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Electrical Calculators

3 Phase Current Calculator

Calculate 3 phase motor current from load power, line voltage and power factor.

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About this calculator

This is the standard formula for calculating the line current of a balanced three-phase load — the same formula used across electrical engineering references for sizing cables, contactors, overload relays and breakers for three-phase motors, screw compressors and other industrial equipment.

Formula

I = (P × 1000) ÷ (√3 × V × PF)
  • PLoad power — kW
  • VLine-to-line voltage — volts
  • PFPower factor — 0–1
  • ILine current — amperes

Worked Example

Given: A 15 kW three-phase screw compressor motor runs on a 415V supply at 0.85 power factor.

Result: I = (15 × 1000) ÷ (1.732 × 415 × 0.85) = 24.6 A.

How to Use

  1. Enter the load power in kW — for a motor, use the input power, not just the shaft (output) HP.
  2. Enter the line-to-line voltage (commonly 415V in India, 400V in Europe, 480V in the US).
  3. Enter the power factor from the nameplate, or use a typical value of 0.8–0.9 for induction motors.
  4. Click Calculate to get the line current in amperes.

Important Assumptions

  • Assumes a balanced three-phase load across all three phases.
  • If you only know motor output HP (not input kW), first divide by motor efficiency, or use the Motor Power Calculator.

Common Mistakes

  • Entering motor output HP directly as kW without converting units or accounting for efficiency.
  • Using phase (line-to-neutral) voltage instead of line-to-line voltage in the √3 formula.

Frequently Asked Questions

Why is there a √3 in the three-phase formula but not single-phase?

√3 (≈1.732) accounts for the 120° phase difference between the three phases when relating line current to total power in a balanced three-phase system.

Should I use motor HP or kW for this calculator?

Use the electrical input power in kW. If you only have HP, convert with the HP to kW calculator first, then divide by efficiency if you want input rather than output power.

What current should I use to size the motor's overload relay?

Use the motor's rated full-load current (FLA) from the nameplate where available; this calculator is for estimating current when only power, voltage and PF are known.

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.