300 HP 480V motor OCPD requirements

[Mavipo0221]

Hey everyone, I'm looking for some assistance in determining the OCPD size for a 300 HP motor

This motor is shown with a VFD starter, 361 amps FLA, with parallel 300kcmil copper conductors, I know the conductors are good, but I'm unsure about the OCPD.

The plans call out for a 400A breaker, but I feel like that is to small.

Thoughts?

 

[augie47]

The VFD manufacturer should have info on the max overcurrent device for their drive. Do you have a make/model; ?

I worked on an Eaton 350HP VFD and the data sheet specified a 500 amp FUSE but that's anecdotal.

 

[petersonra]

Hey everyone, I'm looking for some assistance in determining the OCPD size for a 300 HP motor

This motor is shown with a VFD starter, 361 amps FLA, with parallel 300kcmil copper conductors, I know the conductors are good, but I'm unsure about the OCPD.

The plans call out for a 400A breaker, but I feel like that is to small.

Thoughts?

I'm inclined to agree that the circuit breaker rating is less than what I would supply for a VFD with an input rating of 361 amps. Most cases I would probably be at least 500 amps. Have you looked at what the manual for the VFD says?

An inlet rating of 361 amps would suggest the need for wires with an ampacity rating of about 452 amps. Two 300 mcm copper seems like a lot of wire, but nothing says you can't have bigger wires than needed.

It's entirely possible that somebody did a copy and paste from another drawing and didn't update the details correctly. I would be inclined to just ask them.

 

[paulengr]

It is required to use what the VFD manufacturer specifies as part of its Listing.

Realistically the VFD does overload (as required by NEC). All the OCPD does is short circuit protection. A big consideration though is the short circuit rating. You may find this dictates the OCPD choice or you may find yourself adding a 3% line reactor.

This is where the engineering firm may have to defer to Code. Even if it was an ATL starter there is no way a 400 A breaker would be adequate.

 

[petersonra]

. Even if it was an ATL starter there is no way a 400 A breaker would be adequate.

If it was an across the line starter, there's a very good chance that the circuit breaker would trip just about every time you tried to start the motor.

With a VFD the chances of this cb tripping is pretty remote. The input of a VFD typically does not have enough inrush to Trip the breaker, in the built-in overload protection would prevent it from tripping on overload.

 

[Jraef]

Input conductors must be sized to 125% of the INPUT current rating of the drive. When the drive says it is rated for 361A that is the output current, the manual or tech data should give you the input current rating, which is usually lower than the output current. In a pinch though you could use the output current, you just may end up with larger conductors than necessary.

The OCPD size and type must be selected per the VFD mfr instruction sheet, because what you can use will be determined by their UL listing. Many drives are not UL listed behind circuit breakers, only specific fuses. New rules in UL are making that lean toward fuses only for newer drives (UL listed after 2019) but any existing UL listings are still valid.