Table of Contents
3-Phase Motor Basics
Three-phase electric motors are the workhorses of industry, powering pumps, compressors, conveyors, fans, and machine tools. Knowing the full load amperage (FLA) is essential for selecting the correct wire size, circuit breaker, starter, and overload relay. The FLA depends on the motor horsepower rating, supply voltage, efficiency, and power factor.
Three-phase power delivers a smoother, more constant energy flow than single-phase power, which makes 3-phase motors more efficient, smaller, lighter, and longer-lasting for the same output power. They are available from fractional horsepower to thousands of horsepower.
Amperage Formula
Where I is the full load current in amps, HP is horsepower, 746 converts HP to watts, V is line-to-line voltage, Eff is motor efficiency (decimal), and PF is the power factor (decimal). The square root of 3 (approximately 1.732) accounts for the three-phase power relationship.
Motor FLA Reference Table (460V)
| HP | FLA at 460V | Wire Size (Cu) | Breaker Size |
|---|---|---|---|
| 1 | 1.8 | 14 AWG | 15A |
| 5 | 7.6 | 14 AWG | 15A |
| 10 | 14 | 14 AWG | 25A |
| 25 | 34 | 8 AWG | 50A |
| 50 | 65 | 4 AWG | 100A |
| 100 | 124 | 1 AWG | 175A |
Frequently Asked Questions
Why is actual FLA different from nameplate?
Motor nameplate FLA is measured at rated load, voltage, and frequency. Actual current varies with load: a lightly loaded motor draws less current. Voltage variations also affect current; lower voltage increases current draw for the same mechanical load, potentially causing overheating if sustained.
What is power factor in a motor?
Power factor is the ratio of real power (watts) to apparent power (VA). Induction motors have lagging power factors typically between 0.75 and 0.90 at full load. At light load, the power factor drops significantly. Capacitor banks can be installed to correct low power factor and reduce electricity costs.
How do I size a breaker for a 3-phase motor?
Per NEC Article 430, the branch circuit breaker for a motor is typically sized at 250% of the motor FLA for standard inverse-time breakers. For instantaneous-trip breakers, the multiplier is different. Always verify with local codes and the specific motor controller manufacturer requirements.