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Cable rating

Discussion in 'Electrics UK' started by Dave99, 19 Jul 2003.

This topic originated from the How to page called Choosing the correct size cable.

  1. Dave99

    Dave99

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    If an air conditioner pulls 80A to start up but 16A when running, can this be wired in 2.5mm cable to a 32A breaker? Where do I find info about temporary loads cable can take?
     
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  3. Scoby_Beasley

    Scoby_Beasley

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    2.5 takes around 24 amps.
    Are you sure about the 80amps?
    Thats over 19kw !!
    Would need to be wired in national grid size cable !!
    The "temporary" load on cable would make no difference as your 32amp mcb would trip out straight away (7.68kw).
     
  4. breezer

    breezer

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    that is starting current, motors will quite often draw a huge amount of current to get them started, put it this way, can you drive your car from cold in 4th gear?.

    The same thing aplies to electric motors, form a cold start they need a lot to get them running but once they are, no where near as much
     
  5. Scoby_Beasley

    Scoby_Beasley

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    No prob with the TVR breezer !!
     
  6. Dave99

    Dave99

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    I'm sure about the 80A, I have the installation manual. Syas nothing about the cable or breaker though.

    So the air con should be OK in 2.5mm then? Says 2200W on the side of the unit, although similar units are wired into 32A breakers. How long can 2.5mm sustain this 80A surge for? I have no specs on how long the surge takes.

    Cheers for the quick replies.

    PS Know what you mean about TVR's.....
     
  7. breezer

    breezer

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    not if you use a 32 A mcb it wont. are you now saying that you have one or more air con units os similar same already? you said "although similar units are wired into 32A breakers" why not see what size cable they have
     
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  9. Dave99

    Dave99

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    The aircon units are not exactly the same as the existing ones. Yes they are wired in 2.5mm. I just want to do it properly and not copy something which could be incorrect. My cable run is approx 25 metres on a suspended ceiling.

    @Breezer: Are you saying that my 32A breaker will not hold on an 80A start-up surge?
     
  10. breezer

    breezer

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    in theory it shouldn't as i recal an mcb will trip at 1.2 or was it 1.3 times its rated current
     
  11. Dave99

    Dave99

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    Other breakers are type C. Being an obsolete SquareD box, I could only get a type B which I believe is worse for the start-up current. Type D is about as good as a rusty nail. It's one of those suck it and see situations I think, but I really wondered if anyone knew the calcs for this sort of thing, especially cabling.
     
  12. breezer

    breezer

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    unless you have a current (no punn intended) copy of the iee wiring regulayions it is hard to say.

    You have to take a lot of things into consideration.

    cable run distance, cable type, heat disipation, how the cable is mounted, is it inside or out (ambient temperature)

    you coud always see the iee wiring guide which is here
     
  13. cobweb

    cobweb

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    If your Aircon unit is a motor compressor type - similar to a domestic refrigerator's or freezer's - and it draws 80A at start up then I would think it would be fitted with a capacitor to "jump start" it. This would take some of the load off the ringmain and your MCB.

    Is this a domestic unit, or a commercial one with a voltage inverter to drop it from 3 phase to single?

    I know this isn't precise info but it should help!
     
  14. Dave99

    Dave99

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    It's a factory manufactured Toshiba single phase exterior mounted unit with no inverter. I have now got it up and running on the type B MCB and it appears to be holding well, as these units cut in and out frequently. If it fails, I'll upgrade to a C type MCB I think. Thanks for all who posted help.
     
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