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Insulated ceiling ring final nightmare.

Former Community Member
Former Community Member
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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    I like jimmy's idea. I have a 4HP compressor on a 16A plug to a D32 in the CU. It never will trip of course but does dim the lights a bit (at least it used to, now I have LEDs). Compressors of my size usually have a decompressor in the pressure switch which means the motor is not starting against the tank static pressure, which makes life slightly easier for the supply, switch and motor. The only snag for you Jimmy is that if you switch on too many things together, it will trip the supply, no great problem, don't worry about it because getting the job done is the most important!
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    I like jimmy's idea. I have a 4HP compressor on a 16A plug to a D32 in the CU. It never will trip of course but does dim the lights a bit (at least it used to, now I have LEDs). Compressors of my size usually have a decompressor in the pressure switch which means the motor is not starting against the tank static pressure, which makes life slightly easier for the supply, switch and motor. The only snag for you Jimmy is that if you switch on too many things together, it will trip the supply, no great problem, don't worry about it because getting the job done is the most important!
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