How this calculator works
A horsepower is 746 watts of mechanical output. The motor needs more than that from the supply: the electrical input is the output divided by the efficiency, and the current is that input divided by the voltage and the power factor. A three-phase motor divides by 1.732 as well.
Worked example: a 1 HP motor on 120 V, with 85% efficiency and a power factor of 0.85, draws 8.60 A while running (0.88 kW in for 0.746 kW out), and on 230 V it draws 4.49 A. It may draw 52 A for a moment at start. A 5 HP motor on 230 V single-phase draws 22.4 A, and a 10 HP three-phase motor on 460 V draws 13.0 A.
Estimated motor current (efficiency 85%, power factor 0.85)
Typical running amps. Use the nameplate for a real motor.
| HP | 115 V single-phase | 230 V single-phase | 230 V three-phase | 460 V three-phase |
|---|---|---|---|---|
| 0.5 HP | 4.5 A | 2.2 A | 1.3 A | 0.6 A |
| 1 HP | 9.0 A | 4.5 A | 2.6 A | 1.3 A |
| 2 HP | 18.0 A | 9.0 A | 5.2 A | 2.6 A |
| 3 HP | 26.9 A | 13.5 A | 7.8 A | 3.9 A |
| 5 HP | 44.9 A | 22.4 A | 13.0 A | 6.5 A |
| 10 HP | 89.8 A | 44.9 A | 25.9 A | 13.0 A |
| 20 HP | 179.6 A | 89.8 A | 51.8 A | 25.9 A |
Tips before you buy
- A horsepower is 746 watts of mechanical output. The motor draws more than that from the supply because of losses (efficiency) and reactive current (power factor).
- Use the nameplate full-load amps (FLA) when you have it. It is the figure for the circuit, the breaker and the overload.
- Doubling the voltage halves the current: a motor on 230 V draws half the current it would on 115 V.
- Motors draw 5 to 7 times their running current for a moment when they start, so a long thin extension cord can stall one.
- Breakers and wires for motors follow special code rules (breakers may be sized well above the running current to ride through the start), so ask an electrician.
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Frequently asked questions
How many amps does a 1 HP motor draw?
About 8.6 amps on 120 V and 4.5 amps on 230 V, with 85% efficiency and a 0.85 power factor. The nameplate gives the exact figure.
How do you convert HP to amps?
Multiply the horsepower by 746, then divide by the voltage, the efficiency and the power factor. For three-phase, divide also by 1.732.
How many amps does a 5 HP motor draw?
About 22.4 amps on 230 V single-phase, 13.0 amps on 230 V three-phase and 6.5 amps on 460 V three-phase.
How many watts is 1 horsepower?
746 watts of output. At 85% efficiency, the motor draws about 878 watts from the supply.
Why does a motor draw more current when it starts?
The rotor is not turning at first, so it draws about 5 to 7 times the running current until it comes up to speed, and the wiring and the breaker need to allow for this.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.