3-Phase Motor Amperage Calculator

Calculate the full-load current (FLA) of a three-phase motor from its rated power, line-to-line voltage, power factor, and efficiency using I = P / (√3 × V × PF × η).

Quick Facts

Formula
I = P / (√3 × V × PF × η)
√3 ≈ 1.732 links per-line current to line-to-line voltage in a balanced 3-phase system.
Wire & breaker sizing
NEC 430.22: size at 125% of FLA
Standard rule for continuous-duty, single-motor branch circuits in the US.

Your Results

Calculated
Full-Load Current
-
Motor FLA (amps)
Real Power Input
-
Electrical power drawn (kW)
Apparent Power
-
kVA at rated load
Recommended Ampacity
-
125% of FLA (NEC continuous duty)

Ready

Enter motor power, voltage, power factor, and efficiency, then press Calculate.

About the 3-Phase Motor Amperage Calculator

This tool finds the full-load current (FLA) drawn by a three-phase AC motor from its nameplate rating, using the standard electrical engineering relationship between power, voltage, and current in a balanced three-phase supply.

Understanding the formula

Full-load current is I = P / (√3 × V × PF × η), where P is the motor's rated output power converted to watts, V is the line-to-line supply voltage, PF is the power factor, and η is the motor's efficiency written as a decimal. The √3 (≈1.732) appears because three-phase current is drawn per line while power is delivered across all three lines, which sit 120° apart in phase.

Working with units

  • Power is entered in horsepower (1 HP = 746 W) or kilowatts (1 kW = 1000 W) — use the motor's rated output (shaft) power from the nameplate, not the electrical input power.
  • Voltage is line-to-line (phase-to-phase) RMS voltage — common North American values are 208V, 230V, 460V, and 575V; other regions commonly use 400V.
  • Power factor is a decimal between 0 and 1 (typically 0.80–0.90 for induction motors near full load); efficiency is entered here as a percentage. Both figures are usually printed on the motor nameplate.

Knowing the limits

This formula gives the theoretical full-load current for a balanced, steady-state load at rated output. Measured current can differ because of voltage unbalance, harmonic distortion, or the motor running below its rated load — a lightly loaded motor draws less current and at a lower power factor than shown here. Starting (locked-rotor) current is typically five to eight times the FLA and is not what this calculator reports. Always compare against the nameplate FLA and consult the National Electrical Code (or your local equivalent) before sizing conductors, breakers, or overload protection.

Frequently Asked Questions

What is the formula for 3-phase motor amperage?
Full-load current is I = P / (√3 × V × PF × η), where P is the motor's rated output power in watts, V is the line-to-line voltage, PF is the power factor, and η is efficiency as a decimal. For example, a 10 HP motor (7460 W) at 460V, 0.85 PF, and 90% efficiency draws about 12.2 A.
Why does the formula include the square root of 3?
In a balanced three-phase system, the three line currents are offset by 120°, and the relationship between total power and per-line current works out to a factor of √3 (about 1.732) times the line-to-line voltage. This differs from single-phase power, which is simply V × I × PF.
Should I enter line-to-line or line-to-neutral voltage?
Enter line-to-line (phase-to-phase) voltage, such as 208V, 230V, 460V, or 575V, since that is the value on motor nameplates and utility service labels. Line-to-neutral voltage is roughly 58% of line-to-line voltage and will give an incorrect current if used here.
How much bigger should the wire or breaker be than the calculated current?
For a single continuous-duty motor branch circuit, NEC 430.22 generally calls for conductors and protection sized at no less than 125% of the motor's full-load current. This calculator's Recommended Ampacity result applies that 125% factor, but always confirm against the nameplate FLA and your local electrical code.

Related calculators