Acoustic insulation & cable runs

[Tuttle]

I'm running cables under first floor floorboards. In one place I have a 2.5mm ring main (on a 32A breaker), a couple of 1.5mm lighting circuits (on 10A breakers) and a 1.5mm three core and earth cable for smoke detectors. The floor consists of 170mm deep joists, plasterboard ceiling below, 100mm rockwall insulation, which leaves a 70mm air gap between the top of the insulation and chipboard flooring.

There is not enough room to clip all of the cables clear of the insulation. I plan to clip the ring main cables to the joists above the insulation. For lighting circuits that can not be clipped clear of the insulation I could either lay the cables on top of the insulation, clip to joists below the level of the insulation, or lay them on the plasterboard ceiling completely under the insulation.

Not keen on lying cables on top of the insulation, so I think that my preference is to lay the lighting cables on the plasterboard ceiling below the insulation (ref method 102). I just wondered what others would do in this situation?

 

[telectrix]

on top of the insulation is better for allowing cooling. as you have 70mm above the insulation, that puts the cables > 50mm below floor level, so that's the way i would do it.

 

[Tuttle]

What just lying on top of the insulation?

I wasn't sure if the cables would sink into the insulation over time or maybe compress the insulation reducing its performance.

 

[telectrix]

can't see a problem with that. would need to be seriously heavy cables to sink in.

 

[Engineer54]

What just lying on top of the insulation?

I wasn't sure if the cables would sink into the insulation over time or maybe compress the insulation reducing its performance.

Jesus, ...would you actually lay the cables on the plasterboard and have 100mm of insulation material laying on top of them?? Bit of commonsense needed here, to what sort of level do you think a lighting cable or two is going to compress this 100mm of insulation??


Besides...why can't you clip cables to the other side joist above the insulation??

 

[telectrix]

Jesus, ...would you actually lay the cables on the plasterboard and have 100mm of insulation material laying on top of them?? Bit of commonsense needed here, to what sort of level do you think a lighting cable or two is going to compress this 100mm of insulation??


Besides...why can't you clip cables to the other side joist above the insulation??

think he's running out of space, getting to within 50mm of floor so the wood butchers can nail his cables.

 

[Tuttle]

There's notches for pipes as well, so that takes up some room (I didn't mention that before).

My first thought was to lay the cables on top of the insulation but then I guess I started over thinking it. Got the building inspector and potentially the ELECSA man looking over my shoulder so I started thinking about what they might complain about.

Thanks for the replies. :smiley2:

 

[Octopus]

This must be a very small room if there is only one run for the cable to travel along!

 

[lauriehurman]

We instinctively think that putting the cable in free air is best for cooling but if you read the installation methods carefully you realise that this is not true.

In all cases the highest current capacity comes from contact with a heat conducting material. Burying a cable in plaster is better than leaving it in free air - because plaster is a better heat conductor than air.

There is no point working hard to keep the cables >50mm from the upper floor and then putting them on the plasterboard of the ceiling. The regs make no distinction between top and bottom. If you are <50mm from the surface you must comply.

Laurie

 

[Tuttle]

This must be a very small room if there is only one run for the cable to travel along!

They come through the wall into an extension and end up in the same run for a short distance (1m) before they can go thought a hole in a joist. You are right though, after the first 1m there is no problem.

 

[Tuttle]

We instinctively think that putting the cable in free air is best for cooling but if you read the installation methods carefully you realise that this is not true.

In all cases the highest current capacity comes from contact with a heat conducting material. Burying a cable in plaster is better than leaving it in free air - because plaster is a better heat conductor than air.

There is no point working hard to keep the cables >50mm from the upper floor and then putting them on the plasterboard of the ceiling. The regs make no distinction between top and bottom. If you are <50mm from the surface you must comply.

Laurie

That was my thinking as well, putting a cable under insulation seems like a silly thing to do on the face of it, but it is in contact with the plasterboard so the heat has somewhere to go.

Looking at the cable ratings for 1.5mm T&E, in a stud wall with insulation in contact with plasterboard (ref method 102) is 16A and in conduit or trunking on a wall (ref method B) is 16.5A, which is not a big difference. I'm not sure what the rating would be lying on the thermal insulation, I'm guessing that ref method B would be the closest match (?) so again about 16A.

Also, I think that the 50mm rule only applies to cables "passing through a joist or ceiling support". I agree that it is best to keep cables 50mm away where possible but I don't think that this is essential where it is not possible.