Waliv, Vasai East, Maharashtra +91 8669118834
Gurukrupa Enterprises
Electrical Calculators

Motor Power Calculator

Find a three-phase motor's electrical input power and current from its rated HP and efficiency.

Calculate

Result
—
Copied

About this calculator

A motor's nameplate HP is its mechanical shaft output — the electricity it actually draws from the supply is higher, because of internal losses. This calculator converts rated output HP and efficiency into electrical input power (kW), then applies the three-phase current formula to estimate the running current.

Formula

P_input = (HP × 0.7457) ÷ η, I = (P_input × 1000) ÷ (√3 × V × PF)
  • HPRated shaft output — horsepower
  • ηMotor efficiency — 0–1
  • P_inputElectrical input power — kW
  • ILine current — amperes

Worked Example

Given: A 20 HP motor with 90% efficiency runs on 415V at 0.85 power factor.

Result: P_input = (20 × 0.7457) ÷ 0.9 = 16.57 kW. I = (16.57 × 1000) ÷ (1.732 × 415 × 0.85) = 27.2 A.

How to Use

  1. Enter the motor's rated output in HP from the nameplate.
  2. Enter the efficiency percentage from the nameplate (typically 85–95% for industrial motors).
  3. Enter the supply line voltage and power factor.
  4. Click Calculate to see input power in kW and the estimated line current.

Important Assumptions

  • Assumes the motor runs at rated (100%) load; at partial load, efficiency and power factor both change.
  • Uses nameplate efficiency; if unavailable, IE2/IE3 class motors typically range 85–95% depending on size.

Common Mistakes

  • Using output HP directly in the current formula without dividing by efficiency, which understates the actual current drawn.
  • Assuming efficiency stays constant at all load points — it does not.

Frequently Asked Questions

Why is input power higher than output HP in kW?

Because motors are not 100% efficient — some input energy is lost as heat in windings, iron losses and friction.

What efficiency should I assume if the nameplate doesn't show it?

IE2-class motors are commonly 85–92% and IE3-class 88–95% depending on size; check the manufacturer's datasheet where possible.

Does this calculator account for partial load operation?

No, it assumes full rated load. Efficiency and power factor both fall at partial loads.

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.