Motor Full Load Amps (FLA) Calculator
Calculate the full load amperage (FLA) of AC electric motors.
Find the NEC-required breaker size, wire gauge, and overload protection for single-phase and three-phase motors.
What Is Full Load Amps (FLA)? Full Load Amps (FLA) is the current drawn by an electric motor when operating at full rated load and voltage. It is the benchmark for sizing conductors, circuit breakers, and overload protection. FLA is stamped on every motor’s nameplate — always use the nameplate FLA when available, as it is measured for the specific motor. Calculated FLA provides an estimate when the nameplate is unavailable or for motor sizing during design.
FLA Formulas Single-phase AC motor: FLA = (HP × 746) / (Voltage × Efficiency × Power Factor) Where 746 W = 1 HP (horsepower to watts conversion).
Three-phase AC motor: FLA = (HP × 746) / (√3 × Voltage × Efficiency × Power Factor) The √3 factor (≈1.732) accounts for the three-phase power relationship.
Typical efficiency for induction motors: 85–95% depending on size. Typical power factor at full load: 0.85–0.90.
NEC Ampacity Rules (US National Electrical Code) The NEC requires specific safety margins for motor circuits: Branch circuit conductors: must be sized for at least 125% of FLA (NEC 430.22). Branch circuit breaker or fuse: up to 250% of FLA for standard breakers (NEC 430.52), or 175% for fuses. Overload protection (motor overloads): set at 115–125% of nameplate FLA to protect the motor windings. Disconnect: must handle at least 115% of FLA and must be within sight of the motor.
Motor Service Factor (SF) The service factor is a multiplier indicating how much beyond rated load the motor can handle continuously. SF = 1.0: motor can only handle rated load. SF = 1.15 (common): motor can handle 115% of rated load for short periods. When SF > 1.0, overloads may be set at 125% of nameplate × SF.
NEMA Motor Nameplate Data Every NEMA (National Electrical Manufacturers Association) motor nameplate includes: HP rating, voltage, FLA, RPM, frame size, insulation class, service factor, efficiency, and power factor. The insulation class (A, B, F, H) defines the maximum allowable winding temperature. Frame sizes (143T, 182T, 213T, etc.) define the mounting dimensions.
Horsepower to kW 1 HP = 0.7457 kW. A 10 HP motor outputs 7.457 kW of mechanical power. Electric power input (kW) = Output (kW) / Efficiency. A 10 HP motor at 90% efficiency draws 8.3 kW of electrical power at full load.
Common Motor Applications HVAC compressors: 1–20 HP, three-phase, 208–230/460V. Pump motors: 0.5–100+ HP. Conveyors: 1–50 HP. Residential HVAC: 0.5–5 HP single-phase, 115–240V. Industrial: almost always three-phase, 460V or 480V, for efficiency.