Motor Current Calculator
Estimate the full-load current (FLA) of a single-phase or three-phase AC motor from its horsepower and voltage rating.
How it works
For a three-phase motor: FLA = (HP × 746) / (√3 × V × PF × Efficiency). For single-phase: FLA = (HP × 746) / (V × PF × Efficiency). The factor 746 converts horsepower to watts. Typical power factor for induction motors is 0.80–0.90, and efficiency ranges from 0.85–0.95 depending on motor size and design.
Example
A 10HP three-phase motor operates at 480V with a power factor of 0.85 and 90% efficiency. FLA = (10 × 746) / (1.732 × 480 × 0.85 × 0.90) = 7,460 / 635 = 11.75A. Suggested breaker = 11.75 × 1.25 = 14.7A → use a 15A breaker. Suggested wire: AWG 14 (rated 15A at 75°C).
Frequently asked questions
What is the difference between FLA and LRA?
FLA (Full-Load Amps) is the current the motor draws at its rated load — used for wire and breaker sizing. LRA (Locked Rotor Amps) is the starting current, typically 6–8× FLA, which occurs for a few seconds when the motor starts. Breakers and starters must handle LRA without tripping.
What type of breaker should I use for a motor?
Motors require a breaker rated for motor protection — typically a Motor Circuit Protector (MCP) or an inverse-time breaker rated at 250% of FLA per NEC 430.52. Standard breakers will trip on the high starting current (LRA).
What is power factor in a motor?
Power factor measures how efficiently a motor converts electrical power to mechanical power. A PF of 1.0 is ideal — all power is used. Induction motors typically have PF of 0.80–0.90 at full load. Low PF means the motor draws more current than necessary, increasing wire and breaker costs.
Should I add a service factor to my motor sizing?
Yes. NEC 430.22 requires motor branch circuit conductors to be rated at 125% of the motor FLA. This calculator's suggested breaker already applies the 1.25 factor. For motors with a service factor above 1.15, some engineers size to the service factor current instead.
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