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Cable rating for electric motors

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  1. #1
    4K Club Member Marc's Avatar
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    OK ... drum roll ... I have an electrical question for the electricians.
    If a motor draws say 20 amps at start up but then runs on 15 amps (just an example) does the cable need to be (what is it 4mm?) for 20A or it can be the size required for 15 amp since start up is just a few seconds?
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  2. #2
    1K Club Member Random Username's Avatar
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    Default Cable rating for electric motors

    From - http://www.renovateforum.com/f195/ca...07/#post994645

    My understanding is that the circuit for hard wired motors needs to be sized for the motor's running load (or if there are a number of motors hard wired to a circuit, sized for 100% of the largest motor on the circuit, plus 50% of the total load of all the other motors on the circuit).

    If you sized for startup current draw you'd be connecting everything with thick copper busbars, as the startup current draw can be anywhere from 4-10 times the running current, but (depending on how short a timescale you measure it) maybe 50 times the running current if measured in that tiny instant as the rotor starts to move.

    The startup current is also why circuit breakers come in a range of instantaneous tripping current curves - usually a D curve breaker for larger motors.

    The length of the wiring from the mains supply also comes into it, as you don't want to have too much voltage drop.
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    Community Moderator phild01's Avatar
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    Still like to understand why Fujitsu insist on over rating wire size for their air conditioners! Do they have a technical standpoint.

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    Quote Originally Posted by phild01 View Post
    Still like to understand why Fujitsu insist on over rating wire size for their air conditioners! Do they have a technical standpoint.
    As a Fujitsu dealer myself, ive never got a proper answer out of them. They arent the only ones though dont worry. Just something to be mindful of thats all.

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    4K Club Member Marc's Avatar
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    Quote Originally Posted by Random Username View Post
    From - http://www.renovateforum.com/f195/ca...07/#post994645

    My understanding is that the circuit for hard wired motors needs to be sized for the motor's running load (or if there are a number of motors hard wired to a circuit, sized for 100% of the largest motor on the circuit, plus 50% of the total load of all the other motors on the circuit).

    If you sized for startup current draw you'd be connecting everything with thick copper busbars, as the startup current draw can be anywhere from 4-10 times the running current, but (depending on how short a timescale you measure it) maybe 50 times the running current if measured in that tiny instant as the rotor starts to move.

    The startup current is also why circuit breakers come in a range of instantaneous tripping current curves - usually a D curve breaker for larger motors.

    The length of the wiring from the mains supply also comes into it, as you don't want to have too much voltage drop.
    The aircon thread got me thinking about a water pump that is powered by a big single phase 3hp capacitor started motor. Recently a sparky wired a remote switch inside the kitchen so that when we arrive we don't have to go down the pump room to start it. I had a contactor for this but the electrician told me it was overkill and that he would use a single pole switch that looks like a normal light switch but is rated higher sorry forgot how many amps it is. So the full load goes up the kitchen through the switch and down to the pump. The wire is just 2.5mm yet the circuit breaker is 30 amp. When I asked about this he said that the motor sucks up 25a at start up. If the wire has to be just for the running current it should be ok, if it has to be for the start up current then it is not ok and it should have been 4mm

    PS
    What's that business about me and a violet shrinking? Is that because of the heat from an undersized cable?


    Salu Digby is a native of the planet Imsk, which gives her the ability to shrink down to microscopic proportions, and she can even achieve subatomic size when necessary. She is a longtime member of the Legion of Super-Heroes under the code name Shrinking Violet.

    I look a tad different though ...
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    A. the circuit protection is too large for a 2.5mm cable. That in itself is illegal.

    B. Yes the running current is the important bit. All motors will draw a little extra on start up, but isnt always important in terms of how you rate a cable or circuit.

    Is there a contactor at all in the circuit? If so a 10A switch or similar may be used as the load on that switch is only the coil of the contactor, and the circuit load goes through the contacts.

    and as above the circuit breaker trip curve comes into it, and its the exact reason why there is different trip curves/breaker types. a longer trip time gives you that little bit of grace in motor start up.

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    4K Club Member Marc's Avatar
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    OK so 2.5mm is OK, no contactor, that was my idea to have a contactor, but the electrician did not want to fit a contactor in the switchboard since it is a bit crowded.
    I'll check the circuit breaker and why it is so large. So a lower size with longer trip time would be the go. I'll call him back. I hope the actual switch is the right size. What size it is supposed to be for a 3HP motor and no contactor? Is the capacity stamped on the switch? It is the same size of a domestic 10A switch but has a red light next to it ... well, the light has not much to do with the switch I suppose.
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    Quote Originally Posted by phild01 View Post
    Still like to understand why Fujitsu insist on over rating wire size for their air conditioners! Do they have a technical standpoint.
    The only reason I can see for them doing this is to prevent under-voltage to the unit in a situation when the installation method used might cause an overloading of the cable.
    Anything with a motor in it doesn't like being run under-voltage, and with a motor that failed this way it would be very hard for them (Fujitsu) to prove it was the cause in a warranty claim situation.
    With a bigger cable there is more margin for error in the installation method used.
    So in the end I suppose i'm suggesting that they do it to reduce warranty claims that may be caused by incorrect installation methods, which they have no control over.
    Anyone agree or disagree with this?

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    Quote Originally Posted by Whitey66 View Post
    The only reason I can see for them doing this is to prevent under-voltage to the unit in a situation when the installation method used might cause an overloading of the cable.
    Anything with a motor in it doesn't like being run under-voltage, and with a motor that failed this way it would be very hard for them (Fujitsu) to prove it was the cause in a warranty claim situation.
    With a bigger cable there is more margin for error in the installation method used.
    So in the end I suppose i'm suggesting that they do it to reduce warranty claims that may be caused by incorrect installation methods, which they have no control over.
    Anyone agree or disagree with this?
    I would tend to agree mate. I did however install a Fujitsu ASTG34KMTA this morning, which is their 9.2kW system. (lets not go into this marc) It states in its install manual (i made a point of reading it to check over what i thought id read previously), that it requires a 4mm power supply, on a 32A breaker. HOWEVER it also states that circuit breaker sizing etc needs to be correct depending on what region/country we are in. So obviously under a normal domestic condition you wont put 32A through a 4mm, unless it was running under ground completely or something. In which case i downsize the CB to the standard 25A and use the 4mm cable. Maximum current was listed at around 12A. It is what it is. Its easier to follow their specs than be pulled up down the track, as stupid as it is to require a larger cable than neccesary.

  10. #10
    1K Club Member Random Username's Avatar
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    If it's capacitor start, then their will be (something, typically a centrifugal switch) taking the capacitor out of circuit once the motor gets to speed, as start capacitors die quickly and horribly if left in circuit. (even though some coffee machine repair people believe otherwise).

    So that 2.5mm wire might just be energising the start relay on the motor, and the relay's contacts are taking the start current - the wiring you describe is something I could imagine myself doing after looking at the schematics of a motor but that's not to say I'd actually do it that way, as I've learned that it's better to assume the next person along who plays with the wiring only just passed the "can remember to breathe" test.
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  11. #11
    4K Club Member Marc's Avatar
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    I see Random shares my high regard for coffee machine repairers ... ha ha.
    By the way the little switch for my pump is rated 20A.
    Enjoy present pleasures in such a way as not to injure future ones.
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    Senior Member MorganGT's Avatar
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    Quote Originally Posted by Random Username View Post
    If it's capacitor start, then their will be (something, typically a centrifugal switch) taking the capacitor out of circuit once the motor gets to speed, as start capacitors die quickly and horribly if left in circuit. (even though some coffee machine repair people believe otherwise).
    Try Googling "permanent-split capacitor motor" - you might learn something new. The phrase "duty cycle" is also a relevant consideration, as motors in espresso machines are not designed for continuous running.

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    Quote Originally Posted by MorganGT View Post
    Try Googling "permanent-split capacitor motor" - you might learn something new. The phrase "duty cycle" is also a relevant consideration, as motors in espresso machines are not designed for continuous running.

    Hi
    Yes
    Capacitor start & run motors are commonly used in many applications
    Particularly small 240v water pressure pumps & sump pumps!!

  14. #14
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    Do you mean Google it like this?



    Or possibly like this:



    Duty cycle is not the issue, unless you are expecting a coffee (whatever) to be used for under a few seconds, as a few seconds is all the duty cycle you'll get from a start capacitor. There is a lovely demonstration of the duty cycle of a start capacitor here:

    https://www.youtube.com/watch?v=OMd9QkinXz4

    (I like the one at 6:30 the most)
    Attached Thumbnails Attached Thumbnails run-cap.jpg   run-cap-2.jpg  
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    .
    But I did not have to goggle to find out about them
    For a long time they were referred to as capacitor start & run motors
    The big advantage is higher starting torque
    No Centrifugal switch or thermal time delay on a START ONLY winding ,that are prone to failure & possible burn out of the motor

    They are effectively a two phase motor

    The phase shift for one winding being achieved by a suitable rated Voltage & capacity[uF] capacitor permanently wired in

    In fact there is a doubling of the phase shift between the two windings
    the lagging current in one due to the winding inductance& the leading current in the other due to the capacitor.
    One very common application ,way , way back , was the old external motor & compressors on refrigerators .
    Needed HIGH starting torque motors

    There are many versions of split phase 240V motors
    to name three
    t
    [a]capacitor start, requires either a centrifugal switch or a time delay to save the start winding from burning
    out
    [b]Capacitor start & run[ no switch,or time delay

    [c ]Simple split phase , where the start winding has very few turns compared to the run winding giving a sufficient degree of phase shift for rotation of the respective fields to start the motor turning, use a centrifugal switch or thermal time delay.

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    Marc, just hang it from a steel A-frame, coat it in Xtroll and clock the screws with a blue Bosch and all will be well

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  17. #17
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    The Circuit breaker is there to protect the CABLE First and Not what is connected to it. Each cable has a different rating depending on the installation conditions and length of run. For example 2.5 TPS has a maximum load of 20A and no more. Many manufacturers overrate cable sizes as this reduces voltage drop and current draw.

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    Quote Originally Posted by Marc View Post
    OK ... drum roll ... I have an electrical question for the electricians.
    If a motor draws say 20 amps at start up but then runs on 15 amps (just an example) does the cable need to be (what is it 4mm?) for 20A or it can be the size required for 15 amp since start up is just a few seconds?
    Hi Mark
    A Sum up
    The cable size has to take TWO considerations into account[
    [a]
    The voltage drop under START up current,
    Excess voltage drop could prevent the motor from getting up to speed
    Max current , run length , & cable resistance will determine voltage drop,.
    [b]the max continuous load current



    Fuses /olcbs are a bit more tricky depending on what they are supposed to protect.
    [c]if the device /appliance, they should have a rating equal to the devices continuous full load current
    [d]If the wiring ,they should be no greater than the cable's continuous current rating.

    Run length has NO EFFECT on cable fuse rating.

    But limitations on cable cooling may require derating the cable's
    Current capacity [each TYPE of cable has a specified max operating temp rating]
    The limitations are specified in the relevant AUS STANDARDS

    As stated earlier there are many forms of capacitor use in single phase motors

    [e]capacitor START motors where the capacitor is switched out of circuit by either a centrifugal switch [opens @ around 75% of full speed]
    Or a time delay switch ,normally not much more than a second or two

    [f]Capacitor start & run motors, where the capacitor is permanently in series with one winding ,more for heavy duty use, such as 240v water pumps, washing machines. large single phase refrigeration units, etc [ have a higher starting & continuous torque', less likely to stall than a capacitor start motor

    Finally , duty cycle can be very relevant!!
    Motors in many applications are NOT designed for continuous running
    That can vary from a few minutes up to 30Minutes per hour!!

  19. #19
    4K Club Member Marc's Avatar
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    Thank you 123, it's not that straightforward I see.
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    Quote Originally Posted by ktmtragic View Post
    The Circuit breaker is there to protect the CABLE First and Not what is connected to it. Each cable has a different rating depending on the installation conditions and length of run. For example 2.5 TPS has a maximum load of 20A and no more. Many manufacturers overrate cable sizes as this reduces voltage drop and current draw.
    If this is true why does Table C5 of AS/NZS 3000:2007 state that the protective device for 2.5 mm TPS cable in air is 25 amp and the protective device for 2.5 mm TPS cable mounted either enclosed and buried, or directly buried in the ground is 32 amp?

    Note that in the UK 2.5 mm TPS cable is often protected with 32 amp devices due to the lower maximum air temperature used in the BS (British Standard).
    “What a fool believes, he sees. No wise man has the power to reason away”- The Doobie Brothers

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    4K Club Member Marc's Avatar
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    I think it is because it is colder in the UK and their English millimeters are different from the Australian millimeters.
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    Quote Originally Posted by Marc View Post
    I think it is because it is colder in the UK and their English millimeters are different from the Australian millimeters.
    In the UK: English, Scottish, Welsh and Northern Irish millimetres are identical. Australian millimetres are the same as “UK” millimetres in all respects except of course they are upside down.
    “What a fool believes, he sees. No wise man has the power to reason away”- The Doobie Brothers

  23. #23
    1K Club Member Random Username's Avatar
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    Actually, the Scottish millimeters are the ones we match, except we use them widdershins.
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  24. #24
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    Quote Originally Posted by Marc View Post
    I think it is because it is colder in the UK and their English millimeters are different from the Australian millimeters.
    Marc: That is BS

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    Quote Originally Posted by UseByDate View Post
    If this is true why does Table C5 of AS/NZS 3000:2007 state that the protective device for 2.5 mm TPS cable in air is 25 amp and the protective device for 2.5 mm TPS cable mounted either enclosed and buried, or directly buried in the ground is 32 amp?

    Note that in the UK 2.5 mm TPS cable is often protected with 32 amp devices due to the lower maximum air temperature used in the BS (British Standard).

    Hi
    Your last sentence holds the answer
    clue
    Air temp in Aus versus buried deep in the ground

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    Quote Originally Posted by oldtrack123 View Post
    Hi
    Your last sentence holds the answer
    clue
    Air temp in Aus versus buried deep in the ground
    My question was rhetorical.
    To make my question clear. “ktmtragic” had stated “2.5 TPS has a maximum load of 20A and no more.” I directed him/her to documentation that indicated that his /her statement was not true.
    Note that “ktmtragic” has not replied.

  27. #27
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    Quote Originally Posted by UseByDate View Post
    Marc: That is BS
    What do you mean? It is clearly colder in the UK and their British standard millimeters are different from the Australian ordinary millimeters ... and let's not mention centimeters !
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    I am agreeing with you that it is colder in the UK.

    Quote Originally Posted by UseByDate View Post
    Marc: That is BS
    That is BS(British Standard)
    In Australia the max air temperature used to calculate cable ratings in the AS (Australian Standard) is 40 deg Celsius. In the UK the max air temperature used to calculate cable ratings in the BS (British Standard) is 30 deg Celsius.
    Note that the max temperature used in the BS could have changed since I read it many moons ago.

  29. #29
    4K Club Member Marc's Avatar
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    Clearly so ... don't forget global warming ...
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    Out of curiosity, what's the motor for???

  31. #31
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    Hi Bob, we are just mucking around with Use, but if you want to know see post number 5. It's a water pump.
    Enjoy present pleasures in such a way as not to injure future ones.
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    Somehow got my wires crossed. Subject said cable so I got in my head you were lifting something and somehow the required cable strength related to the motor power...which had me very confused. Whole thread makes much more sense now! My bad.

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    Quote Originally Posted by Marc View Post
    Hi Bob, we are just mucking around with Use, but if you want to know see post number 5. It's a water pump.
    Damn: and I thought I was mucking around with Marc. (Ambiguous BS and all)

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