I am in the process of insulating under our kitchen floor, the plan being to fit the Celotex between the joists and fill any gaps around the edge with PU foam. However, some of these gaps also have cables passing through so just to check does PU foam react like polystyrene with PVC? Secondly, does passing through 100mm of insulation have any significant effect on the cable ratings?
Richard
No, it does not leach the plasticiser in the way EPS does.
Yes, you would need to de-rate the cable by a factor of 0.78 [1]
So take the column C "clipped direct" rating for the cable :
So for example a 2.5mm^2 T&E with a clipped direct rating of 27A would be reduced to 27 x 0.78 = 21.06A (which is only just high enough for use in a 32A ring circuit)
[1]
Should also have added, that it does depend a bit on what you mean bu "in insulation". The worst case is literally that - surrounded on all sides by insulation. Some cases are less onerous however - say wan its a cable against a conductive surface like a wall or plasterboard, with an insulation covering. See the installation methods section here:
Thanks - I did something similar recently, but 50mm celotex, and routed the cables through the floor/insulation using some flexible c.3cm quite heavy duty cable tidy/trunking fixed with foam.
The 'worst case' was 4 cables, and while there could be quite a lot going on (oven, kettle, toaster etc), my method was reasonably neat and seemed like it might be effective. But not, I see now, one of the reference methods. I've yet to finish skirting etc and could easily take out the trunking and clip the cables to the joist a cm or so apart - might that be better?
Don't worry too much about matching the method, there are a much larger number of installation methods documented in the full version of BS7671. Its more the principle to follow.
So cable grouping is one - note that its based on the number of circuits rather than the number of cables. Also if a cable is expected to carry less than 30% of its grouped current rating, then you can ignore it from the count of grouped cables when working out the effects on other cables.
Spacing cable helps. Having them clipped to a thermally conductive surface helps - even if they are covered in insulation.
Unless the cables are stressed already, I'd not think it would. Don't know about reactions, but unless it outgases a lot, I'd imagine the space under there would mean it was pretty diluted, others may feel differently, we never had such worries in my day... I feel like leaning on an old gnarled walking stick near a farm gate looking into the distance... grin. Brian
Thanks John, thinking back now I should have extended the capping that extra 100mm. On the safe side I will either do what Rob did or block the PU foam from contacting the cables.
Richard
Donno but I'd side step the issue by cutting the celotex to be a snug push fit and simply taping any joints. Foamed in would make any future access behind the insulation a right PITA.
Dave I have two joists very close to the wall where the cables are passing below the floor. On one I can do what you suggest and was going to use the PU to fill any gaps. On the other wall there is very little space, although I will try to do as on the other side but suspect the time saving element will be simply to PU the gap.
Richard
When I insulated the loft I ran all but one of the cables (lighting only) across the boards - neater underneath but not as bad as onto exposed joists as they're easier to avoid. The one cable is on top of the insulation and directly under the chipboard. All the others have ~75 mm in insulation; that and a 6A mcb should be OK. Even if all of the LEDs were swapped out for 100W incandescent lamps it'd be like a grill upstairs and unlikely to happen.
Think I'd still try and avoid the PU foam if at all possible and loosely pack the joist/wall gap with some other insulation, rock or sheep wool and tape over the top.
Many thanks - you know your cables!
Yup lighting circuits are unlikely to be a problem since the cable is massively over-specced from a current carrying capability in most cases.
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