Input Contactor and VFD Question

[whiggins]

I'm sure this has a simple answer, but I've not found it in the code.

I have a 50A CB and 45A contactor in an MCC feeding a remote VFD. The wiring is sized
for 125% of the VFD input. The contactor exceeds the full load input current of the drive.
Does the contactor have to match the size of the 50A CB or just be larger than the
input of the drive?

 

[Aleman]

I don't know off hand what code section covers this but since no one else answered your question I will. Which is your contactor needs to
be rated to the circuit maximum set by your over current protection. Of course this all depends on whether anyone will even know which is
highly unlikely but still you should build it right. I have witnessed an exploding contactor first hand. Took out the whole building when the GFI
on the main tripped. Flash bang and all. So even if it is rated below what your VFD requires, the breaker can deliver more than the contactor
can handle and that can possibly be a bad thing.

 

[ptonsparky]

I would size it for the HP of the motor as if the drive was not there. Both the drive and the contactor will have suggestions for fault current protection well above what you have now.

Using the contactor to regularly cycle power to the drive is not recommended as a control method for the VFDs I have used.