CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A single-phase compressor motor has a rated-load current (RLA) of 20 amps and a locked-rotor amperage (LRA) of 110 amps. Per NEC Section 440.22(A), the maximum branch-circuit short-circuit and ground-fault protective device is generally limited to 175% of the motor's RLA. What is the maximum standard breaker size permitted for this circuit?
- A45 amps
- B35 amps
- C20 amps
- D25 amps
Show answer & explanationAnswer & explanation
Correct answer: B. 35 amps
175% of 20A RLA = 35A (20 × 1.75 = 35). Since 35 amps is a standard breaker size, it is the maximum permitted per NEC 440.22(A) without needing to round up to the next standard size.
Why the other options are wrong
- A. 45A exceeds the 175% limit and would provide inadequate short-circuit protection.
- C. 20A equals RLA itself, undersized and would nuisance trip on compressor startup.
- D. 25A is below the 175% calculated maximum and may not handle inrush current.
Motor Branch-Circuit Protection (175% Rule)
NEC 440.22(A) limits the maximum branch-circuit protective device for a single motor-compressor to 175% of its rated-load current (RLA), unless that size is insufficient to handle starting current, in which case it may be increased up to 225%.
- Formula: Max breaker = RLA × 1.75
- Round down to nearest standard size unless 175% isn't sufficient
- May increase up to 225% if needed for starting current
Memory trick: 175% keeps the breaker from tripping on startup surge.