with cool on both sides - just a slight difference in cooling capacity.

I'm not sure I've ever seen it explicitly mentioned - but I maight be wrong. I've seen people working to rules of thumb of a couple of feet.

Reply to
Tim Watts

On Friday 01 February 2013 17:22 John Rumm wrote in uk.d-i-y:

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Thanks John - I had not seen that, despite looking previously...

Reply to
Tim Watts

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