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Insulated ceiling ring final nightmare.

Former Community Member over 4 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 Chris Pearson over 4 years ago

jimmyhorns:

Hm! How is the compressor wired in?

32 Amp radial to Garage consumer unit to 20 Amp MCB to 6mm2 SWA to Single - 3 phase inverter to 3 phase motor of the compressor.

Using a 3 phase motor and running an inverter avoids the horrific start up currents and also allows speed control and avoids on-off cycling when the air drawn off is fairly constant. I designed/built the microcontroller circuitary that controls the inverter to predict / adjust demand.

And with that level of sophistication you worry about a bit of wadding in a ceiling? ??

My 3 hp compressor eats 13 A fuses for breakfast (on start-up), but at least I have not adopted my chum's solution which is to insert a nicely turned bit of brass rod into the fuse holder. It may not be as bad as it sounds. At a steady state, the motor could not overload a 13 A fuse and under fault conditions, the MCB should trip.

Note, please do not do this at home. ?
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0 Chris Pearson over 4 years ago

jimmyhorns:

Hm! How is the compressor wired in?

32 Amp radial to Garage consumer unit to 20 Amp MCB to 6mm2 SWA to Single - 3 phase inverter to 3 phase motor of the compressor.

Using a 3 phase motor and running an inverter avoids the horrific start up currents and also allows speed control and avoids on-off cycling when the air drawn off is fairly constant. I designed/built the microcontroller circuitary that controls the inverter to predict / adjust demand.

And with that level of sophistication you worry about a bit of wadding in a ceiling? ??

My 3 hp compressor eats 13 A fuses for breakfast (on start-up), but at least I have not adopted my chum's solution which is to insert a nicely turned bit of brass rod into the fuse holder. It may not be as bad as it sounds. At a steady state, the motor could not overload a 13 A fuse and under fault conditions, the MCB should trip.

Note, please do not do this at home. ?
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Insulated ceiling ring final nightmare.

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