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Thread: Determining motor ampacity for motors greater than 500 Horespower

  1. #1
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    Determining motor ampacity for motors greater than 500 Horespower

    Hey all,

    The tables specified in section 430.6 to determine motor ampacity only go to 500 horsepower (T430.250 in my case). If have a 657 HP motor I assume I use the data plate amperage? I can't find anything in the book that addresses this.
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

  2. #2
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    Quote Originally Posted by Electron_Sam78 View Post
    Hey all,

    The tables specified in section 430.6 to determine motor ampacity only go to 500 horsepower (T430.250 in my case). If have a 657 HP motor I assume I use the data plate amperage? I can't find anything in the book that addresses this.
    The data nameplate says 657 HP?
    Tom
    TBLO

  3. #3
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    That's what I'm told. I've been asked to size a disconnect for it.
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

  4. #4
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    480 volts, 790 amps, 70 Hz, 2088 RPM
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

  5. #5
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    Quote Originally Posted by Electron_Sam78 View Post
    480 volts, 790 amps, 70 Hz, 2088 RPM

    70 Hz? I guess not a typo since it matches 2088 RPM. Requires a VFD? Or powered by dedicated generator?

  6. #6
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    Running by VFD
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

  7. #7
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    Quote Originally Posted by Electron_Sam78 View Post
    Running by VFD
    That there tells you to look at the drive requirements, at least for the supply circuit. Output I would say size conductors for nameplate amps (x 125%).

  8. #8
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    I would think the drive has some sort of recommendation for protection on the line side of it.
    Tom
    TBLO

  9. #9
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    So if this disconnect needed to be between the VFD and the motor am I to understand the NEC doesn't have guidance for this as far as determing motor ampacity? So revert to data plate ratings?
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

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