National Electrical Code

The NEC and Branch-Circuit Ratings — Part 1

Dec. 7, 2021

How do you know if you have the correct ampere rating (or setting) of an overcurrent device?

Part II of Art. 210 of the 2020 National Electrical Code provides the requirements for rating branch circuits. It’s pretty simple, really. You rate them per the maximum permitted ampere rating (or setting) of the overcurrent device [Sec. 210.18].

But how do you know if you have the correct ampere rating (or setting) of the overcurrent device? You can’t know that until you know the ampacity of the conductors the overcurrent device is protecting. So, we segue into Sec. 210.19, which covers conductor minimum ampacity and size. And that is followed by Sec. 210.20, which covers overcurrent protection.

Branch circuits of more than 600V have different requirements and they are covered by Sec. 210.19(B). Branch circuits of 600V or less are covered by Sec. 210.19(A). These requirements take about five times the space in the NEC that the Sec. 210.19(B) requirements do.

Starting off Sec. 210.19 are three Informational Notes, including the one that is often referenced when people ask, “What is the maximum voltage drop allowed by the NEC?” Except for “sensitive electronic equipment” [Sec. 647.4(D)], the NEC doesn’t mandate voltage drop limits. It does provide a suggestion in this Informational Note and also in Sec. 310.14(A)(1). The NEC was doing that before Art. 647 appeared in its pages. The “correct” amount of voltage drop is a judgment call. You want it as low as you can practically and cost-effectively make it, and deciding what that number is requires balancing many factors.

Or, you can just go with a rule of thumb or best practice to save yourself all that cognitive work The rule of thumb in the NEC is roughly simplified into “3% for branch circuits and 5% for feeders,” and that is the difference in voltage at the source compared to the voltage at the farthest load.

There is one general requirement for conductors 600V or less. It’s stated in Sec. 210.19(A)(1): The ampacity must be at least the larger of two numbers. What are those two numbers?

Where the branch circuit supplies continuous loads (whether mixed with noncontinuous loads or not), the ampacity must be at least the total of all noncontinuous loads plus 125% of the total of the continuous loads on that circuit [Sec. 210.19(A)(1)(a)].
The ampacity can’t be less than the maximum load after applying any adjustment or correction factors per Sec. 310.15 [Sec. 210.19(A)(1)(b].

Two exceptions are listed below this.

After the general requirement is one for branch circuits with more than one receptacle [Sec. 210.19(A)(2)], another for household ranges and cooking appliances [Sec. 210.19(A)(3)], and a set for other loads [Sec. 210.19(A)(B)].

In Part II, we’ll look at what Art. 210 says about overcurrent protection requirements.

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.