Art. 430, Part V provides the requirements for motor feeder short-circuit and ground-fault protection. It specifies protective devices intended to protect motor feeder conductors from overcurrents caused by short circuits or grounds [430.61]. This is in contrast to motor overload protection, which is for protecting the motor from overheating due to overload. With motors, we separate these two functions instead of (as with other circuits) having the overcurrent protection device (OCPD) perform both.
The requirement for the rating or setting of an OCPD for a specific motor load is in 430.61(A). That consists of two paragraphs, two exceptions, and one informational note.
The first paragraph consists of one long run-on sentence. Let’s break it down:
- This applies to a feeder.
- The feeder is supplying a specific fixed motor load.
- The feeder conductors are sized per 430.24.
- The rating or setting of the feeder OCPD cannot exceed the rating or setting of the largest motor branch circuit OCPD for any motor(s) supplied by the feeder plus X.
- X is the sum of the full load currents of the other motors of the group.
- The branch circuit OCPD you are adding to X must be sized per 430.52; if it’s for a hermetic motor, it must be sized per 440.22(A).
The second paragraph basically says that in case of a tie for size champ, pick one to be the OCPD you are adding to X and include the other motor in your calculation of X.
The first exception applies when you have an instantaneous trip breaker. When that’s the case, you assume for your calculation its value doesn’t exceed the maximum percentage of motor full load current permitted by Table 430.52.
The second exception applies when the feeder OCPD also supplies a motor control center. In that case, you must apply 430.94.
The informational note tells you to see Informative Annex D, Example D8. This walks you through the whole thing and is well worth slowly poring over.
Then we get to 430.62(B), which says that if the ampacity of the feeder conductors exceeds what’s required by 430.24, you can base the rating or setting of the OCPD on the ampacity of the feeder conductors.
Part V also addresses the rating or setting of the OCPD when other (nonmotor) loads are involved [430.63]. In this case, the OCPD must have a rating that is at least as large as the sum of the following:
- The sum of the other loads.
- For a single motor, the rating permitted by 430.52.
- For a single hermetic motor, the rating permitted by 440.22.
- For two or more motors, the rating permitted by 430.62.
And there’s an exception: when the feeder OCPD also supplies a motor control center, you must apply 430.94.
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.