© Xiaomin Wang | Dreamstime.com
Dreamstime M 28489449 6329d3866ecb1

NEC: Motors — Part 10

Sept. 20, 2022
Sizing conductors of adjustable-speed drive systems

Article 430, Part X provides the NEC requirements for adjustable-speed drive systems. Notice the word “systems,” which means this covers more than the drive itself.

Size the branch or feeder circuit conductors supplying the power conversion equipment of adjustable-speed drive system to an ampacity at least 125% of the rated input power of the conversion equipment [430.122(A)]. Informational Note No. 1 states this equipment can have multiple power ratings and corresponding input currents. Informational Note No. 2 is new with the 2020 NEC, and it addresses the issue of conductor breakdown susceptibility. It lists some contributing factors; be smart about this and consider these as part of a mitigation strategy.

Size the output conductors (those between the power conversion equipment and the motor) at an ampacity at least 125% of the motor full-load current as determined by 430.6(A) or (B) [430.122(B)]. But you don’t have to follow this rule if power conversion equipment is listed and marked as “Suitable for Output Motor Conductor Protection.” The rule you follow instead is one of the two enumerated in the Exception Note.

If your system has a bypass device, ensure the conductor ampacity for the bypass is at least that determined by 430.6 [430.122(C)]. And if you use a bypass, the circuit conductors supplying the power conversion equipment must be sized at 125% of either the rated input current to the power conversion equipment or the motor full-load current rating as determined by 430.6.

If you have other loads than one motor, ensure the ampacity of the supply conductors conforms to 430.24 [430.122(D)].

If motor overload protection is provided by the adjustable drive system, you don’t need to supply additional motor overloads [430.124(A)]. In fact, it’s better if you don’t. Overtemperature protection of the motor must be provided by the adjustable drive system, if the motor is not rated to operate at the nameplate rated current over the speed range required by the application [430.126]. One reason for this requirement is a fan-cooled motor that is turning more slowly is also cooling more slowly. You’ll find detailed requirements for overtemperature protection in 430.126.

You can relocate the motor disconnecting means to the incoming line to the adjustable drive system [430.128], but only if it has a rating of at least 115% of the rated input current of the conversion unit.

The branch-circuit short-circuit and ground-fault protection requirements for single motor circuits are in 430.130. In essence, you’re going to use the normal sizing process outlined in 430.52(C)(1), (3), (5), or (6). But there’s an exception, and following it are two Informational Notes. And if the manufacturer has protection requirements listed on the equipment, you cannot exceed those values.

The branch-circuit short-circuit and ground-fault protection requirements for circuits including more than a single motor are in 430.131. A key takeaway here is for purposes of sizing the branch-circuit short-circuit and ground-fault protection devices, you consider the conversion equipment to be a motor controller.

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations