Locked Rotor Amps (LRA) is the maximum current a motor draws when the rotor cannot turn, representing the worst-case inrush condition. LRA values are critical for designing electrical protection systems and establishing proper breaker sizing in HVAC applications.
Technical Definition
LRA occurs when full voltage is applied to a stationary rotor with zero speed. This condition exists during motor startup and can occur if mechanical jam develops during operation. LRA values are standardized under NEMA and printed on all motor nameplates alongside Full Load Amperage (FLA). A 5-ton commercial air conditioner compressor might have FLA of 28 amps and LRA of 168 amps. LRA is approximately 3-4 times higher than FLA for induction motors.
Electrical System Design
Breaker ratings must accommodate LRA without nuisance tripping during normal starting. Instantaneous trip settings in magnetic starters typically set at 1.3-1.5 times LRA to prevent false operation. Service entrance capacity calculations should account for simultaneous LRA draws from multiple motors. Undersized electrical service cannot support motor starting, causing voltage sag and potential equipment damage.
Protection Strategy
Dual-element fuses and time-delay breakers distinguish between transient LRA and sustained overload. Soft starters and VFDs reduce effective starting current by limiting voltage ramp rate. Always verify LRA values during equipment selection and electrical design to prevent nuisance breaker trips and ensure reliable motor starting capability in HVAC systems.