# Thread: Oversized VFD / 430.122(A) violation

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## Oversized VFD / 430.122(A) violation

I'm working on a project where the owner wants to use a spare VFD (rated 150HP 480V 180A) to replace an FVNR starter that controls a well pump (100HP 460V 127FLA). The feeder conductors to the motor starter are #3/0 copper and the upstream overcurrent protection is a 200A fused disconnect. Using the 75 deg. ampacity table and no adjustment factors it appears the #3/0 conductors are rated for 200A, which is short of the 225A (125% of 180A) required by NEC 430.122(A). So the question is.... Do I have to replace the #3/0 feeder conductors with #4/0 even though they're protected by 200A fuses?

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The 180 amps you quoted is the FLA for a 150 HP, Table 430.250.

You need to multiply the Drive rated input current * 125%. I would venture a guess that it will be higher than the 180.

The answer is yes, you will need to increase the size of the wire.

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Originally Posted by ptonsparky
The 180 amps you quoted is the FLA for a 150 HP, Table 430.250.

You need to multiply the Drive rated input current * 125%. I would venture a guess that it will be higher than the 180.

The answer is yes, you will need to increase the size of the wire.
According to the literature I have, most of Toshiba's H3 series drives have output ratings that are very close or exactly the same as the FLA values in Table 430.250. The 150HP drive the owner wants to use is rated 180A.

Thanks for your response.

4. Originally Posted by Steamdonkey2011
According to the literature I have, most of Toshiba's H3 series drives have output ratings that are very close or exactly the same as the FLA values in Table 430.250. The 150HP drive the owner wants to use is rated 180A.

Thanks for your response.
The conductors that supply power to the VFD must be have an ampacity of at least 125% of the VFD input amps. Is the 180 amps that the drive is rated at input or output amps?

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Originally Posted by don_resqcapt19
The conductors that supply power to the VFD must be have an ampacity of at least 125% of the VFD input amps. Is the 180 amps that the drive is rated at input or output amps?
Unfortunately the only info I have at the moment is the rated output of the drive. As you pointed out the NEC refers to the input current, hopefully that's stated somewhere on the drive.

Thanks.

6. Unfortunately the NEC does not consider the fact that you possibly can put a smaller motor on the output of the drive than it is rated for. As far as I'm concerrned you could probably even go with 175 amp fuses and 2/0 conductors for 100 hp motor but NEC doesn't seem to think so.

The motor is 100 hp whether connected directly to the line or via the drive. The only increase in power with the drive would be any inefficiencies in the drive - which should not be very much. Poor power factor is not an issue with a drive but is with direct line connection, so that makes up for any other inefficiencies right away.

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Yeah, you've really got to be careful with this code article. We've used drives a little oversized in the past and if you're not paying attention sometimes it bumps the wire size up a notch. Not something you want to find out at inspection time.

Fortunately, I haven't had this come up where I've had to explain to the customer why we have to replace the wire that's been perfectly fine for years just because they want to use a little bit bigger spare VFD....

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