Proper Branch Circuit Protection for a VFD with Multiple Motors

[Jim Colacino]

I’m seeking advice regarding a situation with a large casino and resort built in 2018. Recently, their 34.5kV primary switchgear experienced a failure due to condensation buildup (caused by a failed heater), which led to arcing. Thankfully, Schneider surge protection devices sacrificed themselves, likely saving critical loads. However, one rooftop HVAC unit suffered a catastrophic failure of a VFD, while another seemed to survive.

During troubleshooting, I noticed an interesting setup: the failed VFD powers two 25HP motors. Each motor has its own overload protection but shares a single VFD rated at 50HP at 480VAC, with a main breaker rated at 90A. The feeder conductors to this VFD run approximately 300 feet and are connected to a 90A 3-pole breaker. However, the conductors are #8 THHN, which, in this instance, melted all the way back to the power distribution panel and now need replacement. The rated output current of the VFD is 78A.

While I understand that motors can be over fused in certain cases per the NEC, this setup raised concerns for me. I’ve reviewed the original prints from a reputable electrical engineering firm, and the installation seems to match the design. However, I’m questioning if this is truly NEC-compliant. On a related note, there’s another VFD on the same unit (for return air) fed by a 60A breaker but wired with #10 THHN.

As a contractor who values NEC adherence and prioritizes safety, I want to ensure that replacing the conductors is code-compliant. My specific question is:

If a VFD requires a 90A feed, what conductor size is required to meet the intent of the NEC, especially considering the 300-foot distance? Additionally, does the presence of two motors sharing this VFD affect the calculation?

I appreciate any insights or references to specific NEC articles that would apply to this scenario. I want to ensure this issue is corrected properly and safely before proceeding.



Thank you in advance!
Jim

 

[ptonsparky]

Sounds undersized.
what is the MCA on the label of the rooftop unit?

Min conductor size typically would be 125% of the input amps of the drive. The two motors will not make any difference.

Assuming This is a packaged rooftop unit so you concern yourself with its label MCA.
The VFD will help with VD on start so design for what you are happy with. There maybe a minimum value for the unit but the NEC does not address it other than footnotes.

 

[garbo]

I always ordered VFD'S with input fuses & a handy disconnect on drive panel. Bad ideal to run two important motors that most likely operate 24/7 on a single VFD. We had a remodel at a ambutory care center where they installed a roof top packaged heating & AC unit. They installed four believe 10 HP fan motors on one VFD. Each motor had a combination over current / circuit breaker device. Unit only ran a few months before one motor burnt out. They had to cancel two days in OR'S that did small procedures until motor was replaced. First electrician should have removed the wires from over current device and allow the other three fans provide enough air. This was middle of spring so not much AC needed. Of course the NC contacts on 4 overload devices were wired in series to a stop circuit.

 

[Jraef]

The rated output current of the VFD is 78A.

The rated INPUT current is probably less, probably around 72A, but not low enough that 8ga THHN was appropriate. 430.122 requires that the conductors feeding the drive must be sized at 125% of the input current MINIMUM. That means wire sized for 90A is the smallest allowable for this. That’s not 8ga.