Mike Dolt Enterprises
Mike Dolt Enterprises
User: textmi, Location: California
Asked on: Thursday, December 26, 2024 at 3:20 pm EST
Answered on: Thursday, March 6, 2025 at 6:41 am EST
Question: I have been reading previous threads and can't find a definite answer to this question.I have a 75 hp VFD with an input rating of 96A. Per NEC 430.122, I should size the line side conductor to 125%*96A=120A. This can be done with #1 conductors and #6 ground (will handle 125A).However, the breaker from the panel serving this VFD is a 150A and the VFD also comes with a 250A thermal magnetic breaker. Which of the following is correct?1) I can use line-side conductors per NEC 430.122 (handle up to 125A) although the line-side breaker ratings are all higher amperage than what my conductor can handle.2) I will need to upsize my conductors to handle the OCPD (150A) or replace the OCPD at the feeding panel to be 125A so that my conductors are protected.
You're right, this is a common point of confusion with VFD installations. Let's break it down and get you a definite answer
The correct approach is #2: You will need to upsize your conductors to match the upstream OCPD (150A) unless you can reduce the OCPD to a suitable level that protects the conductors.
Here's why: NEC 240.4(B) - Protection of Conductors
This is the key section of the NEC. It states that conductors must be protected against overcurrent in accordance with their ampacity ratings unless one of the exceptions applies. While there are exceptions for certain situations, none of them apply to this general VFD installation scenario. Crucially, the overcurrent protective device (OCPD) must protect the conductor.
NEC 430.122 - Conductors - Minimum Size and Ampacity
This section dictates how to size conductors for VFDs. It specifies the 125% of the rated input current calculation (96A 1.25 = 120A in your case).
However, this calculation is the minimum ampacity allowed. It doesn't override the general requirement of 240.4(B). It's a sizing calculation for the load. OCPD Coordination
The OCPD must protect the conductor based on its ampacity. If your conductor ampacity is less than the OCPD rating, the conductor is vulnerable to damage from an overload.
Therefore, you have two valid options
1.
Upsize Conductors
Increase the conductor size to a rating that equals or exceeds the OCPD at the panel feeding the VFD (150A). You'd need to check your conductor ampacity tables (NEC Table 310.16) to determine the appropriate size, considering insulation type, ambient temperature, and any derating factors. #1/0 conductors will most likely be able to handle 150A in accordance with table 310.
16. Also, upsize the ground to #4 copper.2.
Reduce OCPD
Replace the 150A breaker at the feeding panel with a smaller breaker that matches or is slightly lower than the conductor ampacity. If you want to use the #1 conductors, you would need to replace the 150A breaker with a 125A breaker or smaller. This will provide the appropriate level of protection for the conductors.
Regarding the 250A Breaker on the VFD
This is typically a short-circuit protective device internal to the VFD. It's designed to protect the VFD's internal components in the event of a major fault. It's not intended to provide overload protection for the conductors feeding the VFD. The upstream breaker (the 150A or resized breaker) is responsible for protecting the line-side conductors.
In summary: Prioritize conductor protection based on NEC 240.4(B). Use NEC 430.122 as a guideline for minimum conductor size, but not an excuse to under-protect them.* Your primary goal is to ensure the conductors are protected by the upstream OCPD.I hope this clarifies things! Let me know if you have any further questions. Always consult with a qualified electrical engineer or inspector to ensure your installation meets all applicable codes and safety standards. Flag for review