2020 National Electrical Code Changes

Chapter 9 of the NEC — Part 5

March 16, 2021

Calculating voltage drop with help from Table 8.

The term “voltage drop” is often misused to mean the difference between the voltage measured at Point A in a circuit and that measured at Point B. Typically, Point A is the supply breaker, and Point B is utilization equipment. Voltage drop is actually the result of a calculation, and it is a design consideration.

The catch is that to calculate voltage drop you need the circular mils of the conductor for which you are making the calculation. Table 8 provides that value for conductors ranging from 18 AWG to 4/0. When you go larger than 4/0, the conductor size is expressed in thousands of circular mils (kcmil). A 4/0 conductor is 211.6kcmil, and the next size up is 250kcmil.

The NEC does not specify a maximum voltage drop for general circuits. It does provide a figure for “sensitive electronic equipment” [647.4(D)]. You will also find suggested figures in an Informational Note in 210.19(A) and in 310.14(A)(1). Also, manufacturers of some utilization equipment may specify a figure.

You can usually correct for voltage drop by increasing the conductor one size.

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.