HP to Amps Converter
A 5 HP three-phase motor at 240V with 0.85 power factor and 90% efficiency draws about 11.7 amps at full load. This HP to amps converter translates mechanical horsepower into the electrical current a motor needs, which is essential for sizing breakers, wire gauge, and motor starters. Enter the horsepower, supply voltage, power factor, and efficiency, and the tool returns the full-load current in amps along with the input and output power in watts. The calculation works for both single-phase and three-phase motors. Three-phase motors draw less current per phase because the power is distributed across three conductors, which is why large industrial motors almost always run on three-phase supply. The converter uses the standard 746 watts per mechanical horsepower and accounts for real-world losses through the efficiency and power factor inputs.
Quick answer
One mechanical horsepower equals 746 watts of mechanical output power.
What this tells you
- •One mechanical horsepower equals 746 watts of mechanical output power.
- •The electrical input is always higher than the mechanical output because motors lose energy to heat, friction, and magnetic losses.
- •Efficiency (typically 85 to 95%) accounts for those losses. A 90% efficient motor needs about 829 watts of electrical input per horsepower of output.
- •Power factor (typically 0.80 to 0.95) accounts for the phase difference between voltage and current in AC motors.
- •Single-phase: amps = (HP x 746) / (volts x PF x efficiency).
- •Three-phase: amps = (HP x 746) / (volts x PF x efficiency x 1.732). The 1.732 is the square root of 3.
How to Use
- 1Enter the motor's rated horsepower from the nameplate.
- 2Enter the supply voltage. Common values are 120V, 240V, 208V, 480V, and 600V.
- 3Enter the power factor. If the nameplate does not list it, use 0.85 as a typical default.
- 4Enter the motor efficiency as a decimal. If not listed, use 0.90 for a standard motor.
- 5Select single-phase or three-phase to match your electrical supply.
- 6Click Calculate to see the full-load current in amps plus the input and output power in watts.
How It Works
Formula
Single-phase: I = (HP x 746) / (V x PF x eff)
Three-phase: I = (HP x 746) / (V x PF x eff x sqrt(3))The conversion starts with the mechanical output: 1 HP equals 746 watts. The electrical input is higher because no motor converts all electrical power to mechanical power. Dividing the output watts by the efficiency gives the input watts. For single-phase motors, dividing the input watts by the product of voltage and power factor gives the current. For three-phase motors, the same division includes an additional factor of the square root of 3 (approximately 1.732) because power in a three-phase system is distributed across three conductors. The result is the full-load current, which is the steady-state current the motor draws at rated output. Starting current is much higher, typically 5 to 7 times the full-load current, and is not covered by this calculation.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
1 HP single-phase motor at 120V
Output is 1 x 746 = 746 W. Input is 746 / 0.90 = 828.89 W. Current is 828.89 / (120 x 0.85) = 8.11 amps. A 15A breaker on a dedicated circuit handles this motor.
5 HP three-phase motor at 240V
Output is 5 x 746 = 3,730 W. Input is 3,730 / 0.90 = 4,144.44 W. Current is 4,144.44 / (240 x 0.85 x 1.732) = 11.74 amps. The three-phase supply keeps per-phase current lower than a single-phase equivalent.
10 HP single-phase motor at 240V
Output is 10 x 746 = 7,460 W. Input is 7,460 / 0.92 = 8,108.70 W. Current is 8,108.70 / (240 x 0.90) = 37.54 amps. This motor needs a 50A circuit with appropriate wire gauge.
2 HP three-phase motor at 480V
Output is 2 x 746 = 1,492 W. Input is 1,492 / 0.90 = 1,657.78 W. Current is 1,657.78 / (480 x 0.85 x 1.732) = 2.94 amps. Higher voltage means lower current for the same power, which is why industrial motors use 480V.
Full-Load Amps for Common Motor Sizes
Approximate full-load current for typical motor horsepower ratings at common voltages, using 0.85 power factor and 90% efficiency.
| HP | Single-phase 120V | Single-phase 240V | Three-phase 240V | Three-phase 480V |
|---|---|---|---|---|
| 1 | 8.1 A | 4.1 A | 2.3 A | 1.2 A |
| 2 | 16.2 A | 8.1 A | 4.7 A | 2.3 A |
| 3 | 24.3 A | 12.2 A | 7.0 A | 3.5 A |
| 5 | 40.6 A | 20.3 A | 11.7 A | 5.9 A |
| 7.5 | 60.9 A | 30.4 A | 17.6 A | 8.8 A |
| 10 | 81.2 A | 40.6 A | 23.5 A | 11.7 A |
These are estimates using the stated assumptions. Nameplate values from the motor manufacturer take precedence. NEC tables may show different values for code-compliant sizing.
Common mistakes
- Using output watts instead of input watts. The motor draws more electrical power than it delivers mechanically because of efficiency losses. Always divide output by efficiency to get the correct input.
- Forgetting the square root of 3 for three-phase motors. Omitting this factor overestimates the current by 73%, leading to oversized breakers and wire.
- Confusing full-load current with starting current. Motors draw 5 to 7 times their full-load current during startup. Breaker and wire sizing must account for this surge separately.
- Using 1,000 watts per horsepower. The correct conversion is 746 watts per mechanical horsepower. Using 1,000 overestimates current by about 34%.
- Assuming power factor is 1.0. AC induction motors have a lagging power factor, typically 0.80 to 0.95. Using 1.0 underestimates the actual current draw.
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