Horsepower Is Output Power
One mechanical horsepower is approximately 746 W. A 10 hp motor therefore delivers about 7,460 W of mechanical output at its rated load.
Because no real motor is 100% efficient, electrical real-power input must be higher:
Electrical input kW = hp × 0.746 ÷ efficiency
Current Formulas
For a single-phase motor:
I = hp × 746 ÷ (V × efficiency × PF)
For a balanced three-phase motor:
I = hp × 746 ÷ (√3 × V × efficiency × PF)
Efficiency and power factor must be entered as decimal values. For example, 90% efficiency is 0.90.
Worked Example
For a 10 hp, 460 V three-phase motor at 90% efficiency and 0.85 power factor:
I = 10 × 746 ÷ (1.732 × 460 × 0.90 × 0.85)
The estimated running current is approximately 12.24 A. Actual nameplate current may differ because the motor's exact design and ratings differ from assumed values.
Efficiency vs. Power Factor
Efficiency compares mechanical output power with electrical real-power input. Power factor compares electrical real power with apparent power. They describe different losses or burdens and should not be used interchangeably.
| Quantity | Ratio | Main effect |
|---|---|---|
| Efficiency | Mechanical output / real input | Energy converted to heat and losses |
| Power factor | Real power / apparent power | Current required to deliver real power |
Starting Current and Voltage Dip
Across-the-line motor starting current can be several times full-load current. Starting performance depends on motor design, driven-load torque, source impedance, transformer size, conductor drop, and starting method.
Soft starters and variable-frequency drives can change current and torque behavior, but they introduce their own ratings, harmonics, and control considerations.
Which Current Should Be Used?
Nameplate current, manufacturer data, and code-table current may serve different purposes. Rules for conductors, overload protection, short-circuit protection, and disconnects do not necessarily use one identical current value.
A formula estimate is useful for understanding and early checks, not as the sole basis for a motor branch-circuit design.