A technician is installing a single-phase, 120-volt, 15-ampere hermetic refrigerant motor-compressor. What is the maximum rating of the branch-circuit overcurrent protective device (OCPD) permitted if the motor-compressor's rated-load current (RLC) is 12 amperes?
- A20 amperes
- B25 amperes
- C15 amperes
- D30 amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 20 amperes
According to NEC 440.22(A), the branch-circuit short-circuit and ground-fault protective device for a hermetic refrigerant motor-compressor shall be capable of carrying the starting current of the motor-compressor and not exceed 175% of the motor-compressor rated-load current (RLC). So, 12 A * 1.75 = 21 A. If the protective device selected is not sufficient for the starting current, it can be increased, but shall not exceed 225% of the RLC. However, 21A is not a standard size, so the next higher standard size, 20A, is allowed as it is below 225% (12A * 2.25 = 27A).
Why the other options are wrong
- B. This exceeds 175% of the RLC (21A) and is not the next standard size.
- C. This is below the calculated maximum of 21A (175% of 12A).
- D. This exceeds 225% of the RLC (27A) and is not permitted under normal circumstances.
Hermetic Motor-Compressor OCPD Sizing
The branch-circuit short-circuit and ground-fault protective device for a hermetic refrigerant motor-compressor must be sized between 175% and 225% of the motor-compressor's rated-load current (RLC), with the lower value (175%) being the starting point, rounded to the next standard size.
- NEC 440.22(A) governs this sizing.
- Initial sizing is 175% of RLC.
- Can be increased up to 225% of RLC if needed for starting current.
- Must use standard OCPD sizes (NEC 240.6).
Memory trick: Cool compressor needs its own special protection math: 175 to 225.