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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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    Andy, Compressor starting is one of the high start current items because the motor must give maximum torque (the running torque) at startup, ie zero revs. This is not the same as your pressure washer, because that load is controlled by the flow, which is essentially zero. Yes, yours was a low capacity start capacitor, but that was a fault, these compressors are not faulty, and changing the capacitor to a larger value would not help at all, in fact it would make the starting worse. An accurate 90-degree phase shift is required to start at all. The decompressor means that there is little air pressure on the piston for a period after switching on, It is connected via the little hose coming from the air pressure switch to the cylinder which also operates the pressure switch when sufficient pressure is in the tank. Bigger piston compressors have a similar decompressor, often driven by a centrifugal clutch to allow normal operation when running speed is achieved.
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  • 0
    Andy, Compressor starting is one of the high start current items because the motor must give maximum torque (the running torque) at startup, ie zero revs. This is not the same as your pressure washer, because that load is controlled by the flow, which is essentially zero. Yes, yours was a low capacity start capacitor, but that was a fault, these compressors are not faulty, and changing the capacitor to a larger value would not help at all, in fact it would make the starting worse. An accurate 90-degree phase shift is required to start at all. The decompressor means that there is little air pressure on the piston for a period after switching on, It is connected via the little hose coming from the air pressure switch to the cylinder which also operates the pressure switch when sufficient pressure is in the tank. Bigger piston compressors have a similar decompressor, often driven by a centrifugal clutch to allow normal operation when running speed is achieved.
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