Joist Insulation + Electrics

Hi,

I've insulated underneath my floor (between the joists) using 100mm celotex insulation sheets. The space is in the cellar and full or rubble so the room above gets very cold in the winter. The space has some electrical wiring in it (from the main fuse box) and some gas pipes (the main feed to the house)

Wherever possible I've tried to avoid putting the insulation over electrical cables but there have been a few points where it's been necessary (mainly where the wires go up to the room above through the floorboards). I have re-pinned as many of the wires as possible so that they sit on the joists themselves.

My question is this, is this ok or is this likely to prove to be a fire risk.



The red arrow indicates a cable that goes the length of the rafters and up though a hole the other end (around 2m).

We really need to now what the CSA of the cables are, the MCB/Fuse that is protecting the cable and the distance the cables is surrounded by the insulation.
Would then be able to give you a better idea of the current rating efffect that the insulation is have on the cable.

I'm not sure what the wires are connected to unfortunately so not sure how i can give this information..

Thanks for any advice

I wiil try and help you out as much as I can given the limited information.
Cables should be run in and fixed in spaces, so they are not coverd in insulation as this can/will effect the cables current carrying capacity.
When it is impossible to run/fix cables without them being covered in insulation, a calculation can be made on the effect it will have depending on the length the cable is run through the insulation.
So you insulation 100mm thick, if we say the cables are running straight through, thay are covered by a maximum of 100mm.
So de-rating factor of 0.78 is applied.
This means the cables current carry capacity is reduced to 78% of it's original rating.
your standard flat grey T&E cables can carry
1.0mm=16amps derated to 12.48amps
1.5mm=20amps derated to 15.6amps
2.5mm=27amps derated to 21.06amps
4.00mm=37amps derated to 28.86amps.

So it now depends what breakers/fuses are protecting these cables, as the protective device should be lower than the cables rating.

You are probably okay on lighting circuits, which are usually fairly geneoursly rated cable, but immersion heaters and electric showers, can be rather borderline in terms of cable rating if enclosed in insulaton.

Socket rings on 2.5mm² should also be OK, as the cable needs to have a rating of at least 20A, as long as you have MCBs, not rewirable fuses. If you've got the latter then all all the values above have to be multiplied by 0.725, so 2.5mm² drops to 15.3A.