HP to Amps Calculator
Estimate a motor's current draw in amps from its horsepower. Enter the horsepower, the supply voltage, the efficiency and power factor, and choose single-phase or three-phase to get the full-load current and electrical input power.
Example: with Motor power (hp) 5 · Voltage (V) 230 · Phase Single-phase · Efficiency (%) 90 · Power factor 0.85 → Full-load current: 21.2 A (single-phase).
- Electrical input power4.14 kW electrical input
- Sizing noteSize conductors and overloads above this; nameplate FLA and NEC tables govern the final wire and breaker.
Computed by the calculator below using its default values. Change any input to see your own numbers.
One horsepower is 746 watts of output. Single-phase current I = hp × 746 / (V × eff × PF); three-phase divides by an extra √3 because the power splits across three lines.
Turning horsepower into amps
A motor rated in horsepower is rated by its mechanical output — 1 hp is 746 watts at the shaft. To find the current it pulls from the line, you first divide by efficiency to get the electrical input power, then divide by the voltage. But a motor is not a pure resistor: it draws more current than the real power alone suggests, and the power factor accounts for that gap. So I = hp × 746 / (V × efficiency × PF).
Three-phase motors add one twist. Their power is delivered over three lines, and the line current relates to power through a √3 factor, so the same horsepower draws less current per line at the same voltage. That is a big reason industrial motors are three-phase: thinner conductors carry the same power. Real installations still size wire and overloads from nameplate full-load amps and code tables, which include safety margins this estimate does not.
How it’s calculated
Single-phase: I = hp × 746 / (V × eff × PF). Three-phase: I = hp × 746 / (√3 × V × eff × PF), with √3 ≈ 1.732. Efficiency entered as a percent above 1 is divided by 100; power factor is a fraction from 0 to 1. Electrical input power = hp × 746 / efficiency.
A rough design estimate, not a substitute for nameplate full-load amps or NEC ampacity tables, which build in safety and starting-current margins. Assumes balanced voltage and steady, full-load running.
Approximate single-phase current at 230 V (90% eff, 0.85 PF)
| Motor | Full-load current |
|---|---|
| 1 hp | 4.2 A |
| 2 hp | 8.5 A |
| 3 hp | 12.7 A |
| 5 hp | 21.2 A |
| 7.5 hp | 31.8 A |
| 10 hp | 42.4 A |
Computed with I = hp × 746 / (V × eff × PF); rounded. Real installs use NEC full-load amps.
Common mistakes
- Leaving out efficiency and power factor, which understates the real current a motor draws.
- Using the single-phase formula for a three-phase motor — three-phase needs the extra √3.
- Entering power factor as a percent like 85 without meaning 0.85 (the tool converts, but check it).
- Sizing wire straight from this estimate instead of the nameplate full-load amps and code tables.
Frequently asked questions
How do you convert horsepower to amps?
Use I = hp × 746 / (V × efficiency × PF) for single-phase power. For three-phase, divide by an extra √3. One horsepower equals 746 watts of output.
Why is three-phase current lower for the same hp?
Three-phase power is split across three lines and relates to line current through a √3 factor, so each line carries less current than a single-phase supply at the same voltage.
What efficiency and power factor should I use?
Typical induction motors run around 85–92% efficient with a power factor near 0.8–0.9 at full load. Use the motor nameplate values when you have them.
Can I size my wire from this number?
No. This is a design estimate. Size conductors and overload protection from the motor's nameplate full-load amps and the NEC tables, which include required margins.