Q. How do I size the conductor and overcurrent protection for an air-conditioner? (Answer based on 2011 NEC.)
Find the Answer
A. The short circuit and ground fault protective device for a single motor compressor must not be more than 175% of the motor compressor current rating. If the protective device sized at 175% isn’t capable of carrying the starting current of the motor compressor, the next size larger protective device can be used, but in no case can it exceed 225% of the motor compressor current rating [440.22(A)].
Let’s work through an example to make sure you understand how to make this type of calculation.
Example: What size conductor and protection are required for a 24A motor compressor connected to a 240V circuit?
Step 1:
Determine the branch-circuit conductor [Table 310.15(B)(16) and 440.32]
24A x 1.25 = 30A, 10 AWG, rated 30A at 75°C [Table 310.15(B)(16)]
Step 2:
Determine the branch circuit protection [240.6(A) and 440.22(A)]
24A x 1.75 = 42A, next size down = 40A
If the 40A short circuit and ground fault protective device isn’t capable of carrying the starting current, then the protective device can be sized up to 225% of the equipment load current rating.
24A x 2.25 = 54A, next size down 50A
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.