CSLB C-20 HVAC ContractorControls, Electrical and SafetyHard
A blower motor is rated 1/2 HP with a full-load amperage (FLA) of 5.0 amps and a service factor (SF) of 1.15. What is the maximum current this motor can safely draw on a continuous basis?
- A5.0 amps
- B5.75 amps
- C6.5 amps
- D4.35 amps
Show answer & explanationAnswer & explanation
Correct answer: B. 5.75 amps
The service factor indicates how much above the rated FLA a motor can be safely loaded on a continuous basis. Multiplying FLA by the service factor gives the maximum safe continuous amperage: 5.0 A × 1.15 = 5.75 A. Exceeding this value risks overheating and premature motor failure even though it is above the nameplate FLA.
Why the other options are wrong
- A. This is the nameplate FLA, not the maximum allowed with the service factor applied.
- C. This overstates the allowable current beyond what the service factor supports.
- D. This divides rather than multiplies by the service factor, giving an incorrect lower value.
Motor Service Factor
Service factor (SF) is a multiplier indicating how much above the nameplate FLA a motor can be safely operated continuously without overheating.
- Formula: Max current = FLA × SF
- Common SF values: 1.0, 1.15, 1.35
- Running above the SF-adjusted amperage shortens motor life
Memory trick: Multiply FLA by SF for the safe ceiling.