running electric cables to sockets in PIR boards

[JP_]

So, plan is 50mm insulation board, with 35mm backboxes, plasterboard, and skimmed.
My idea, which I hope will maintain vapour control and also give space for cable to avoid overheating risk, is to totally remove the 50mm board where the cables run from below the floor to the socket, and replace with 25mm board, leaving a 25mm gap, which along with the 10mm (or so) plasterboard will provide the right size space for the back box, and give room for cable and channel to run.

Does this sound like a good idea? How do people usually run cables over, or through, PIR? The insulation and plasterboard will both be stuck with foam, so no batten spaces etc.

Here's a pic I just made ...

 

[ban-all-sheds]

Will the areas of 25mm leave you with a low enough average U-value?

 

[JP_]

Yes, it will only be a few cm wide and about 40cm high.

 

[EFLImpudence]

Do you need to make a channel?

The cable touching the plasterboard allows higher current than not.

 

[JP_]

there will be a gap of 25mm between the insulation board and the plasterboard. I could run it through a bit of plastic pipe if that would make it safer?

 

[EFLImpudence]

I am saying there is no need for the 25mm. gap so that the cable touches the plasterboard.

No, the conduit will reduce the current carrying capacity too much for a standard ring circuit if that is what it is.

 

[JP_]

Ah. But, then the plasterboard wont fit perfectly onto the insulation board if the cables are running between the two.
Don't those PVC cable channels act as a conduit? They are used all over the house.

 

[oldbutnotdead]

If the cables are clipped to a wall (or in contact with the plasterboard) then the wall/plasterboard is deemed to act as a heatsink so the cables don't get hot. If the cables are enclosed in conduit then that heatsinking is deemed to not happen. Your best bet (well BCO was happy when I did it anyway) was clip the cables to the masonry wall and cut holes in the insulation/pb where the socket boxes were (insulation bonded to pb and the whole lot dot and dabbed to the wall, 70mm PIR so well within regs for U value of the overall scheme).

 

[ban-all-sheds]

Ah. But, then the plasterboard wont fit perfectly onto the insulation board if the cables are running between the two.

It doesn't have to.

A link to PVC cable channels

 

[JP_]

link was changed, should be: https://www.screwfix.com/p/tower-pvc-channel-25mm-x-2m-40m-pack-of-20/46474

 

[EFLImpudence]

Ah. But, then the plasterboard wont fit perfectly onto the insulation board if the cables are running between the two.

Won't it push the cable in and make a dent? Or just dig out enough for the cable still to be touching the plasterboard.

Don't those PVC cable channels act as a conduit? They are used all over the house.

The channel is only meant to hold the cable in place on masonry walls for plastering. It then has the masonry and plaster to absorb any heat.

Acting like conduit is not a good thing. It derates the current carrying capacity of the cable.



I may now be sounding over cautious; I am just saying that you do not need to bother fitting the 25mm. insulation.

 

[ban-all-sheds]

http://www.awin1.com/cread.php?awin...r-pvc-channel-25mm-x-2m-40m-pack-of-20/46474?

 

[JP_]

Looks like the forum turns any Screwfix link into an affiliate link. Then the break for some reason.

 

[JP_]

Efl, I'll be using foam to glue the boards together, the cable could be embedded in insulation and foam. That would be bad wouldn't it?

 

[ban-all-sheds]

You really should be discussing the routing and exact installation methods with your electrician. Cables can, if you wish, be completely surrounded by PIR insulation - they just have to be sized accordingly.