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Thread: VFD Multi Motor Input Conductor Size

  1. #1
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    VFD Multi Motor Input Conductor Size

    Looking at a mechanical equipment control panel for a packaged piece of equipment it indicates two motors at 40HP connected to a single VFD with independent O.L.’s . When I research the VFD provided it turns out they utilized a 100HP drive [assuming because 2@40H=80HP odd size so they used the next common size] however this then makes the drive rated at 140A input current when looking at the input conductors and reading 430.122 the conductors need to be sized for not less than 125% of the input rating that would make the conductors sized at 175A or 2/0, the driver for this being the drive they selected…does this make sense???

  2. #2
    Join Date
    Jan 2016
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    Does the panel have a nameplate with rated current?

    there was a recent thread about something similar
    perhaps a multi vfd control panel

    you will likely get a few different approaches



  3. #3
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    Aug 2004
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    Quote Originally Posted by Mike01 View Post
    Looking at a mechanical equipment control panel for a packaged piece of equipment it indicates two motors at 40HP connected to a single VFD with independent O.L.’s . When I research the VFD provided it turns out they utilized a 100HP drive [assuming because 2@40H=80HP odd size so they used the next common size] however this then makes the drive rated at 140A input current when looking at the input conductors and reading 430.122 the conductors need to be sized for not less than 125% of the input rating that would make the conductors sized at 175A or 2/0, the driver for this being the drive they selected…does this make sense???


    Part X. Adjustable-Speed Drive Systems
    430.120 General. The installation provisions of Part I through
    Part IX are applicable unless modified or supplemented by
    Part X.
    430.122 Conductors — Minimum Size and Ampacity.
    (A) Branch/Feeder Circuit Conductors. Circuit conductors
    supplying power conversion equipment included as part of an
    adjustable-speed drive system shall have an ampacity not less
    than 125 percent of the rated input current to the power
    conversion equipment.
    It seems to me that there is no wiggle room in 430.122 (A). The minimum ampacity of the branch and feeder conductors has to be 125% of the rated VFD input current.
    Bob

  4. #4
    Join Date
    Jul 2006
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    Quote Originally Posted by petersonra View Post
    It seems to me that there is no wiggle room in 430.122 (A). The minimum ampacity of the branch and feeder conductors has to be 125% of the rated VFD input current.
    Yes, correct.

    So what's the concern? Keep in mind that the VFD protects the motor and circuit, so the conductors from the VFD to each motor can be the normal size you would use for a 40HP motor. 430.122 only relates to the INPUT current rating.
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