3 Phase Motor Amps Calculator

Three-Phase Motor Full-Load Amps

Full-load amperage (FLA) is the current a three-phase motor draws at rated horsepower, voltage, efficiency, and power factor. Use FLA to size conductors, overload relays, and breakers.

FLA Formula

$$\text{FLA} = \frac{\text{HP} \times 746}{\sqrt{3} \times V \times \eta \times \text{PF}}$$

Where HP is horsepower, $V$ is line-to-line voltage, $\eta$ is efficiency (0 to 1), and PF is power factor (0 to 1). One horsepower equals 746 watts of mechanical output.

Example: 5 HP at 480 V with 85% efficiency and 0.85 PF gives about 6.2 A FLA. Size wire with 220V Wire Size Calculator or 100 Amp Wire Size Calculator.

Frequently Asked Questions

What is a typical motor efficiency?

Small three-phase motors are often 80% to 90% efficient. Use nameplate efficiency when available; 85% is a reasonable default for estimates.

What power factor should I use?

Induction motors commonly run 0.80 to 0.90 PF at full load. Use 0.85 unless the nameplate lists a different value.

Line or phase voltage?

Enter line-to-line voltage (e.g., 480 V or 208 V). The $\sqrt{3}$ factor converts three-phase power to line current.

How does FLA relate to breaker size?

Branch circuits often use an inverse-time breaker up to 250% of FLA for standard motors. Always follow NEC Article 430 and local code.

Why multiply HP by 746?

746 watts equals one mechanical horsepower. The formula converts shaft power to electrical input power before solving for current.

Report

3 Phase Motor Amperage Calculator

Three-Phase Motor Full-Load Amps

FLA Formula

Frequently Asked Questions

Related Tools

Recent Visits