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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
  • 0
    Chris Pearson:
    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. ?






    Failed capacitor?


    I cut the moulded plug off a commercial pressure washer and connected it using a 16 amp plug and socket protected by a C16 MCB, because it kept blowing fuses only to find it tripped the new MCB.


    It was pulling around 57 amps on startup until the service guy replaced the capacitor.


  • 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.
  • 0
    Hi Jimmy, have you looked into acoustic mats? eg Acoustic Mat - Sound Proofing Mats - Sound Solution Consultants Ltd can be laid under carpets or flooring in the new bedroom. I don't think as a standalone solution it would suffice but may help reduce the amount of wadding between joists, or it might let you put one layer of sound insulation above the plasterboard as per Dave's suggestion. If the new bedroom is yours and you have a living room below with surround sound TV etc, you might want all the sound insulation you can get :)

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