CSLB C-20 HVAC ContractorControls, Electrical and SafetyHard
A compressor motor is rated with a full-load amperage (FLA) of 12 amps and has no internal thermal overload protection. Per standard electrical protection practice, what is the maximum overload device trip setting (at 125% of FLA)?
- A13.5 amps
- B18.0 amps
- C15.0 amps
- D12.0 amps
Show answer & explanationAnswer & explanation
Correct answer: C. 15.0 amps
Overload protection for a continuous-duty motor without built-in thermal protection is typically sized at 125% of the motor's FLA: 12 amps × 1.25 = 15 amps. Setting it at the FLA itself (12A) would nuisance trip under normal starting conditions, while 18A (150%) exceeds standard overload protection allowances for this type of motor.
Why the other options are wrong
- A. This value doesn't correspond to the standard 125% calculation (12 × 1.25 = 15).
- B. 18A represents 150% of FLA, which exceeds standard overload protection sizing for this motor type.
- D. Setting overload at exactly FLA would cause nuisance tripping under normal load.
Motor Overload Sizing (125%)
Overload protection devices for continuous-duty motors without integral thermal protection are typically sized at 125% of the motor's full-load amperage.
- Formula: Overload setting = FLA × 1.25
- Prevents motor damage from sustained overcurrent
- Different from short-circuit/branch-circuit protection sizing
- Motors with built-in protection may use different sizing rules
Memory trick: Quarter more than full load — that's the overload code.