Feeder Sizing for Packaged VFDs

[fuse_junkie]

We have some confusion in our office about how to protect the conductors on the line side of a packaged VFD. These VFDs come with OCPD in the box, sized by the manufacturer (typically a breaker). We always feed these from distribution panel breakers. From my understanding, we use 430.122(A) to size the conductors feeding the VFD package, and then we size the panel breaker per 430.130(A) using the motor load. Here's an example of what this setup would look like for a 20HP, 480V, 3Ø motor:

Panel -> 60A breaker -> #8awg -> VFD input breaker (per mfr) -> VFD wiring (per mfr) -> VFD

In this example, my lead engineer would have concerns about the #8 conductors between the two breakers. He doesn't think those wires are protected by the 60A breaker in the panel and upsizes them so they match the panel breaker size (in this example it would go to #4, which is a big difference, considering now we have to integrate extra box sizing rules into conduit runs). It's hard to get a specific reasoning from him, but he's mentioned something about breaker to breaker wiring with nothing in between needs to be sized based on the breaker size to protect the conductors. I can't find anything in Part X of 430 to back this up.

So I guess this all boils down to this: when we have a packaged VFD (that includes a branch SC/GF breaker), are the panel breaker and conductors upstream of that package still considered to be the motor branch circuit, which (in the above example) would allow me to use 430.122 to size the conductors to #8 and 430.130 to size the panel breaker at 60A? Thoughts?

 

[JoeStillman]

Article 430 freaks people out all the time because it distinguishes between "short circuit and ground fault protection" and "overload protection". Often, (as in your case) the SC/GF protection is at the supply end and the overload protection is at the load end of the wire. You are almost always permitted to have wires with an ampacity lower than the handle rating of the breaker - and that makes people very nervous. But the wires are protected at their ampacity by the motor overload protection (the VFD). Breakers in motor circuits are (almost) never the overload protection. They get sized according to the rules for SC/GF protection, which are different.

 

[petersonra]

look at 430.62 for ocpd sizing of the feeder.

 

[augie47]

Your referencing to Part X of Art 430 is the correct method using 430.122 for conductors and 430..130 for circuit protection.