CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A client requests a new dedicated 20-ampere, 120-volt branch circuit for a single-phase, hermetic refrigerant motor-compressor. The motor-compressor nameplate indicates a rated-load current (RLC) of 15 amperes and a branch-circuit selection current (BCSC) of 20 amperes. What is the minimum required ampacity for the branch-circuit conductors?
- A15 amperes
- B20 amperes
- C25 amperes
- D30 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 25 amperes
For hermetic refrigerant motor-compressors, the branch-circuit conductors must be sized based on either 125% of the rated-load current (RLC) or 100% of the branch-circuit selection current (BCSC), whichever is greater (NEC 440.32). In this case, 125% of RLC is 15 A * 1.25 = 18.75 A. The BCSC is 20 A. The greater of these two is 20 A. Therefore, the minimum required ampacity for the conductors is 20 amperes.
Why the other options are wrong
- A. This is the rated-load current (RLC) by itself, not the calculated minimum ampacity.
- B. This is the branch-circuit selection current (BCSC), which is the greater value after comparing 125% of RLC versus BCSC. However, the question asks for the minimum required ampacity for the conductors. The calculated value is 20 A. Therefore, the conductor must have an ampacity of at least 20 A. A 14 AWG conductor is rated for 20 A (75°C). So 20 A is the minimum required ampacity.
- D. This value is higher than the minimum required ampacity.
Hermetic Motor-Compressor Conductor Sizing
Branch-circuit conductors for a hermetic refrigerant motor-compressor must have an ampacity not less than 125% of either the motor-compressor's rated-load current (RLC) or the branch-circuit selection current (BCSC), whichever is greater.
- Applies to hermetic motor-compressors (HVAC, refrigeration).
- Compare (RLC × 1.25) vs. BCSC.
- Take the greater of these two values.
- Multiply that greater value by 1.25 for final conductor ampacity.
- Refer to NEC Article 440.32.
Memory trick: Hermetic's Current, Compare and Multiply.