Motor Current Calculator
Enter motor horsepower, voltage, phase count, efficiency, and power factor to create a transparent U.S. English preliminary motor running-current estimate. The model uses 746 watts per mechanical horsepower and does not reproduce NEC motor tables, replace a full-load-current table lookup, or select a compliant conductor, starter, overload setting, or overcurrent device.
- Estimated shaft power
- W
- Efficiency ratio
- decimal
- Estimated electrical input power
- W
- Estimated apparent power
- VA
- Estimated running current from entered horsepower
- A
Calculation details
- Phase multiplier
- x
- Calculation basis
- NEC table boundary
- Nameplate boundary
- Power factor boundary
- Phase model boundary
- Equipment selection boundary
Recent results
Formulas
- Shaft power watts = motor horsepower x 746
- Efficiency ratio = efficiency percent / 100
- Input power watts = shaft power watts / (efficiency percent / 100)
- Apparent power VA = input power watts / power factor
- Phase multiplier = 1 for single-phase; sqrt(3) for balanced three-phase
- Estimated running current amps = apparent power VA / (motor voltage x phase multiplier)
When to use this calculator
Estimate motor shaft power, electrical input power, apparent power, and approximate running current from horsepower, voltage, phase, efficiency, and power factor.
What the results mean
Review the calculated values together with the displayed units and assumptions. The result is a calculation aid for the stated inputs.
Formula basis
shaft power watts = motor horsepower x 746 input power watts = shaft power watts / (efficiency percent / 100) apparent power VA = input power watts / power factor phase multiplier = 1 for single-phase; sqrt(3) for balanced three-phase estimated running current amps = apparent power VA / (motor voltage x phase multiplier)
Before you use the result
This is a U.S. English preliminary engineering estimate, not an NEC motor full-load-current table lookup, conductor-sizing calculation, overload calculation, overcurrent-protection selection, starter selection, service calculation, or code-compliance result. Actual current varies with motor design, voltage, load, speed, starting condition, efficiency, power factor, temperature, harmonics, nameplate data, and local requirements. Verify the nameplate, manufacturer data, adopted NEC edition, AHJ direction, equipment ratings, and project conditions with a qualified electrical professional before design or installation.
FAQs
How does this motor current calculator estimate amps?
It converts horsepower to shaft watts, adjusts for the efficiency and power factor you enter, then divides apparent power by voltage and the selected phase multiplier. The result is an approximate running-current estimate.
Does this calculator use NEC motor full-load-current tables?
No. It uses transparent arithmetic and your entered assumptions. NEC motor tables, nameplate rules, conductor ampacity, overload protection, and overcurrent protection require separate review for the actual installation.
What should I use for efficiency and power factor?
Use motor nameplate, manufacturer, or measured operating data when available. Generic assumptions can be useful for early screening but should not replace equipment-specific data.
Can this result be used for conductor, breaker, overload, or starter selection?
No. The running-current estimate is from entered horsepower, efficiency, voltage, phase, and power factor. Conductor sizing, overload protection, overcurrent protection, starter selection, equipment ratings, and code-compliance decisions require separate review.