Figure. Branch-Circuit Conductor = 18A × 125% = 22.50A. Use a 10 AWG conductor rated 30A at 60°C.

Stumped by the Code? NEC Requirements for Sizing Branch Circuit Conductors for a Single Motor Compressor

April 5, 2022

Answering your NEC questions.

All questions and answers are based on the 2020 NEC.

Q. What does the NEC require as the minimum size of branch-circuit conductors for a single motor compressor for air conditioning or refrigeration equipment?

A. Branch-circuit conductors to a single motor compressor for air-conditioning and refrigeration equipment must have an ampacity not less than the greater of [Sec. 440.32]

(1) 125% of the motor-compressor rated-load current.

(2) 125% of the branch-circuit selection current.

Branch-circuit conductors for a single motor compressor must have short-circuit and ground-fault protection sized between 175% and 225% of the rated-load current [440.22(A)].

Let’s look at an example scenario to show you how this works.

What size conductor and overcurrent device are required for an 18A motor compressor for air-conditioning equipment (See Figure)?

Solution:

Step 1: First, determine the branch-circuit conductor [Sec. 440.32 and Table 310.16].

Branch-Circuit Conductor = 18A × 125% = 22.50A

Use a 10 AWG conductor rated 30A at 60°C [Sec. 110.14(C)(1)(a)(2) and Table 310.16]

Step 2: Next, determine the branch-circuit overcurrent protection [Sec. 240.6(A) and Sec. 440.22(A)].

Branch-Circuit Over Current Protection = 18A × 175% = 31.50A

Use the next size down, which is 30A. If the 30A short-circuit and the ground-fault protective device cannot carry the starting current, then the protective device can be sized up to 225% of the equipment load current rating.

Branch-Circuit Over Current Protection = 18A × 225% = 40.50A

Use the next size down, which is 40A.

Author’s notes:

A 30A or 40A overcurrent device can protect a 12 AWG conductor for an air-conditioning circuit. See Sec. 240.4(G) for details.
The branch circuit selection current is not required to be marked on the nameplate unless the internal overload equipment permits continuous operation at a current that exceeds 156% of the rated load current. The minimum circuit ampacity that is found on the equipment nameplate is 125% of the rated load current and the conductor selection can be made directly from that value as shown on the nameplate.

Q. What is the Code requirement for the listing of stationary generators?

A. Stationary generators rated 600V and less must be listed [Sec. 445.6].

Exception: One-of-a-kind or custom-manufactured generators are permitted to be field labeled by a field evaluation body.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.