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Motor Current Calculator (FLA)

Quick answer

Get full load amps for single and three phase motors: I = P/(V x eff x PF) or I = P/(root3 x 400 x eff x PF).

  • Includes kW to HP conversion and input power.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How the Motor Calculator Works

I = P / (√3 × V × PF × η) for 3-phase

Calculates motor electrical parameters from mechanical power output. Accounts for efficiency losses and power factor to determine actual current draw from the supply. Worked example: a 5 HP (3.73 kW) 230 V single-phase motor at 88 percent efficiency and PF 0.85 draws I = 3730/(230 x 0.88 x 0.85) = 21.7 A. NEC 430 lists 28 A for this class - size conductors from the table. Convert either way instantly: 5 HP equals 3.73 kW, and a 2.2 kW motor at 90 percent efficiency consumes 2.44 kW electrical while delivering full rated shaft output.

  1. 1

    Step 1

    Select phase type (single or three-phase)

  2. 2

    Step 2

    Enter motor power in kW or HP

  3. 3

    Step 3

    Enter supply voltage and efficiency

  4. 4

    Step 4

    Enter motor power factor

Use Cases

Sizing motor starters and contactors

Calculating cable and breaker requirements

Estimating motor operating costs

Designing motor control centers

Tips

  • 1

    1 HP = 0.746 kW (approximate)

  • 2

    Typical motor efficiency: 85-95%

  • 3

    Typical motor power factor: 0.8-0.9

  • 4

    Full load current determines cable and protection sizing

Common Mistakes

  • Confusing mechanical power (output) with electrical power (input)

  • Using nameplate HP without converting to kW

  • Forgetting efficiency when calculating input power

  • Not accounting for power factor in current calculations

FAQs

What is motor efficiency?

Efficiency is the ratio of mechanical output power to electrical input power. A 90% efficient motor drawing 10 kW delivers 9 kW of mechanical power; 1 kW becomes heat.

Why do motors have low power factor?

Motors are inductive loads that require magnetizing current to create the magnetic field. This reactive current doesn't do useful work but still flows, lowering power factor.

How do I convert HP to kW?

Multiply HP by 0.746 to get kW. For example, 10 HP = 7.46 kW mechanical output. To find electrical input, divide by efficiency: 7.46 / 0.90 = 8.29 kW input.

How do I calculate motor full load current?

Single-phase: I = P/(V x eff x PF). Three-phase: I = P/(root3 x V x eff x PF). A 3 kW 400 V motor at 0.85 PF and 90 percent efficiency draws 5.66 A; NEC 430 tables list 9 A for 3 kW across-the-line.

What wire does a motor circuit need?

NEC 430.22: conductors at 125 percent of FLA. A motor with 20 A FLA needs 25 A conductor ampacity - 10 AWG copper. The branch short-circuit protector may be larger (175-250 percent FLA) to ride through inrush.

Why is motor current higher at start?

Locked-rotor current is 5-8x FLA until the rotor reaches speed. Star-delta or VFD starting cuts this to 2-3x. A 20 A FLA motor may briefly draw 120 A, so use D-curve or motor-start breakers.

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