Jim Colacino
New User
- Location
- Rochester, New York
- Occupation
- Electrical Contractor
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
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
