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NEC national electrical code

NEC: Phase Converters

Oct. 3, 2023
A summary of Art. 455 requirements

Article 455 provides the requirements for installing phase converters [Sec. 455.1]. Before installing a phase converter, read the nameplate to ensure the unit you have matches the drawings and specifications. The nameplate must be permanent and provide the six bits of info required by Sec. 455.2 (seven requirements if it’s a rotary-phase converter).

How do you determine the ampacity of the conductors? It depends on whether the loads are variable or fixed. If the loads are:

  • Variable, the conductor ampacity must be at least 125% of the phase converter nameplate single-phase input full-load amperes [455.6(A)(1)].
  • Fixed, determine if the conductor ampacity meets the requirements for variable loads. If it’s less than that, the ampacity must be at least 250% of the sum of the full-load, 3-phase current rating of the motors and other loads served but only if the input and output voltages (of the converter) are identical. If these voltages differ, determine the current per Sec. 455.6, and multiply by the ratio of output to input voltage [Sec. 455.6(A)(2)].

The manufactured phase conductors must be identified at all accessible locations with a distinctive marking that is consistent throughout the system and premises [Sec. 455.6(B)].

How do you determine overcurrent protection? As with conductor ampacity, it depends on whether the loads are variable or fixed. If the loads are:

  • Variable, the conductor ampacity must be at most 125% of the phase converter nameplate single-phase input full-load amperes [Sec. 455.7(A)].
  • Fixed, protect the conductors per their ampacity. But don’t exceed 125% of the phase converter nameplate single-phase input full-load amperes [Sec. 455.7(B].

What about a disconnecting means? It’s required, and you have to install it in sight from the controller and make it readily accessible [455.8(A)]. It has to be a switch (rated in horsepower if motor loads are used), a circuit-breaker, or a molded-case switch [Sec. 455.8(B)]. Its ampere rating must be at least 115% of the rated maximum single-input full-load amperes [Sec. 455.8(C)]. But if its used for specific fixed loads you can select the rating from Sec. 455.8(C)(1) or (2), which are for a current rated disconnect or a horsepower rated disconnect respectively.

If you connect single-phase loads on the load side of a phase converter, do not connect them to the manufactured phase [Sec. 455.9].

Part II is brief and it applies specific provisions to different types of phase converters. For example, if you have a rotary-phase converter then power can’t be supplied to the utilization equipment until the converter has been started [Sec. 455.21].

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.

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