Sizing Parallel Conductors for a VFD & Motor

[minerman]

I have a 480V feed from a 700A switchgear to a VFD, disconnect and motor.

The motor is rated at 515 full load amps.

Someone else sized the equipment for this job and I'm trying to make sense of it and choose wiring.

The switchgear breaker is rated at 700A, which seems fine to me. The VFD's internal breaker is also rated at 700A, which also seems fine...

But, the VFD is being shipped with 2 parallel 350MCM conductors per phase on the load side (connected to the load terminals). I've been advised to use the same conductors from the VFD to the motor. This seems too low to me. That would be rated for 620A which is less than 1.25 x 515A, right?. Also, someone chose a 600A disconnect. That also seems too low for the same reason.

Am I missing something here?

 

[petersonra]

Where did you get the 515A number from?

 

[tkb]

What is the HP rating of the disconnect?

 

[infinity]

Would this apply?

430.122 Conductors ? Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. 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 to the power conversion equipment.

 

[don_resqcapt19]

The conductors to the VFD has to be sized to at least 125% of the rated input amps. There is nothing in Part X of Article 430 that addresses the conductors between the motor and the VFD, so per 430.120 the standard conductor sizing rules apply. You need to use conductors with an ampacity of at least 125% of the motor full load current as found in the tables in Part XIV of Article 430.

 

[minerman]

Where did you get the 515A number from?

I have a 450HP motor, NEC says 515A. Motor datasheet says that too.

What is the HP rating of the disconnect?

The spec sheet says that the typical HP rating of this switch is 400HP. But I did not find information on the specific switch.

Would this apply?

Yeah, I think that would apply to the wiring upstream of the VFD. Thank you.

The conductors to the VFD has to be sized to at least 125% of the rated input amps. There is nothing in Part X of Article 430 that addresses the conductors between the motor and the VFD, so per 430.120 the standard conductor sizing rules apply. You need to use conductors with an ampacity of at least 125% of the motor full load current as found in the tables in Part XIV of Article 430.

That's what I was thinking too. The disconnect they have already ordered doesn't seem adequate and the suggested wiring does not either. I think it will probably work, but it doesn't meet code.