A client requests a new dedicated 20-ampere, 120-volt branch circuit for a single-phase, hermetic refrigerant motor-compressor. The motor-compressor has a nameplate rated-load current (RLC) of 15 amperes and a branch-circuit selection current (BCSC) of 18 amperes. What is the minimum continuous current rating required for the branch-circuit overcurrent protective device (OCPD)?
- A25 amperes
- B20 amperes
- C22.5 amperes
- D15 amperes
Show answer & explanationAnswer & explanation
Correct answer: B. 20 amperes
For motor-compressors, the OCPD must be sized based on the branch-circuit selection current (BCSC) or rated-load current (RLC) if BCSC is not marked. The OCPD must be capable of carrying the motor's starting current, but not exceed 225% of the BCSC or 175% of the RLC if no BCSC is marked. The minimum continuous rating for the OCPD is 125% of the BCSC or RLC, whichever is greater, but cannot be less than the next standard size.
Why the other options are wrong
- A. This is the next standard OCPD size above 22.5A, which would be the maximum permitted, but not the minimum continuous rating for a 20A circuit.
- C. This is 125% of the BCSC (18A * 1.25 = 22.5A), which is the minimum continuous current the OCPD must be able to handle. However, the OCPD itself will be a standard size.
- D. This is the rated-load current, not the value used for OCPD sizing.
Hermetic Motor-Compressor OCPD Sizing
The overcurrent protective device (OCPD) for a hermetic refrigerant motor-compressor branch circuit must be sized based on the branch-circuit selection current (BCSC) or rated-load current (RLC), ensuring it can handle starting current but also protect against overcurrent.
- OCPD must be rated for at least 125% of the BCSC or RLC.
- Maximum OCPD size is 225% of BCSC or 175% of RLC.
- Must use the next higher standard OCPD size if calculated value is not standard.
Memory trick: Cool circuits need precise protection.