Motor Full Load Current Calculator
Calculate the full load current (FLA) drawn by a single-phase or three-phase AC motor from its horsepower, voltage, efficiency, and power factor. Used for selecting fuses, breakers, wire gauges, and motor starters.
About this calculator
The full load current (FLA) of an AC motor is derived from the power balance equation. Motor output power in watts equals horsepower × 746 (the watt-equivalent of one horsepower). The electrical input power must be higher to account for motor losses, so it is divided by efficiency (η) expressed as a decimal. For a three-phase motor, power is shared across three lines, introducing the √3 factor from line-to-line voltage geometry. The complete formula is: FLA = (HP × 746) / (V × η × PF × √3) for three-phase, or FLA = (HP × 746) / (V × η × PF) for single-phase, where PF is the power factor. Knowing FLA is essential for NEC-compliant circuit protection: branch circuit conductors must be sized at 125% of FLA (NEC 430.22) and the overcurrent device at 250% of FLA for inverse-time breakers (NEC 430.52).
How to use
Calculate FLA for a 10 HP, three-phase, 460 V motor with 91% efficiency and 0.88 power factor. FLA = (10 × 746) / (460 × 0.91 × 0.88 × √3) = 7,460 / (460 × 0.91 × 0.88 × 1.732) = 7,460 / 641.5 ≈ 11.6 A. For NEC compliance, size the branch circuit conductor for 11.6 × 1.25 = 14.5 A minimum, and select a circuit breaker up to 11.6 × 2.5 = 29 A — a standard 30 A breaker is appropriate.
Frequently asked questions
What is the difference between full load current and locked rotor current for an electric motor?
Full load current (FLA) is the steady-state current drawn when the motor is running at its nameplate horsepower output. Locked rotor current (LRC), also called inrush or starting current, is the high current spike — typically 6–8 times FLA — that flows in the instant before the motor accelerates. LRC determines the size of the overcurrent protective device (fuse or breaker) needed to allow the motor to start without tripping. FLA determines the conductor and overload relay sizing for continuous operation. Both values must be considered when designing a motor branch circuit.
How does a lower power factor affect the full load current drawn by an AC motor?
Power factor represents how effectively the motor converts apparent power (VA) into real work (W). A motor with a low power factor (e.g., 0.70) draws significantly more current from the supply than a high-PF motor (e.g., 0.92) producing the same shaft output. From the formula FLA = (HP × 746) / (V × η × PF × √3), you can see that FLA is inversely proportional to PF — halving the power factor doubles the current. This excess current heats conductors, trips protective devices, and increases utility demand charges. Lightly loaded motors typically have poor power factors, which is why running a motor at or near full load is recommended.
When should I use the nameplate full load current instead of a calculated value?
Always prefer the nameplate FLA for the specific motor you are installing, as it reflects the manufacturer's measured value for that exact design, including construction tolerances. Calculated FLA from horsepower, efficiency, and power factor is an estimate useful at the design stage when the specific motor hasn't been selected yet, or for cross-checking nameplate data. NEC Table 430.248 and 430.250 provide standard FLA values by HP and voltage that are used for sizing protective devices when the nameplate is unavailable. Once the motor is on hand, the nameplate value governs overload relay settings and conductor ampacity verification.