Conductors Supplying VFD

[Flapjack]

I have a VFD that has a rated input current of 285 amps. The VFD has a 600 amp circuit breaker and overloads in the drive enclosure, and the VFD will be fed from a MCC with a 600 amp circuit breaker. Is it code compliant to size the supply conductors at 1.25 x drive input per 430.122 which would be 500 kcmil, or do the conductors need to be sized for 600 amps so they are protected by the breaker?

I've seen other threads asking a similar question, but came across mixed answers.​

 

[petersonra]

Take a look at 430.120.

Then look at 430.130.

 

[Jraef]

... The VFD has a 600 amp circuit breaker and overloads in the drive enclosure, ...

Overloads? Where and for what? Is this VFD so old that it was before they had built in OL protection? Not exactly what you were asking, but I'm curious because one application where you WOULD need OLs is if you have multiple motors behind the VFD. That then becomes problematic in that the NEC rules for sizing the OCPD create a conflict with the motor size being significantly smaller than the VFD size. By that I means that per 430.130 your OCPD must still be sized per the motor, but the VF may allow a much larger OCPD that that permits, setting up a conflict.

 

[310 BLAZE IT]

Sounds like the VFD was a retrofit which explains the larger breaker size and overloads that were left in place.

Reduce the breaker size to protect the appropriately sized conductors. You can probably just pull the overloads out.

One thing to look out for when sizing conductors is heavy duty and standard duty, since this will effect the continuous output.

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