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Code Calculations

Jul 23, 2004 6:45 PM, By Mike Holt

The requirements for protecting the branch-circuit conductors, control apparatus, and circuits that supply hermetic refrigerant motor-compressors against short circuits and ground faults are noted in 440.22. The size and type of short-circuit and ground-fault protection device for air conditioning and refrigeration equipment are often marked on the equipment nameplate. The manufacturer calculates these ratings per 440.22 and 440.32. If the equipment nameplate specifies “Maximum Fuse Size,” use a one-time or dual-element fuse. If the nameplate specifies “HACR Circuit Breaker,” use a HACR-rated circuit breaker [110.3(B)].

Short-circuit and ground-fault protection can’t exceed the nameplate ratings. If the equipment doesn’t have a nameplate that specifies the size and type of protection device, how do you size those devices? That depends on whether you’re sizing for multiple motors or a single motor.

Where the equipment incorporates more than one hermetic refrigerant motor-compressor, or a hermetic refrigerant motor-compressor and other motors or other loads, size the equipment short-circuit and ground-fault protection with the “largest load” method. The rating of the branch-circuit short-circuit and ground-fault protective device can’t exceed the largest motor-compressor short-circuit ground-fault protection device plus the sum of the rated-load currents of the other compressors. Size the branch-circuit conductors at 125% of the larger motor-compressor current plus the sum of the rated-load currents of the other compressors (440.33).

When working with only one motor compressor, the short-circuit and ground-fault protection device for the motor-compressor conductors must be capable of carrying the starting current of the motor. The protection device also can’t exceed 175% of the equipment load current rating. If the protection device sized at 175% isn’t capable of carrying the starting current of the motor-compressor, you can use the next larger protection device if it doesn’t exceed 225% of the motor-compressor current rating.

Test your knowledge with this question: What size conductor and protection device must you use for a 24A motor-compressor on a 240V circuit (Figure above)?

Step 1: Size the branch-circuit conductor per Table 310.16 and 440.32. 24A x 1.25 = 30A Per 110.14(C)(1)(a) and Table 310.16, you would select a 10 AWG conductor, rated 30A at 60°C.
Step 2: Size the branch-circuit protection device per 240.6(A) and 440.22(A). 24A x 1.75 = 42A

Selecting the next size down protection device yields a 40A device. If a 40A protection device isn’t capable of carrying the starting current, you can size the protection device up to 225% of the equipment load current rating (24A x 2.25 = 54A, next size down 50A).

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