Feeder conductor sizing to VFD

[dave121]

If I had a VFD with a 250 amp breaker, single phase in - 3 phase load out, and a calculated load of 150 amps on the single phase lines, wouldn't a 1/0 copper be big enough to feed the VFD? Next, the time delay fuse protecting those 1/0 conductors could be 300 amp fuses? 150 x 1.75 = 262- next size up being 300?
Thanks for your help.

 

[dave121]

Just remembered the 125%, which makes the 1/0 feeder go to 3/0 cu.? Could the fuse protecting the conductors be as low as 200 amp?

 

[petersonra]

I don't know what you mean by "calculated load". The code requires you use the rated input current of the drive. There is no calculation.

See 430.122 (A) for conductor sizing.

430.130 covers OCPD ratings.

 

[dave121]

Okay, I thought that was what was used, thanks for pointing that out. Then with a input rating of 180 amps x 1.25 gets me to 225 a conductors feeding the VFD, with a 225a maximum fuse protecting the 4/0. (240.4.B)?

 

[texie]

Okay, I thought that was what was used, thanks for pointing that out. Then with a input rating of 180 amps x 1.25 gets me to 225 a conductors feeding the VFD, with a 225a maximum fuse protecting the 4/0. (240.4.B)?

The type and size of the OCPD will be indicated by the drive manufacturer.

 

[dave121]

The VFD does have its own OCPD in the form of a 250 A breaker. I was trying to get the proper size conductor feeding the VFD, which I think we now have. ( 430.122 ) thank you. What I need is to know how to size the OC to protect the conductors feeding the VFD 4/0 conductors. I apologize for not saying what I mean. Would that be 240.4.B?

 

[drktmplr12]

The VFD does have its own OCPD in the form of a 250 A breaker. I was trying to get the proper size conductor feeding the VFD, which I think we now have. ( 430.122 ) thank you. What I need is to know how to size the OC to protect the conductors feeding the VFD 4/0 conductors. I apologize for not saying what I mean. Would that be 240.4.B?

first size the ungrounded conductors to the VFD per 430.122(A), then size the OCPD to protect them per 240.4. then select the size of the EGC per 250.122.

side note for 250.122: if you upsize conductors for any reason, the ground must also be upsized by same ratio-except if you are on 2014 or older NEC, where it is only required for voltage drop. eg if you go from 300 to 400 kcmil feeder circuit, you must upsize the ground by 33% (400/300 kcmil= 1.33). Assuming your ground was #3 (5.26 kcmils), the new minimum ground is 1.33 x 5.26 = 7.0 kcmil or #1.

 

[dave121]

Thank you!

 

[petersonra]

A VFD is a motor circuit. You don't have to size motor circuits short circuit and ground fault devices by anything in article 240.The built-in overload protection on the VFD provides overload protection so the circuit breaker or fuse only provides short circuit protection which is covered in article 430 not article 240.

 

[Jraef]

A VFD is a motor circuit. You don't have to size motor circuits short circuit and ground fault devices by anything in article 240.The built-in overload protection on the VFD provides overload protection so the circuit breaker or fuse only provides short circuit protection which is covered in article 430 not article 240.

He's right.
Article 430 trumps Article 240, it says so right in 240.3. So you size the conductors per 430.122, then you size the OCPD per the VFD mfr listing (as called for in 110.3.B).

Also, pay close attention to the EXACT wording of the VFD installation requirements, you cannot assume to substitute Fuse for Circuit Breaker unless the VFD mfr. expressly states that a Circuit Breaker by itself is allowable. More and more we are seeing VFDs requiring fuses no matter what in order for them to pass UL listing and SCCR requirements, i.e. even if you have a breaker, the VFD still needs fuses (or without fuses, the SCCR is only 5kA and it's nearly impossible to get the Available Fault Current that low).

 

[drktmplr12]

He's right.
Article 430 trumps Article 240, it says so right in 240.3. So you size the conductors per 430.122, then you size the OCPD per the VFD mfr listing (as called for in 110.3.B).

To be clear, the OCPD you are referencing is the motor branch short circuit and ground fault protection described in 430 Part IV and is typically provided as fuses or inst. trip breaker in the VFD control panel, correct?

I am mistaken in applying 240.4 to the inverse time circuit breaker for the VFD feeder (430 Part V). Typically this would be the breaker in a panelboard, switchboard, MCC, etc. This results in a breaker that is always less than the maximum Table 430.52 stipulates for inverse time breakers. That said, I may have been limiting myself by not considering an instaneous trip breaker to adequately protect the feeder conductors, provided the motor controller's overloads are set properly. TIL thanks Jraef

 

[petersonra]

To be clear, the OCPD you are referencing is the motor branch short circuit and ground fault protection described in 430 Part IV and is typically provided as fuses or inst. trip breaker in the VFD control panel, correct?

I am mistaken in applying 240.4 to the inverse time circuit breaker for the VFD feeder (430 Part V). Typically this would be the breaker in a panelboard, switchboard, MCC, etc. This results in a breaker that is always less than the maximum Table 430.52 stipulates for inverse time breakers. That said, I may have been limiting myself by not considering an instaneous trip breaker to adequately protect the feeder conductors, provided the motor controller's overloads are set properly. TIL thanks Jraef

The short circuit protection can be a variety of things. The overload protection is provided by the VFD itself.

 

[dave121]

The VFD panel is a complete unit including it's own 250 amp. breaker to which I will connect the feeders on the line side. The ocd I'm needing information how to size will be ahead of the feeders, the service disconnect.

 

[petersonra]

The VFD panel is a complete unit including it's own 250 amp. breaker to which I will connect the feeders on the line side. The ocd I'm needing information how to size will be ahead of the feeders, the service disconnect.

it is not the service disconnect. it is just a feeder.

have you looked at 430.62?

 

[dave121]

It will be the service disconnect ahead of the feeder conductors going to the VFD. Meter- service conductors-service disconnect-feeders-VFD breaker. 430.62 says not larger than the scd for any motor. That scd is the 250 amp breaker that is part of the VFD panel. With 430.62 saying not larger, can I go smaller and if so what are the limitations? Are services for motor loads limited to 80%?