Moving Cables above 270mm Loft Insulation to Avoid Overheating


Postby Benchillian » Mon Jan 25, 2016 9:07 pm

I'm planning to upgrade my loft insulation to 270mm.

I've read much on this site and elsewhere saying that cables should be above the insulation to avoid overheating. And of course, lots say it's not worth bothering.

For safety, I'd prefer to move cables above the insulation, but as ever, there's not enough slack to do it.

So, is it acceptable practice to extend cables using junction boxes and extra lengths of cable? Or is there a better way?

Any advice welcome.
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Postby ericmark » Tue Jan 26, 2016 6:42 pm

With lighting cables at 1mm they are rated at 10.5 amp Reference Method 101 (above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness) so in real terms not a problem as the ceiling rose is used as a junction box and that is only rated at 6 amp so we never use a MCB rated over 6 amp or at least we shouldn't. And using LED lighting you are really talking about very few amps my upstairs lamps in total are under 230W in fact more like 60W and at 230W it uses 1 amp so really not a problem.

If you were really worried then using maintenance free junction boxes there would be no problem extending.

However power cables is another story. Although maintenance free works great with 5A at 45A used with for example a shower they are really no good and any joint has a risk of poor connections and as a result fire and the cable is so easy to over load. 16mm cable is still only rated at 46A Reference Method 101 (above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness) and 6 mm cable is down to 34 amp.

The normal method to extend power cables is to add a device. For a shower at 45A then adding an isolator is a good method to allow you to extend the cable.

What I would consider is with LED lights it is often better to have the lights spread around the room, clearly easier to add extra lamps before the installation goes in.
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Postby Benchillian » Wed Jan 27, 2016 11:31 am

Many thanks, ericmark.

Checked in the loft last night, and the only cable other than lighting is for the immersion heater. There's enough slack in that to move it above the insulation.

So just the lighting to deal with. Step 1 will be to replace the 50w halogen bulbs with LEDs.

I'll read up on some the things you've mentioned, but might come back with a couple of questions to check my understanding, if that's OK.

Thanks again.
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Postby ericmark » Wed Jan 27, 2016 9:50 pm

Start a new thread if you have more questions, as when I see answered thread I often don't look again.
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