Loft Ring Main touching insulation one side

[frank999]

Hi,
I have a loft ring main, I am hoping to clip the cable to the floor joists, and where it crosses another joist (ie the solid blocking) trail it under the blocking/joist and back up the other side continuing to clip as I go (rather than drill a hole), where the cable is pushed under the 45mm width joist it presses against a Knauf Earth wool mineral quilt, this situation also occurs where the cable will run under a 203mm steel beam (ie it presses against the insulation one side and steel t'other).
Is this a problem regarding de-rating, its only 45mm, and the 203 steel would actually act a a heat shunt as thats always cool to touch.
Thanks

 

[EFLImpudence]

Do you mean a ring as in socket circuit or is it the lighting?

If socket circuit, don't let it run through any insulation.

Lighting circuits are very overrated for the loads but even a socket circuit will still be acceptable as you describe.

 

[PrenticeBoyofDerry]

General if the RFC is protected by 32A breaker and of 2.5mm2 T&E then the circuit should be carrying no more than 21A, a unbroken ring if loaded correctly would carry 20A, so you should be okay.
But it would be best advised to clip or lay the cables above insulation.

 

[frank999]

Sorry ring, guess I could push a bit of 1x1cm trunking over it where it passes under the joist ?

 

[PrenticeBoyofDerry]

Installing cables in containment such as trunking will also factor in a de-rating, so you would still be in the same situation.

 

[viewer]

The steel joist will act to disperse any heat, so better let it touch the metal rather than separate (no sharp edges of course).

 

[frank999]

Umm I guess if its small trunking (16x16 or 25x16mm) there's not much free air moving around that.

I read its not a perfect science, so the more air around the cable the better.
Point taken with the steel, but one side is pressing against the Earthwool, would it not be better to still have it away from that somehow, maybe in 25x16 trunking with the lid off to the steel side ?

 

[EFLImpudence]

Just put a piece of wood under the cable.

 

[frank999]

You suggest wood rather than use plastic trunking, - is that because plastic can melt ? and current practice is to clip to joists (wood) ? the only air gap around the cable then would be the depth of the cable as it will be sandwiched between the steel and the wood.

 

[EFLImpudence]

I don't think you need to do anything if it is above the insulation and touching the beam.

However, rather than go to all the trouble of conduit/trunking which itself would derate the cable just separate the cable and insulation with a piece of wood, a piece of metal or remove some of the insulation (this, perhaps, not ideal).

You just need to ensure the cable can cool.