Cable surrounded by thermal insulation

[T10Ken]

Hi guys

I live in a first floor maisonette and have just taken the floorboards up to put thermal insulation (Rockwool) under the floor in an attempt to soundproof my flat from the neighbours downstairs. Unlike my loft where I was able to lay the cables on top of the insulation, it proved impossible to keep the insulation away from the cables under the floor. Whilst the noise is no longer a problem I’m now concerned about the cables possibly overheating.

The cables that I’m concerned about is a ring circuit serving the lounge, hall and bedroom (2.5mm T&E protected by a 32A mcb). Basically the entire cable is surrounded by the insulation for virtually it’s entire length of approx 25m. I’ve done a bit of reading about cable selection, etc and note that the current carrying capacity for T&E installed in a thermal wall (I'm assuming under a floor is the same thing) is 18.5A before correction factors are applied for ambient temp, grouping, thermal insulation, etc. The tables say that if the cable is run for more than 500mm in thermal insulation that it is to be derated by 50%. Am I correct in thinking that it is 18.5A x .50 = 9.25A? Or does the fact that you have already started off with the cable at 18.5A take into account its being surrounded by thermal insulation? If the latter is the case and the fact that it is a ring circuit, would I be safe to continue using the 32A mcb that is currently fitted? If not should I consider having a smaller MCB fitted to protect the circuit (which shouldn’t be a problem as these rooms are unlikely to draw a large current anyway)?

Thanks for your help.

 

[JohnD]

you could make some little channels ot troughs in plasterboard (which is fireproof and very easy to work) and put them over (or under) yor cables so they are uninsulated on at least one side. You can seal the joints with a little patching plaster. You have to make an enclosure or the insulating material might flop over them. It does not matter if they have an uninsulated path to the floor or the ceiling.

6A lighting circuits in 1.5mm are so lightly loaded it doesn't matter if they run in insulated. Electric showers may be running at max capacity for extended periods and it would be incredibly expensive to increase their cable CSA so it is much better to move them out of insulation.

 

[T10Ken]

Thanks... I realise the lighting cables are not a problem and there are no shower cables in that area. My only concern is the ring circuit serving the living room, bedroom and hallway. I realise that ideally it would be best to keep the insulation away from the cables. However due to the amount of insulation needed to effectively soundproof the flat and the amount of added work it would take to construct some sort of barrier, I was hoping to just protect the circuit with a smaller mcb. Again this circuit would be unlikely to draw a high current so a 20A (or possibly even a 16A) MCB shouldn't be a problem. My main question being the method that you derate the cable by. I appreciate that it is derated by 50%. But is this from a starting point of 18.5A (the rating for installing it in a thermal wall)? Which in a way seems like you are derating the cable for thermal insulation twice. Hope that makes sense.