Reference method for cables above insulation

[grahamgibson]

Folks,

Apologies if this has been covered already, but I've been struggling to find a definitive answer to this.

What reference method should be applied to cables clipped along the tops of joists in a loft (perpendicular to the joists) with insulation underneath?

100 and 101 have the cable in contact with a conductive surface (wood or plasterboard) - what if that conductive surface is air?

On the one hand, wood/plasterboard may conduct heat away better than air, but on the other hand the CCC of cables is higher in free space than clipped direct.

Sorry if I'm being incredibly thick here, but I would have thought that cables above insulation in this way would be better than having them beneath the insulation albeit in contact with wood/plasterboard.

Cheers folks,
Graham

 

[PrenticeBoyofDerry]

As the cables are above the insulation and not within it, I would opt for clipped direct, ref method C.

 

[riveralt]

What type of circuit are you talking about here?
A lighting circuit with 1mm² T&E in insulation is still going to be more than enough for standard lighting.
It tends to be the more power hungry circuits - like a shower or a ring final circuits for example - that you need to be more aware/concerned.

 

[JohnW2]

As the cables are above the insulation and not within it, I would opt for clipped direct, ref method C.

FWIW, so would I.

Kind Regards, John

 

[securespark]

Not IM 100 or 101?

If it were the 16th, I would say it was installation method 15: Sheathed cables installed directly in a thermally insulating wall or above a thermally insulating ceiling, the cable being in contact with a thermally conductive surface on one side (otherwise as Reference Method 4).

But that's passed, so I would say 100 or 101, depending on the insulation.

Clipped direct is only for situations where there is no insulation involved, isn't it?

 

[JohnW2]

Not IM 100 or 101? ... But that's passed, so I would say 100 or 101, depending on the insulation. ... Clipped direct is only for situations where there is no insulation involved, isn't it?

Maybe we're interpretting the OP differently. I read it as meaning that the cable was clipped to the top of joists, but with insulation below it (presumably between the joists) and with the cable not in contact with the insulation. If that's the case, I would have said that insulation was not 'involved' and therefore that it would count as 'clipped direct' (just as it would if the insulation weren't there). However, maybe I've interpreted the situation wrongly.

Kind Regards, John

 

[grahamgibson]

The insulation is about the same height as the joists (between the joists), so in a worst case scenario the cable would be touching the insulation on one side.

Intuitively, I would have thought that the CCC would be higher if there is insulation beneath the cable and nothing above it, rather than insulation above it and wood or plasterboard beneath it, but maybe not.

I guess I'm wondering if air above conducts heat away better than wood or plasterboard below, if you know what I mean.

 

[JohnW2]

The insulation is about the same height as the joists (between the joists), so in a worst case scenario the cable would be touching the insulation on one side.

Arguably 'marginal', then, I suppsoe. If there were a small airgap bewteen cable and the insulation below, I would personally be happy to call it 'clipped direct'.

Intuitively, I would have thought that the CCC would be higher if there is insulation beneath the cable and nothing above it, rather than insulation above it and wood or plasterboard beneath it, but maybe not.

I don't think there is anything wrong with your intuition.

I guess I'm wondering if air above conducts heat away better than wood or plasterboard below, if you know what I mean.

It's obviously complex. Whilst most solid materials (wood, masory, plaster, plasterboard etc.) will usually be better conductors of heat than (still) air, the existance of convection, and the fact that air can move, means that 'being in free air' results in pretty good heat removal.

Kind Regards, John