Electrical cables underneath loft boarding...

16 Mar 2009
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United Kingdom
I am currently re-boarding my loft (previous owner - botched job, nuff said) and taking the opportunity to lay more loft insulation in the process.

The existing insulation is only a few inches deep so I am topping up with 150mm deep Spaceblanket over the top. These are unrolled easily down the length of the joists and I have enough slack in all electrical cables to lift them to run on top of the new insulation.

Having added this top up insulation, it is proud of the joist top by 20-30mm in places. My plan is to lay the boards on top and just compress the insulation in the process (have tested this and it works no problem).

My question is - is this okay from an electrical cable perspective. Cables are on top of all the insulation, but will be pretty tight against the underside of the loft boards because I have compressed the insulation slightly. Is that okay?

Any thoughts welcome!
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In the regulations we have four scenarios
called 100
called 101
called 102 and
called 103.

From your description you are using 102 this gives a 2.5mm cable a rating of 21A however if you are using 103 the rating drops to 13.5A in all cases the cables are fixed i.e. they have cleats of some type holding them to a beam or ceiling or floor. Cables are not free to go where they like.

Once on hides cables they come under the 50mm rule and if not 50mm deep with need protection which can be done by fitting an RCD or by using Ali-tube cable or by using conduit. In the latter case then it comes under another set of installation rules not the four shown.

Basic idea cables get hot and must be able to cool by touching something which can conduct the heat away. A beam or circulating air will do this. And the current you can pass through a cable depends on cooling. With lights rated 6A on a 1.5mm cable there is unlikely to be a problem. With power especially a shower supply cable one has to be very careful. PVC cable is designed to run at up to 70degC and some of the others for example Ali-tube cable can run up to 90degC and of course as well as cable damage due to over heating we must also consider what it is attached to.
you do realise in compresing the insulation by any amount, you are reducing its effectiveness to insulate
Thanks both.

I was aware it reduces the effectiveness of the insulation - but I assume it will still be a lot better than what was there before (which was practically nothing).

Sounds like I am okay with the cabling in terms of heat - they are all lighting cables (no showers or anything else high rated). I hadn't considered the 50mm rule though and I guess that with the insulation pressing the cables up to touch the boarding I had better put the cables into conduit for protection.

Thanks both for the pointers.
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Sorry - one more question...

I have an RCD on the main house fuse box (which all the aforementioned cables ultimately feed to). Is that good enough - or do I need to use conduit as well?

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