VFD Feeder and Branch Conductors

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philly

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I wanted to confirm my understanding for sizing Feeder and branch conductors for a VFD application.

I'm looking at a 500HP VFD which will be feeding a 400HP 480V motor.

When calculating the branch circuit between the VFD I look in Table 430.250 and see that a 400HP motor has a FLA of 477A. Because this is a continuous duty motor I need to multiply the motor FLA by 125% which gives 477A x 125%= 597A. Therefore I would select (2) Sets of 350MCM having an ampacity of 610A between the VFD and the motor for the branch circuit.

When calculating the feeder to the VFD I looked at the VFD input rating and found it to be 577A. Looking at 430.122 the feeder circuit is required to be rated at 125% of the VFD input rating. This would give 577A x 125% = 722A. Therefore I would select (2) sets of 500MCM having an ampacity of 760A between the Switchgear and the VFD input.

I'll also mention that the drive has a bypass option. Based on this do I need to do anything based on 430.122(B)? I don't think so since this seems to be refereeing to the feeder to the drive which in my case is already the larger of the values presented in options (1) and (2)

Am I on the right track here?
 
in general the 125% thing on motors has little to do with being continuous. It is just a requirement for all motors. I think there may be some minor exceptions to the rule, but the general rule is that motors conductors are sized at 125%, continuous or not.

I see nothing wrong in your sizing, presuming you came up the the correct FLA number for the motor and drive.

the bypass means that you have to account for the contactor when you figure out how to meet the required SCCR.
 
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philly said:
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I'm looking at a 500HP VFD which will be feeding a 400HP 480V motor.

When calculating the branch circuit between the VFD I look in Table 430.250 and see that a 400HP motor has a FLA of 477A.

When calculating the feeder to the VFD I looked at the VFD input rating and found it to be 577A.
Click to expand...

Must be an Asian drive then, Mitsubishi? Toshiba? Yaskawa? The reason is, they don't seem to completely grasp the concept of HP changing with voltage ratings, either that or they don't want to bother thinking about it because they don't care. On most VFDs, the maximum input current rating is LOWER than the output current rating, because the VFD corrects the PF as seen by the input side, but has to SUPPLY the reactive + active current on the output side. Yet most Asian drives tend to be the other way around. Took me a while to figure that out, but it's because they UL list the drive as 400HP, whether it is 400HP at 460V, or 400HP at 380V, so the maximum INPUT current rating is listed at the lower voltage, which makes US have to use wire that is a LOT bigger because of the NEC rules that changed way back in 2002.

But apparently they don't really care.
 
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