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Thread: VFD Load Side Conductors - Multiple Motor

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    VFD Load Side Conductors - Multiple Motor

    We have applications at times that utilize one VFD for up to 4 motors (overhead cranes). If I branch out the output of the VFD to each motor through thermal overload relays to meet 430.124(C), is there an additional code section that addresses the conductor sizes in this situation? For instance, if I have (4) 25A motors and a 100A VFD output, directly out of the VFD I'd size conductors per that 100A, but I think due to the tap rules, each of the 4 motor branch conductors would also need to be rated for 100A (likely longer than 25' runs per 240.21 and would not meet 240.21(4)). I think if I did size per the 25A on each circuit, I'd need appropriate branch OCPD. Thoughts?

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    Quote Originally Posted by aNerdyEngineer View Post
    We have applications at times that utilize one VFD for up to 4 motors (overhead cranes). If I branch out the output of the VFD to each motor through thermal overload relays to meet 430.124(C), is there an additional code section that addresses the conductor sizes in this situation? For instance, if I have (4) 25A motors and a 100A VFD output, directly out of the VFD I'd size conductors per that 100A, but I think due to the tap rules, each of the 4 motor branch conductors would also need to be rated for 100A (likely longer than 25' runs per 240.21 and would not meet 240.21(4)). I think if I did size per the 25A on each circuit, I'd need appropriate branch OCPD. Thoughts?
    Or the nearest size up maybe.
    Is there any diversity in the running of the motors?

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    Aside from mechanical defects and tolerances, the are intended to always be the same size, manufacturer, model, etc.

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    Quote Originally Posted by aNerdyEngineer View Post
    Aside from mechanical defects and tolerances, the are intended to always be the same size, manufacturer, model, etc.
    I meant do all the motors always run at the same time?

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    Quote Originally Posted by Besoeker3 View Post
    I meant do all the motors always run at the same time?
    Yes, they are all driven from the same VFD and any overload relay or motor klixon/thermal would stop the drive.

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    Quote Originally Posted by aNerdyEngineer View Post
    Yes, they are all driven from the same VFD and any overload relay or motor klixon/thermal would stop the drive.
    Are they all loaded at the same time?

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    Yes, always should have equal loads on all motors.

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    Quote Originally Posted by aNerdyEngineer View Post
    Yes, always should have equal loads on all motors.
    I'd size each of the four cables for each motor and have individual fuse switch and protection for each motor that would shut down the VFD. Belts and braces.

    But I live in UK so NEC doesn't generally apply.

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    Either or; meaning if you use conductors sized per the 25A motor loads, then they must have an SCPD in addition to the OL device. If you don't want an SCPD, you must size all conductors based on 100A.

    Rockwell came out with a special version of their MPCBs that are designed to be used on the output of VFDs (up to 400Hz) for this very reason.
    https://literature.rockwellautomatio...p004_-en-p.pdf
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