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Tip of the Week: Is your voltage imbalance excessive?

Jan. 22, 2018
More than 2% is excessive for motors; for other types of load, more than 4% is too much.

Suppose you are measuring the voltage for the nominal 480V supply to a motor. If you measured the same voltage on each phase (e.g., A-B, A-C, B-C), then clearly there’s no voltage imbalance.

Suppose you measure 481V, 480V, and 479V. That’s an imbalance, but is it significant? The general rule of thumb for motors is you want 2% or less imbalance. For other types of load, you’re looking for 4% or less.

That percentage isn’t a percentage of the nominal voltage. Nor is it the simple difference between the highest and lowest voltage; the actual calculation is a bit more complicated.

Here’s an approach for seeing if your circuit is within the desired limit.

Measure the voltage on each phase. Suppose you measure 469V, 474V, and 481V on a motor supply circuit.

Multiply your desired percentage by the lowest voltage. This gives you the maximum allowable voltage difference between the lowest measured voltage and the highest measured voltage.

In this case, the maximum difference is 9.38V. But the difference between 481V and 469V exceeds that, so you’ve got excessive voltage imbalance. If the circuit had been serving a non-motor load (other than a “sensitive electronics” load), voltage imbalance would be within normally acceptable limits.

The limitation of this approach is that it doesn’t tell you what your actual voltage imbalance is, only whether it’s within the desired limits.

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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