VFD Feeder Size

[JTProE]

NEC 2005: Article 430.122(A) states that the feeders of a VFD are required to be sized at 125% of the rated input of the drive. If I don't plan on using the full rating, is there a way around this rule? Why can't I just fuse the input conductors to a smaller size (assuming the conductors can handle the starting current)?

Thanks.

 

[dpc]

The only way around it is to apply for a variance with local inspector and get it approved. But I wouldn't get your hopes up.

The problem is that when someone gets a bright idea that they can put a larger motor on the drive, the upstream fuses blow and larger fuses are installed, but the conductor remains the same. Also, the fuses and conductors must be adequately sized to handle whatever output the drive decides to deliver to the motor when it is accelerating.

 

[jraef]

Besides, if you think that concept through a little more and leave out the VFD, you might make the same argument for ANY circuit right? "I have a 30HP motor, but I only need 10HP for the load, can't I just use a circuit capable of the 10HP and fuse it for 10HP?" Doesn't sound as reasonable now, does it?

The NEC is not just about what you want to do today, but that it is right for whatever someone else who DIDN'T know your original intent may try to do later.