Derating cable under internal wall insulation

[StuB]

I have a 10mm T&E cable on a solid brick wall under a steel channel running from the ceiling to half way down the wall and then exiting through the brick wall to a 32A 7.4kw car charger that may run for several hours at a time. It's on a 40A MCB.

I'm looking to install wood fibre internal wall insulation on this solid brick wall and the insulation will likely be about 75mm thick, certainly less than 100mm.

If the cable was left in place and the insulation placed over the top of it presumably the cable would be derated but what derating reference method should be used to work out by how much it is derated?

If needed the cable could be moved away from the brick to in front of the insulation and just behind the plasterboard in which case I think would be method 102 would apply?

Alternatively as it's sandwiched between brick and insulation do they class the brick as insulation in which case method 103 would apply.

Any thoughts?

 

[blup]

Leaving the cable against the brick wall will significantly reduce the derating requirement as it against, and touching, a cold external wall.efficiency of the insulation behind it, What do you calcs show for each method?

 

[StuB]

First off I'm not a sparky just wanting to install internal wall insulation and want to make sure I'm not causing problems.

From what I can see a 10mm cable clipped direct will carry 64A and under method 102 it would be 47A both of which are fine for the 32A it'll carry and the 40A MCB but that is designed for just plasterboard between the cable and the internal room.

I was under the impression that being against the brick would be good thing and when the sparky installed the car charger I asked them about potential insulation over the cable and they said they'd be happy with it

However I had a conversation in passing with another sparky who said they thought it would be more insulated than method 102 as while brick is more conductive than plasterboard there is a lot more of it and he wasn't sure what the derating would be hence me asking here.

If it gets classed as method 103 with insulation all around it then the cable rating drops to 32A which means the MCB would need changing.

Ideally I'd like to just leave the cable where it is assuming it's safe to do so.

 

[davelx]

I was under the impression that being against the brick would be good thing and when the sparky installed the car charger I asked them about potential insulation over the cable and they said they'd be happy with it

However I had a conversation in passing with another sparky who said they thought it would be more insulated than method 102 as while brick is more conductive than plasterboard there is a lot more of it and he wasn't sure what the derating would be hence me asking here.

If there's a "lot more of it" then it will absorb a lot more heat for a given temperature rise. It's a bigger heat sink!
Also the thermal conductivity of normal bricks at about 0.6W/m/K is a lot better than plasterboard at about 0.038W/m/K and their specific heat capacity is also a lot higher so that they can absorb much more energy per unit mass for a given temperature rise

 

[StuB]

If there's a "lot more of it" then it will absorb a lot more heat for a given temperature rise. It's a bigger heat sink!
Also the thermal conductivity of normal bricks at about 0.6W/m/K is a lot better than plasterboard at about 0.038W/m/K and their specific heat capacity is also a lot higher so that they can absorb much more energy per unit mass for a given temperature rise

Thanks for the reply.

The brick being a bigger heat sink was my original thinking and to me it makes sense that it can dissipate lots of heat.

Their argument was that while the thermal conductivity of brick is much higher at about 0.6W/(m·K) and normal plasterboard is around 0.15W/(m·K) the solid double brick is 225mm thick while the plasterboard in only 12.5mm (plus a bit more for plaster) therefore it would dissipate the heat to internal room faster than the brick would to the outside.

0.038W/(m·K) would be the thermal conductivity of wood fibre insulation but we're talking about moving the cable in front of the insulation but behind the plasterboard.

It's not a sparky I've used before however and it was a comment in passing but I thought I'd check while it's currently easy to alter things if needed.

 

[davelx]

But the brick's much higher thermal capacity would more than offset that, I would think.

There are so many variables that the only way to know for sure would be to experiment with the specific conditions of your case.