Breaker/wire size for hard starting 3 phase motors
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  1. #1
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    Default Breaker/wire size for hard starting 3 phase motors

    I've got an American Rotary ADX-20 (digital series 20 hp idler, rated to start 10 hp). It's fed from a 70 amp single phase breaker, and 4 gauge wire, per AR's spec sheet. No issues there.

    For awhile,it's just been powering a CNC press brake with a 7.5 hp motor, 20.2 amp nameplate. Knowing that I would later add additional machines, I wired the brake with a 60 amp panel mount breaker and 6 AWG wire, so that I could later run a 3 phase subpanel off that breaker/wire, then run branch circuits to the machines.

    That time has come.

    I've since acquired an old Niagara flywheel mechanical shear with a 5 hp motor (15 amp nameplate rating). I've had it temporarily wired into the phase converter for a short time, and it's a hard starting SOB. Despite being a 5 hp motor on a 20 hp idler, it actually triggers the start light and you can hear the start capacitor in the panel engage when starting it.

    Also just purchased a Geka ironworker with a 7.5 hp motor, nameplate rating of 20.3 amps.

    I started to order 30 amp breakers and 10 gauge wire for the two 7.5 hp machines, but given that they are hydraulic loads, I'm not sure if that's adequate for starting.

    Likewise, the flywheel load on the 5 hp shear seems like a 20 amp/12 gauge might not be happy.

    Can anyone give an idea of how much I should upsize these branch circuits?

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    Not sure about AR specs, but my 20hp Phase a matic calls for a 125amp breaker. I've heard more than a few stories of hard starting motors that model X rpc should be able to start, but would not, standard answer is a larger rpc. Have you tried running the brake while trying to start shear?

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    It starts the shear fine, just triggers the start capacitors/light. I've been using the shear for a couple months with no problems.

    When I called AR to order the phase converter, I inquired about running 2 machines at once. At the time, I was trying to buy a 10 hp press brake, and was also looking at shears in the 7.5 - 10hp range, and they claimed that for hard running loads (flywheel and hydraulic loads), I should treat the max starting capacity as the max running capacity. As such, I've always avoided running both machines together even though they are both smaller than what I originally inquired about.


    The manual that came with my phase converter specified 4 AWG for my 1 phase input, hence the 70 amp breaker to protect that (AR had no breaker spec listed). Interestingly enough, the manual on their website for the same machine now specs a 60 amp breaker and 3 AWG for the 1 phase input. Go figure.

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    To experiment, I did just go start the shear with the press brake motor running. It started it without triggering the start light/capacitors on the phase converter panel, so that's a result.

    I did get a slightly different buzzing sound during starting, though. Not sure which machine it was coming from, or if my ears were just being over sensitive.

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    Interesting that AR specs almost half what PAM specs, I wonder why? I ran the PAM on a 70a breaker in Tx, but as I've added some machines I recently looked at specs again and they say 125A for max capacity.

    Cannot recommend wire sizes as I'm no sparky, when in doubt go with larger size tends to work

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    Quote Originally Posted by dalmatiangirl61 View Post
    Interesting that AR specs almost half what PAM specs, I wonder why? I ran the PAM on a 70a breaker in Tx, but as I've added some machines I recently looked at specs again and they say 125A for max capacity.
    I just looked at Phase-A-Matic site, and the 20 hp unit allows up to a 60 hp combined load of lightly loaded motors. AR only allows up to the idler nameplate (20 hp) for combined loads. That might explain it.


    I just called American Rotary to inquire about the shear triggering the start light, and was informed that it's doing that by design. Not only does the red light indicate starting, it also is used to show that the "CTR technology" is doing it's thing.

    The "CTR technology" is basically the selling point for the ADX over the AD model - it uses some sort of electronic wizardry/capacitors to stabilize voltage during extreme hard starting. So that's a non issue - the phase converter is not unhappy starting the shear.



    As for my original question, I'm finding a number of online charts that are showing it as acceptable to put a 40 amp breaker on 10 gauge wiring for motor loads (same for all wire/breaker sizes - breaker can be one size above what is normally allowed to prevent nuisance tripping), as the magnetic starter on the motor will provide the overload protection for the machine, and the breaker is mainly protecting against a dead short in the wiring.

    As such I'm going to give it a shot with 10 gauge and 40 amps to the brake and ironworker, and try a 30 amp and 12 gauge on the shear, unless someone has an objection.

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