VFD breaker / conductor coordination

[Aldoni]

I'm designing a circuit for a VFD driven 75HP motor and am questioning my work. I'd appreciate feedback on the following:

The power input rating of the VFD is 95 Amps. I am sizing the incoming conductors based on 430.122(A)
"Circuit conductors supplying power conversion equipment included as part of an adjustable-speed drive system shall have an ampacity not less than 125 percent of the rated input current to the power conversion equipment."

This gets me a minimum design current rating of 95x1.25 = 118.8 A.
The VFDs terminals are rated for 75C so the minimum wire size for THHN should be #1 (130A.)

The OCPD is used to protect the VFD from fault currents and was sized per 430.130(A)(1)
"The rating and type of protection shall be determined by 430.52(C)(1),..., using the full-load current rating of the motor as determined by 430.6(A) or (B)."

430.6(A) leads me to 430.250 for the full load current rating of the motor (460V 3P) of 96 Amps.

Using 430.52(C)(1), the maximum breaker size is 96*2.5=240A (next size up = 250A). 250A is also listed as the maximum value in the VFD manual.

All that to ask - the 250A breaker does not protect my #1 conductor. Should I decrease the breaker rating as its a code maximum, and/or increase the conductor as its a code minimum? Or is it acceptable to leave as is- since the VFD has internal overload protection?

 

[infinity]

I thought that the OCPD ahead of the drive is typically determined by the drive manufacturer. Size the conductor to 125% of the input current as you've stated and then the OCPD is sized by the equipment manufacturer.

 

[Tip DS]

Aldoni, I just researched this same issue. Leave your OCPD at 250% of the motor FLA (or the maximum size specified by the VFD manufacturer, whichever is less.) Don't worry about the conductor size when considering the motor branch circuit. The code allows this apparent discrepancy between the OCPD and conductor size, because the VFD includes an overload function. The overload protects the conductor from an overload condition the same way a breaker normally protects the conductor in NON-motor branch circuits.

 

[petersonra]

The next size up rule does not apply for VFD power input. The instructions will tell you specifically what the max rating can be. You cannot go up.