Keeping cable away from insulation in joist void

[marshr02]

Any thoughts on this; 200mm joists with 100mm of mineral wool quilt to be installed in lower half. Have cables going across joists through centrally drilled holes. Left as is the mineral wool will be squeezed/touching against this cable, meaning the cable CCC is too low. I was going to fix some 9mm ply under the cables to hold off the insulation. My question is; either I fit some ply with an air gap of say 20mm to the cable, or my preference is the ply fits tight to the underneath of the cable, with the insulation below - would this second option still be ref B (cables in void). Thanks for any help.

 

[telectrix]

what size cable and what is the Ib?

 

[marshr02]

4mm cable, worst ref method should be 'B' (in void) leading to CCC of 30A, cb downrated from 32A to 20A to be on the safe side. So I need to ensure ref method B is the worst case,

 

[telectrix]

there is no ref. method B for T/E. table 6D1 in red osg. gives 32A ref. B. but in your case, the cable is not enclosed in trunking or conduit: there is no insulation above the cable ( heat rises ): i would rate the 4mm at 32A. tin hat on.

 

[marshr02]

Thanks for reply. 32A is clipped direct in the open, not sure it's defendable to say cable installed in a small void touching insulation is still method 'C'. Anyhow .... didn't want to get drawn into that old chestnut of t&E not having a reference method 'B'. For the purposes of my assumptions, and the question if the insulation wasn't there then this is ref method B.

 

[Richard Burns]

I would say it is a multi core cable in a void and therefore ref B.
Since the insulation is effectively the surface on which the cable rests it can be discounted from consideration (unless you are using a reference method based on thermal conductivity of a surface).
If you have 4mm cable on a 20A breaker then you are fine, I think.
Make sure they are not going to top up the insulation over your cable!

 

[Spoon]

The recommended thickness of insulation is 270mm. Someone is going to upgrade from 100mm at some point, but I suppose you have to do calcs to what is there now and not what might possibly happen in the future.

 

[marshr02]

The recommended thickness of insulation is 270mm. Someone is going to upgrade from 100mm at some point, but I suppose you have to do calcs to what is there now and not what might possibly happen in the future.

Yes, for a loft. This is separating floor in the middle of property. The 100mm is for acoustic and fire protection. Cheers

 

[Tuttle]

I've recently done a similar thing, I drilled the center of the joists, ran the cables above the insulation and assumed ref method B.

 

[marshr02]

Richard - thanks for reply. (Although breaker is 20A, I would be happy to uprate to 32A, if load requires it later so .... want to ensure ref methods are okay for 32A - or near enough) I thought that 'C' and 'B differ in how much air movement and thermal conductivity is possible, with B relying on SOME thermal conductivity through the conduit/trunking or surface being clipped to .... so if insulation was one of the surface then this is compromised. Obviously this is splitting hairs to some degree - thanks for your opinion - thinking of using ply sheet tight under the cables with insulation tight below...

 

[marshr02]

I've recently done a similar thing, I drilled the center of the joists, ran the cables above the insulation and assumed ref method B.

thanks for your reply -

 

[Tuttle]

If there was no insulation then cables running through joists would be in free air, not touching anything. Even if they were lying on a ceiling they would probably only touch here and there as they are not perfectly straight and flat. I would have thought that situation wouldn't be much better than running above insulation, but that's just my opinion.

 

[marshr02]

Thanks Tuttle - it's opinions I'm after My concern is the cables will nestle in the insulation rather than skim over top (which I would be happy with)

 

[Tuttle]

Thanks Tuttle - it's opinions I'm after My concern is the cables will nestle in the insulation rather than skim over top (which I would be happy with)

I know what you mean. I found a picture of what I did in case it helps at all.

 

[Richard Burns]

The problem you have is that the only installation methods that permit 32A and above for 4mm are reference method C and better in terms of heat removal.
Reference method B is working on limited air flow restricting heat loss, but the difference from B to A (conduit in insulated wall) is small so having a larger air space should be better than A and since normal conduit is B having a 100mm space should give you more CCC, but then you are into designing the circuit for a specific instance which would require more capability than I would be prepared to apply at my skill level.
I am assuming you are talking about a mid level floor and the insulation is acoustic rather than heat.
In a lot of cases it is considered that running cable in between floors is reference method C (which I sort of agree with even though the regs do not) so you might consider this situation as C in this case but it would only be backed up by common practice.
Other than that try a 25A breaker which would be fine and give more flexibility.

 

[marshr02]

I know what you mean. I found a picture of what I did in case it helps at all.

View attachment 18957

Thanks Tuttle - that's exactly where I'm at. Looks okay.

 

[marshr02]

Thanks Richard. Yes - mid level floor - acoustic & fire protection. Don't seem to be able to get 25A RCBOs for this contactum CU. Looking at Tuttle's picture, this is good enough for B in my opinion (which I would be happy to put 4mm on a 32A breaker as CCC quoted at 30A is near enough given other positive variables) My problem is I'm installing from below, and so later when the insulation is installed who knows how far up it could be pushed. Summary - I would be happy with Tuttle's installation, but am going to put ply under cables to prevent the insulation being squashed higher into cabling....hope that makes sense....thanks for all the great replies/help