Cable rating for insulated blockwork

I'm in the process of adding some thermal insulation to the single leaf wall of a utility room. The existing power sockets are wired as a radial circuit with 2.5mm^2 T&E fed from a 20A MCB. I'll be sticking 65mm foam insulated plasterboard over the cable and am uncertain what derating factor I should apply to the cable.

None of the installation methods shown in the cable section of the DIY Wiki show an insulated masonry wall . I'm assuming that the heat loss through the blockwork will be less than through a thin layer of plasterboard and that this arrangement would be equivalent to something between methods 102 (in a stud wall with thermal insulation with the cable touching the inner wall surface) and 103 (in a stud wall with thermal insulation with the cable NOT touching the inner wall surface).

If method 102 applies then I'd be Ok with the 2.5mm^2 and 20A MCB but if it's as bad as 103 then I'd need to go for 4.0nn^2 and a 16A MCB.

What derating factor should I be assuming for this arrangement?

Reply to
Mike Clarke
Loading thread data ...

I *think* that it's method 20. Essentially all the heat is lost to the masonary - so reducing the little that is lost to the air is neither here nor there.

I don't think that's right. There's lots of blockwork to absorb the heat. There's hardly any plasterboard, and once it's warmed up, there's nowhere for the extra heat to go.

An expert will be along shortly to confirm.

(First rule of Usenet: The fastest way to get a response is to post something factually incorrect.)

Reply to
Martin Bonner

I would be included to treat it as Reference method A - that will be slightly more pessimistic that what you actually describe.

formatting link

So that would mean you are ok with the present cable CSA and protective device.

Reply to
John Rumm

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.