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