How do I calculate motor current from kW?

For three-phase motors, use A = kW × 1000 ÷ (√3 × volts × power factor × efficiency). For single-phase motors, use A = kW × 1000 ÷ (volts × power factor × efficiency).

How do I calculate motor current from horsepower?

Convert horsepower to kW first using kW = HP × 0.746, then calculate current using voltage, phase, power factor, and efficiency.

What is full-load current?

Full-load current is the approximate current a motor draws when delivering its rated output power. It is different from starting current, which can be much higher.

Why do I need efficiency for motor current?

Motor nameplate kW or HP usually describes mechanical output power. The electrical input must be higher because no motor is 100% efficient.

Why do I need power factor for AC motors?

AC motors draw current that includes real and reactive components. Power factor tells how much of the apparent power becomes real input power.

What power factor should I use for a motor?

Use the motor nameplate value where possible. For rough estimates, many three-phase motors are around 0.8 to 0.9, but small motors can be lower.

What efficiency should I use for a motor?

Use the motor nameplate efficiency where possible. For rough estimates, small motors may be 60% to 85%, while industrial three-phase motors are often 85% to 95%.

Can this calculator size a breaker for a motor?

No. It estimates motor current only. Breaker, overload, cable, and starter sizing must follow the motor nameplate and local electrical standards.

Is starting current included?

No. The calculator estimates running current. Starting current can be several times higher and depends on the motor design and starter type.

How many amps does a 7.5 kW motor draw?

At 400 V three-phase, 0.85 power factor, and 90% efficiency, a 7.5 kW motor draws about 14.15 amps.

Engineering

Motor Current Calculator

Estimate motor full-load current from kW or horsepower, or calculate approximate motor output power from amps. This calculator supports DC, single-phase AC, and three-phase AC motors with power factor and efficiency.

Motor output power (kW)

Phase

Voltage

Power factor

Efficiency %

Estimated full-load current

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Input power—

Apparent power—

Horsepower—

Formula used—

Motor current formulas

This calculator estimates motor full-load current from motor output power, voltage, phase, efficiency, and power factor. It is useful for early generator sizing, load estimates, and understanding motor nameplate values.

DC motor current: A = Output kW × 1000 ÷ (V × Efficiency) Single-phase AC motor current: A = Output kW × 1000 ÷ (V × PF × Efficiency) Three-phase AC motor current: A = Output kW × 1000 ÷ (√3 × V × PF × Efficiency) Horsepower conversion: kW = HP × 0.746

Worked example: 7.5 kW motor at 400 V three-phase

Given: Motor output = 7.5 kW Voltage = 400 V three-phase Power factor = 0.85 Efficiency = 90% = 0.90 A = 7.5 × 1000 ÷ (√3 × 400 × 0.85 × 0.90) A = 14.15 A

This estimate is close to the running full-load current. Starting current may be much higher, depending on motor type and starting method.

Motor current vs starting current

Motor current calculators usually estimate running current, often called full-load current. Starting current can be several times higher for direct-on-line starts. Soft starters, VFDs, star-delta starters, and reduced-voltage starters can reduce starting demand, but exact values depend on the motor and starter.

For final cable, overload, breaker, and contactor sizing, use the motor nameplate and applicable electrical code tables rather than relying only on a general calculator.

Typical power factor and efficiency assumptions

Motor / load type	Rough PF	Rough efficiency
Small single-phase motor	0.65 – 0.85	60% – 85%
Industrial three-phase motor	0.80 – 0.92	85% – 95%
Large efficient motor	0.85 – 0.95	90% – 96%

Common questions

For three-phase motors, use A = kW × 1000 ÷ (√3 × volts × power factor × efficiency). For single-phase motors, use A = kW × 1000 ÷ (volts × power factor × efficiency).
Convert horsepower to kW first using kW = HP × 0.746, then calculate current using voltage, phase, power factor, and efficiency.
Full-load current is the approximate current a motor draws when delivering its rated output power. It is different from starting current, which can be much higher.
Motor nameplate kW or HP usually describes mechanical output power. The electrical input must be higher because no motor is 100% efficient.
AC motors draw current that includes real and reactive components. Power factor tells how much of the apparent power becomes real input power.
Use the motor nameplate value where possible. For rough estimates, many three-phase motors are around 0.8 to 0.9, but small motors can be lower.
Use the motor nameplate efficiency where possible. For rough estimates, small motors may be 60% to 85%, while industrial three-phase motors are often 85% to 95%.
No. It estimates motor current only. Breaker, overload, cable, and starter sizing must follow the motor nameplate and local electrical standards.
No. The calculator estimates running current. Starting current can be several times higher and depends on the motor design and starter type.
At 400 V three-phase, 0.85 power factor, and 90% efficiency, a 7.5 kW motor draws about 14.15 amps.