Lighting wiring in/around/over loft insulation

[FarmerJon79]

After reading around this website regarding lighting cable in lofts I am very confused. Advice varies from loose cables over insulation to clip on joists under rockwool and "its not right but it'll be OK".

I am boarding the central part of my loft to allow access to a new boiler. The joists have been filled with rockwool, and then 2 layers of Knauf Space Board/Polyfoam has been laid, topped with 18mm chip. Other areas will have joists filled and then cross laid with 170mm rockwool.

What is the CORRECT method for installing 1.5mm T&E for lighting given that the flooring is for access.

I am really looking for a definitive answer, what the regs say, not what I can get away with.

 

[ColJack]

it's not just the lighting that's to be worried about..
it's the shower cable, the upstairs sockets ( if they come down the walls rather than up from the floor void ) etc..

it's all to do with cable correction factors..

if the cable is close to it's limit, then covering it with insulation prevents any heat from escaping, so the cable has more chance of melting and needs to have less load on it..

given that the average lighting circuit is 6A in a domestic situation ( or 10A at most ), and that 1.5mm cable can take 14.5A if clipped to a joist and covered with insulation ( ref method 15, installed above a thermally insulating ceiling and being in contact with a thermally conductive surface on one side ), then I recon your fine ..

 

[FarmerJon79]

Thats just it, you "reckon"! This is a domestic situation, 1.5mm, 6A MCB,
5x60W pendants (to be fitted with CFLs) 4xGU10s and 1x45w extractor.

Don't mean to sound rude, and I realise there are calculations, but there must be a best practice.

 

[ColJack]

exactly like I said.. clipped to the side of the joists, with the insulation put in the gap as normal..

 

[doormouse1]

Test results and best practice recomendations.

http://www2.theiet.org/Publish/Wire...rent_ratings_cables_in_thermal_insulation.pdf

 

[ColJack]

that makes sense than, since if it's at the bottom of the joist, and in contact with the ceiling also then it has 2 thermally conducting surfaces

there is the added possibility that the plasterer will stick a screw or nail through it when boarding the ceiling if you run the wires at carcase stage in a new build, but for domestic where the ceilings are already in then that looks like the way to go..

next house I do, I'm gonna wrap the joist in tinfoil and clip the cable to that.. big heatsink...