Any electricians that can answer a wires through insulation question please?

First off yes I have an electrician doing the work, but it’s the weekend so can’t ask him at the moment.

I’m in the process of boarding one of my rooms with 100mm insulated plasterboard. Is it acceptable to drill up from underneath before fitting the board to the wall, into the front of the insulation against the plasterboard?

Effectively I’ll end up with a 400mm channel with 3 sides surrounded by insulation and the 4th side will have the plasterboard infront of it.

The wiring will then be run under the floor and pulled up to each socket.

i know derating is thing but not sure how relevant it is in this situation?

Cheers

It only really becomes an issue if it’s a load heavy piece of equipment you’re supplying and the run (within insulation) is long. It’s only 400mm though so you should be fine.

Is there a risk that the insulation can damage the cable sheathing over time or did I just make that up?

Could you just put some conduit into the channel and run the cable through that if in doubt?

You’re into the world of reference methods and current carrying capacity of wires.

Ideally you don’t want to be inside the insulation as any heat built up inside the wire has nowhere to go. More than 300mm inside insulation and you have to de-rate the circuit.

It only really becomes an issue if it’s a load heavy piece of equipment you’re supplying and the run (within insulation) is long. It’s only 400mm though so you should be fine.

Practically yeah but I want to make it up to current regs so want to do it properly.

Is there a risk that the insulation can damage the cable sheathing over time or did I just make that up?

Apparently so in polystyrene, PIR is OK I think?

Could you just put some conduit into the channel and run the cable through that if in doubt?

This was my original plan, to run some plastic pipe up the hole/channel, but then I started Googling and it suggested that trunking also has a derating factor.

You’re into the world of reference methods and current carrying capacity of wires.

Ideally you don’t want to be inside the insulation as any heat built up inside the wire has nowhere to go. More than 300mm inside insulation and you have to de-rate the circuit.

Thanks, I think this might be the definitive answer I need.

So if I go straight through the insulation and out the back I’ll only be going through 65mm (100mm insulation minus 35mm back box) and all the concerns go away.

This will make it slightly more difficult to fish the wires through but should be OK.

(Unfortunately I’m doing things in slightly the wrong order, otherwise this wouldn’t be an issue).

Trunking is a different reference method, so yes affects the current carrying capacity.

The best scenario is called “Clip Direct” which assumes the cable is directly clipped to something that helps dissipate the heat (the clipping isn’t always possible).

There are ways to fish the wires through that can be fiddly but sounds like that’s the better solution. If you can run bits of string through your electrician will thank you 🙂

Don’t worry, these things are often in the wrong order but so long as it’s not too frustrating that’s fine.

I did it in the right order! Clipped to the existing plastered masonry wall and then we cut a channel in the back of the insulation and foil taped the cuts so the boards would fit properly. Boards were Kingspan k118 65mm insulation with 12mm PB mechanically fixed plus foam adhesive.

Did have some intumescent back box liners to make airtight but they were a pain to fit so just used the foam adhesive.