Determining motor ampacity for motors greater than 500 Horespower

[Electron_Sam78]

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.

 

[ptonsparky]

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?

 

[Electron_Sam78]

That's what I'm told. I've been asked to size a disconnect for it.

 

[Electron_Sam78]

480 volts, 790 amps, 70 Hz, 2088 RPM

 

[GoldDigger]

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?

 

[Electron_Sam78]

Running by VFD

 

[kwired]

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%).

 

[ptonsparky]

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

 

[Electron_Sam78]

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?