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Motor Full-Load Current (Three Phase)

This gives the full-load current for a three-phase motor. You divide the motor power by the square root of three times the line voltage times the power factor times the efficiency. For example, a 10 kW motor on 400 V with 0.85 power factor and 0.92 efficiency draws about 18.5 A. This formula is essential for sizing three-phase motor circuits, including cables, contactors, and protective devices. It is the three-phase counterpart of the single-phase motor current formula.

Written by the Vorzep Team

Electric Machines Motor Sizing

Motor Full-Load Current Calculator

Full-load current, protection ratings, starting current and power-factor correction for three-phase, single-phase and DC motors.

3-phase · 1-phase · DCConductor & overloadBreaker / fuse sizeStarting current
I = P / (√3 · V · cos θ · η)

Inputs

Results

Variables

Symbol Quantity Unit
I Full-load current A
P Output power (mechanical) W
V Line-to-line voltage V
cos θ Power factor dimensionless
η Motor efficiency dimensionless (0 to 1)

What it means

For a three‑phase motor, the full‑load current is given by I = P / (√3 × V × cosθ × η), where P is the output power (W), V is the line‑to‑line voltage, cosθ is the power factor, and η is the efficiency. The √3 factor accounts for the three‑phase configuration. This current is used for selecting overload relays, contactors, and cable sizing. Example: A 5kW, 400V, three‑phase motor with power factor 0.85 and efficiency 90% draws I = 5000 / (1.732 × 400 × 0.85 × 0.9) = 5000 / (529.9) ≈ 9.43A. This is typical for a motor of this rating.

Worked example

Three‑Phase Motor FLC – Practical Example

Real‑World
Scenario: A 15 HP (11.19 kW) three‑phase motor runs at 400 V, efficiency 90%, pf 0.86. Find the full‑load current.
ParameterValue
P11,190 W
VL400 V
cos θ0.86
η0.90
FormulaI = P / (√3 × VL × cos θ × η)
1√3 × 400 × 0.86 × 0.90 = 1.732 × 400 × 0.774 = 536.4
2I = 11190 / 536.4 ≈ 20.86 A
Final Design I ≈ 20.9 A ✓ Full‑load current
Why: Three‑phase motor current is less than single‑phase for the same power due to √3 factor – important for cable and protection sizing.

Common mistakes

Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.

Applications

Motor full‑load current (three phase): I = P/(√3×V×cosθ×η) calculates the line current for three‑phase motors. This is widely used for industrial motor installations. Engineers use it to size feeders, breakers, and starters. This formula is essential for three‑phase motor protection and control.

  • Three‑phase motor circuit design and protection
  • Motor starter and contactor sizing
  • Cable sizing for industrial motors
  • Power factor and efficiency analysis
  • Educational understanding of three‑phase motor currents

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  • Published July 10, 2026
  • Last updated August 9, 2026