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Insulated ceiling ring final nightmare.

Former Community Member over 3 years ago

I am having a bit of a nightmare trying to adequately size cable for a ring final. The existing circuit is 2.5mm2 T&E which runs in the ceiling void between ground floor and first floor (this is a residential property). Currently there is no ceiling insulation installed. An extension means a new ceiling and significant modification to the ring final, and also (to meet regs) sound proofing insulation installed in the ceiling. This has a similar U value to thermal insulation in most cases.

100mm of insulation is to be installed throughout the new ceiling - which is 50% old circuit.

My first thought were to re-wire the whole circuit in 4mm T&E as with 2.5mm2 and the 18th ed tables its method of installation in all places wont give me 20A on each leg of the ring.

Great I thought, problem solved with plenty of overhead. More of a pain at 2nd fix, but not the end of the world.

Except now when I am looking through it seems like 4mm2 might not be enough if its going to run the risk of being covered with insulation. Joists are 170mm, so while the cable will not be covered completely the insulation will tend to balloon around it, so it looks like I will have to de-rate by a factor of 0.5 which would mean needing 6mm2 cable which seems crazy.

Any thoughts? Where am I going wrong in my assumptions / calcs?

Thanks in advance

James

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0 Alcomax over 3 years ago

OK. The OP has quite correctly come to the conclusion that some of the existing RFC 2.5mm will be sandwiched between 2 lots of 100mm insulation type material. So imagine 2.5 mm twin/earth in pipe lagging, but the lagging is 100mm rockwool or similar.

Yes, some will be resting on the plasterboard, some may be clipped to side of joists, but likely a significant amount is routed through joist holes for some distance between joists. This is all existing. Suddenly this cable is derated. I know many wish that away, but it derates all the same. If you are adding to this circuit or altering it, you need to take this into account. After all you are aware that insulation is going to be installed?

If this was new work you could use all sorts of exotic and time consuming construction methods, or you could install as normal , holes through joist but wire the ring final in 4mm with a 25 amp OCD . 2 x 4mm twins in a socket is no big deal; many contractors routinely use 4mm for rings now. 25 amp covers all bases, 32 amp is fine with careful routing of cable.
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0 Alcomax over 3 years ago

OK. The OP has quite correctly come to the conclusion that some of the existing RFC 2.5mm will be sandwiched between 2 lots of 100mm insulation type material. So imagine 2.5 mm twin/earth in pipe lagging, but the lagging is 100mm rockwool or similar.

Yes, some will be resting on the plasterboard, some may be clipped to side of joists, but likely a significant amount is routed through joist holes for some distance between joists. This is all existing. Suddenly this cable is derated. I know many wish that away, but it derates all the same. If you are adding to this circuit or altering it, you need to take this into account. After all you are aware that insulation is going to be installed?

If this was new work you could use all sorts of exotic and time consuming construction methods, or you could install as normal , holes through joist but wire the ring final in 4mm with a 25 amp OCD . 2 x 4mm twins in a socket is no big deal; many contractors routinely use 4mm for rings now. 25 amp covers all bases, 32 amp is fine with careful routing of cable.
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Insulated ceiling ring final nightmare.

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