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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Cables running in contact with thermal insulation https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1359408346 Message started by yoggo on Jan 28^th, 2013, 9:25pm

Title: Cables running in contact with thermal insulation
Post by yoggo on Jan 28^th, 2013, 9:25pm

Two situations I have found in my son’s recently purchased house:

Ring-main cables running side-by-side on top of concrete floor (suspended rather than in contact with ground – house was convertion from a former industrial building). On each side of the cables there is 25mm solid microfoam insulation with silvery foil coating – looks similar to material used in cavity walls. On top of that 35mm rockwool and on top of that floorboards. So the cables are in an insulated ‘tunnel’. Is this ok?

Lighting cables in loft. There is no thermal insulation at present. Packing on rockwool insulation just bought says “electric cables not to be covered, must be lifted carefully so that they run on top of insulation”. Most of the cables are clipped to the sides of the joists between 30 – 50mm down from top of joists and in many places pass through drilled holes in the joists and cross-bracing timbers. The rockwool will come to the top of the 170mm joists. To comply with instruction would be a major rewire of 18 50w halogen lamps. The circuit with the most lamps is serving 8 lamps. The cable is 1mm rather than 1.5 which I think is now commonly used. Some people, who may not be up to date on these matters, have told me lighting cables can be covered with any amount of insulation provided they are in contact with ceiling, joists, or masonry - ie I can ignore the insulation manufacturer’s instruction for lighting but NOT power cables.

I’d welcome your views on these two situations please.

Title: Re: Cables running in contact with thermal insulation
Post by Lectrician on Jan 28^th, 2013, 9:38pm

Lighting cables are usually fine - especially 1.5mm It is more shower cables and socket cables that the warning on the rockwool is aimed at.

The sockets situation is not ideal - The NIC did an article on this, and clearly showed that it was non-compliant for a 32amp ring circuit with 2.5mm in thermal insulation. 4mm 32amp rings are just about OK, or down rate the 2.5mm ring to a 20amp circuit (still as a ring).

Title: Re: Cables running in contact with thermal insulation
Post by yoggo on Jan 28^th, 2013, 10:36pm

Many thanks for info - at least its easy to make safe the ring main quite simply as you suggest.

Thinking about the lighting - I am not quite sure if you are saying I can carry on with laying insulation in the loft against the 1mm cables clipped to the sides of the joists. The lighting circuit with the most lamps (8 x 50w) is controlled by a single switch and fed from its own 6a mcb. It is unlikely the lamps would be uprated to a higher wattage and downrating would not provide enough light.

Incidentally the insulation will be kept away from the lights (which get quite hot) with galvanised loft insulation support boxes from Click Lighting. I have no intention of covering the boxes with insulation, just cutting insulation to fit round them.

Title: Re: Cables running in contact with thermal insulation
Post by Lectrician on Jan 29^th, 2013, 10:57am

Even 1.0mm² can carry around 10amps when run through thermal insulation. A 6amp lighting circuit will be fine on 1.0mm² or 1.5mm². It is things like shower circuits which would be affected.

For example, a 9.5kw shower may be fine on 10mm² clipped direct, but would need uprating to 16mm² if run in insulation upto 100mm thick. Above 100mm thick, you would need to increase the size further. This is why loft insulation companies make a point of saying cables should be above the insulation.

So, to tie up, lighting cables would be fine, but other cables need consideration.

Title: Re: Cables running in contact with thermal insulation
Post by yoggo on Jan 29^th, 2013, 8:44pm

Yes, I can get on with the job with confidence now - your help much appreciated.