Thanks for the feedback!
There is no bypass, just the VFD.
The VFD came "factory assembled" and mfgs always default to worst case. Since the drive can output up to 200% over current depending on the setup parameters, they size the OCPD for that. VFDs always seem to be a bit of a sticking point since they can be setup and configured for their specific application. Input currents can differ simply based on whether you're running in normal duty or heavy duty mode. But code doesn't take that into consideration. That same drive i mentioned in my initial post can be set to run in a mode that limits input current to 170A, but the way the code reads, we'd still have to size the incoming feeders at 125% of 207A since both 207A and 170A are on the drive nameplate.
The 200% does not factor into it, drives will only deliver that 200% for 2-3 seconds before they shut down to protect themselves. If you think about it, if I size conductors for FVNR starting of a motor, I size them at 125% of the NEC table of FLCs for that HP size. But that motor may take
600% current for 2-3 seconds on startup, yet I am not required to oversize the conductors. That is baked into the sizing rules, so 200% for 3 seconds is irrelevant to those rules.
When you USE a drive at its lower Constant Torque drive current rating, you use the lower input current rating of that drive in sizing the feeder conductors. Yes, both input current values may be shown, but they will apply to
different HP ratings for the motor. If you use a CT rated drive on a VT load of the same HP as the CT rating of the drive, yes, you will have to size everything based on the HP used, which was your decision in using more VFD than was necessary. It’s the same if you use a VFD that is 2x the size of the motor FLA because you are feeding it with single phase, your input conductors will have to be sized for the input current rating of the VFD (something that lots of people miss).
The OCPD put into a combo VFD unit made by the mfr is more of an economic decision than anything else. For THEM, there is going to be no difference in cost between a 300A breaker and a 400A breaker, so they might as well use 400A just in CASE the user wires their own bypass in the field. From a protection standpoint though, it’s all somewhat irrelevant in that the VFD has better protection in it for the
motor circuit than the OCPD is going to provide, and VFDs had to be listed AS the motor OCPD starting back in 2005. So the breaker is really only providing protection for the VFD itself and since by definition of anything goes wrong inside of the VFD it is ALREADY toast, all you are really doing is preventing a fire at that point.