National Electrical Code

Chapter 9 of the NEC — Part 6

April 6, 2021

Use Table 9 to determine electrical reactance values.

Electrical reactance is a measure of the opposition to current flow from inductance or capacitance.

Inductive and capacitive components store energy. With resistance, the energy is continuously dissipated as heat. With reactance, that stored energy is returned to the circuit and the result includes phase changes. Knowing the reactance of a coil or other device (or even an entire circuit) is important in some applications. For example, determining what phase changes to expect can help you select the right transformer.

If you start a motor, the inductive reactance is initially low but rapidly increases. This is why we have inrush current. We also have inrush current in non-resistive lighting technologies (e.g., fluorescent).

The total reactance of a circuit is the difference between its inductive reactance and its capacitive reactance. You may recall the vector diagrams from your electrical theory classes if you haven’t been using them on the job (yet).

With alternating current (AC), even the conductors have reactance. The amount varies with the conductor size and the raceway containing the conductors. Need to know the values? See Table 9 in Chapter 9.

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.