Electrical
Three-phase motor current estimation
Estimate motor input current from horsepower or kilowatts while keeping efficiency, power factor, rated voltage, nameplate current, and code sizing tables in their proper roles.
- Product
- Three-phase AC motors
- Level
- Intermediate
- Read time
- 11 min
- Reviewed
- 2026-07-15
What to establish before troubleshooting
Motor shaft output power is lower than electrical input power because efficiency is below one. Current also depends on power factor, so both terms belong in a first-pass three-phase estimate.
The estimate is a reasonableness check. Actual nameplate current, manufacturer data, and applicable code tables govern different design and protection decisions.
Abbreviated worked example
Estimate a 25 hp motor at 460 V
Assume 25 hp output, 460 V line-to-line, 90 percent efficiency, and 0.85 power factor.
- 1Mechanical output = 25 x 746 = 18,650 W.
- 2Current = 18,650 / (1.732 x 460 x 0.90 x 0.85).
Result: Estimated input current is approximately 30.6 A.
Caution: Use the actual nameplate and the applicable electrical code method for overloads, conductors, and branch-circuit protection.
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