hp to Amps Calculator
A 10 hp single phase motor on 230 V draws about 42 amps when running at full load.
Quick answer: Multiply the horsepower by 745.7, then divide by the voltage, efficiency, power factor and phase factor. For example, 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF is 42.381 A, 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF is 14.07 A and 25 hp on 460 V, three phase, 92% efficiency, 0.88 PF is 28.901 A.
The hp to amps formula
Current in amps equals horsepower times 745.7, divided by voltage, efficiency, power factor and the phase factor.
The phase factor is 1 for single phase and 1.732 for three phase.
Take 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF. 10 × 745.7 ÷ (230 × 0.9 × 0.85) gives 42.381 A.
Take 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF. 10 × 745.7 ÷ (400 × 0.9 × 0.85 × 1.732) gives 14.07 A.
From shaft power to line current
The horsepower is the shaft power. Multiply by 745.7 to get watts, then divide by the voltage to find the current.
Efficiency and power factor raise the current because the supply must cover losses and carry apparent power (Wikipedia: Power factor).
How to convert hp to amps step by step
- Turn the horsepower into watts with 745.7.
- Divide by the line voltage.
- Divide by the efficiency and the power factor as decimals.
- For three phase, divide once more by 1.732.
Example 1: 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF
- Start with 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF.
- 10 × 745.7 ÷ (230 × 0.9 × 0.85) = 42.381.
- The current is 42.381 A.
10 hp on 230 V, single phase, 90% efficiency, 0.85 PF = 42.381 A
Example 2: 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF
- Start with 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF.
- 10 × 745.7 ÷ (400 × 0.9 × 0.85 × 1.732) = 14.07.
- The current is 14.07 A.
10 hp on 400 V, three phase, 90% efficiency, 0.85 PF = 14.07 A
Example 3: 25 hp on 460 V, three phase, 92% efficiency, 0.88 PF
- Start with 25 hp on 460 V, three phase, 92% efficiency, 0.88 PF.
- 25 × 745.7 ÷ (460 × 0.92 × 0.88 × 1.732) = 28.901.
- The current is 28.901 A.
25 hp on 460 V, three phase, 92% efficiency, 0.88 PF = 28.901 A
The three examples below use common sizes.
Each one shows the sum in full.
hp to amps conversion table
| Motor and supply | Current (A) | Input power (kW) |
|---|---|---|
| 1 hp on 120 V, single phase, 80% efficiency, 0.8 PF | 9.71 A | 0.9321 kW input |
| 2 hp on 230 V, single phase, 85% efficiency, 0.8 PF | 9.536 A | 1.755 kW input |
| 5 hp on 230 V, single phase, 88% efficiency, 0.85 PF | 21.672 A | 4.237 kW input |
| 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF | 42.381 A | 8.286 kW input |
| 15 hp on 230 V, single phase, 90% efficiency, 0.85 PF | 63.572 A | 12.428 kW input |
| 5 hp on 400 V, three phase, 88% efficiency, 0.85 PF | 7.195 A | 4.237 kW input |
| 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF | 14.07 A | 8.286 kW input |
| 20 hp on 400 V, three phase, 92% efficiency, 0.86 PF | 27.207 A | 16.211 kW input |
| 25 hp on 460 V, three phase, 92% efficiency, 0.88 PF | 28.901 A | 20.264 kW input |
| 50 hp on 460 V, three phase, 94% efficiency, 0.9 PF | 55.315 A | 39.665 kW input |
| 75 hp on 460 V, three phase, 95% efficiency, 0.9 PF | 82.1 A | 58.871 kW input |
| 100 hp on 575 V, three phase, 95% efficiency, 0.92 PF | 85.669 A | 78.495 kW input |
The highlighted row is the default of the calculator. The last column gives kW input.
A quick mental shortcut for hp to amps
On a 230 V single phase supply, each hp needs roughly 4 amps. A 5 hp motor is about 22 A.
On a 460 V three phase supply, each hp needs roughly 1.2 amps.
Single phase versus three phase
A three phase supply shares the load across three conductors. That is why the 1.732 factor lowers the current per line.
Use the line to line voltage for three phase, such as 400 V or 460 V (Electricity and Electronics: Three-phase AC circuits).
An estimate, not a code value
Real wiring rules use tabulated full-load currents, which are often higher than a plain calculation. Use the plate on the motor and your local code.
Have a qualified electrician choose wire size and breakers.
What the plate may show
Many plates list amps, volts, efficiency and power factor. If the plate gives amps, trust it over this estimate.
Use the calculator to check that the plate looks sensible.
Quick conversion answers
| Conversion | Result |
|---|---|
| 5 hp on 230 V, single phase, 88% efficiency, 0.85 PF | 21.672 A |
| 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF | 42.381 A |
| 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF | 14.07 A |
| 50 hp on 460 V, three phase, 94% efficiency, 0.9 PF | 55.315 A |
Where this conversion is used
Electricians estimate current for a motor circuit.
Owners check if a generator can start a pump.
Common mistakes to avoid
- Using the wrong voltage. Three phase needs the line to line value.
- Skipping efficiency and power factor. The current will look too low.
- Using this for wire sizing. Codes use their own tables and safety margins.
- Forgetting the 1.732. Three phase divides by it once.
Related converters
More motors and supplies
0.5 hp on 120 V, single phase, 70% efficiency, 0.7 PF is 6.341 A. 1 hp on 120 V, single phase, 75% efficiency, 0.75 PF is 11.05 A. 3 hp on 230 V, single phase, 85% efficiency, 0.8 PF is 14.3 A.
7.5 hp on 230 V, single phase, 89% efficiency, 0.85 PF is 32.14 A. 3 hp on 400 V, three phase, 86% efficiency, 0.8 PF is 4.693 A. 7.5 hp on 400 V, three phase, 89% efficiency, 0.85 PF is 10.67 A.
15 hp on 460 V, three phase, 91% efficiency, 0.86 PF is 17.94 A. 30 hp on 460 V, three phase, 93% efficiency, 0.88 PF is 34.31 A. 60 hp on 575 V, three phase, 94% efficiency, 0.9 PF is 53.1 A.
Doubling the first number
1 hp on 120 V, single phase, 70% efficiency, 0.7 PF is 12.68 A. 2 hp on 120 V, single phase, 75% efficiency, 0.75 PF is 22.09 A. 6 hp on 230 V, single phase, 85% efficiency, 0.8 PF is 28.61 A.
15 hp on 230 V, single phase, 89% efficiency, 0.85 PF is 64.29 A. 6 hp on 400 V, three phase, 86% efficiency, 0.8 PF is 9.387 A. 15 hp on 400 V, three phase, 89% efficiency, 0.85 PF is 21.34 A.
30 hp on 460 V, three phase, 91% efficiency, 0.86 PF is 35.88 A. 60 hp on 460 V, three phase, 93% efficiency, 0.88 PF is 68.62 A. 120 hp on 575 V, three phase, 94% efficiency, 0.9 PF is 106.2 A.
A note on safety
This page does the arithmetic only. Electrical work can cause shock, fire and injury.
Have a qualified electrician choose wire, breakers and protection for any real circuit.
How voltage changes the current
10 hp on 120 V, single phase, 90% efficiency, 0.85 PF is 81.23 A. 10 hp on 230 V, single phase, 90% efficiency, 0.85 PF is 42.38 A. 10 hp on 400 V, three phase, 90% efficiency, 0.85 PF is 14.07 A.
10 hp on 460 V, three phase, 90% efficiency, 0.85 PF is 12.23 A.
Frequently asked questions
How many amps is a 10 hp motor?
At 230 V single phase, 90 percent efficiency and 0.85 power factor, about 42.4 A.
How do I convert hp to amps for three phase?
Use the same formula and divide once more by 1.732.
Which voltage do I use for three phase?
Use the line to line voltage.
Can I size a breaker with this?
No. Breaker and wire sizes follow your local electrical code. Ask a qualified electrician.
Why does a lower power factor raise the amps?
The supply must carry more apparent power for the same real power.
What if my plate shows amps?
Use the plate value. This page is an estimate.
Sources
- Wikipedia: Horsepower (en.wikipedia.org)
- Wikipedia: Power factor (en.wikipedia.org)
- Electricity and Electronics: Three-phase AC circuits (control.com)
