What this converter does
This converter estimates a motor’s full-load current in amps from its horsepower rating, for single-phase and three-phase supplies. Enter the voltage, power factor and efficiency, pick the phase, and read the current — or swap to go from amps back to HP. It updates as you type.
Horsepower is mechanical output, so the motor draws more electrical power by its efficiency: input watts = HP × 746 ÷ η. Always size cables and overloads to the nameplate current where available. For the power itself, see the HP to kW converter.
The units it covers
Motor current depends on four inputs beyond the horsepower — phase, voltage, power factor and efficiency.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Motor power | hp | P | Mechanical output at the shaft |
| Full-load current | A | I | Line current the motor draws |
| Voltage | V | V | Line-to-line for three-phase |
| Efficiency | η | eta | Output ÷ input, typically 0.85–0.95 |
The formula
Convert HP to input watts through efficiency, then to current like any load:
A = HP × 746 ÷ (√3 × V × PF × η) — drop √3 for single-phaseWhere:
- HP = motor mechanical output
- V = supply voltage (line-to-line for 3-phase)
- PF = power factor, 0 to 1
- η = motor efficiency, 0 to 1
Worked example
A 10 HP three-phase motor at 400 V, 0.85 PF, 0.9 efficiency. Find the current.
A = HP × 746 ÷ (√3 × V × PF × η)7457 ÷ (1.732 × 400 × 0.85 × 0.9) = 14.07 AThe motor draws roughly 14 A at full load — check the nameplate to confirm.
The units in this example
The mechanical output at the shaft. Because a motor is not perfectly efficient, its electrical input is HP × 746 ÷ efficiency, which sets the current.
- 1 hp = 745.7 W output
- input W = HP × 746 ÷ η
- 1 hp, 230 V, 1ph, 0.85 PF, 0.9 η = 4.24 A
- 1 hp ≈ 0.746 kW
The line current a motor draws at rated load. Estimated here from HP; use the nameplate FLC for cable and overload sizing where possible.
- A = HP × 746 ÷ (V × PF × η) — 1ph
- A = HP × 746 ÷ (√3 × V × PF × η) — 3ph
- lower η or PF → more current
- √3 = 1.732