This HP to amps calculator converts a motor's horsepower into full-load current (amps) for single-phase, three-phase, and DC circuits, factoring in voltage, efficiency, and power factor. It's an everyday tool for sizing wire, breakers, and overloads, or checking a motor nameplate against measured draw.
HP to Amps Formula
Three-phase: Amps = (HP × 746) ÷ (Volts × 1.732 × Eff × PF)
One electric horsepower equals exactly 746 watts of mechanical output. But a motor must draw more electrical power than it delivers, because it isn't perfectly efficient and AC circuits carry reactive current described by the power factor. Dividing the output watts by voltage, efficiency, and power factor (and √3 for three-phase) gives the real current the motor pulls from the supply.
Why Efficiency and Power Factor Matter
Efficiency (η) accounts for heat and friction losses inside the motor — a 90%-efficient motor needs about 11% more input power than its rated output. Power factor (PF) describes how well voltage and current stay in phase in an AC circuit; a PF of 0.85 means only 85% of the apparent power does useful work. Both sit in the denominator, so a less efficient or lower-PF motor draws more amps for the same horsepower. Always read these values from the motor nameplate when you can.
Single-Phase vs Three-Phase
Three-phase motors carry the √3 (1.732) factor because power is delivered across three conductors, sharing the load and drawing less current per leg than an equivalent single-phase motor. That's why nearly all industrial and commercial motors above a few HP are three-phase: lower current means smaller wire and breakers for the same power. For sizing conductors, the NEC requires using table full-load currents and applying a 125% factor for continuous motor loads.
Approximate Full-Load Amps (three-phase, 0.9 Eff, 0.85 PF)
| HP | 208 V | 230 V | 460 V |
|---|---|---|---|
| 1 | 2.7 | 2.5 | 1.2 |
| 5 | 13.6 | 12.3 | 6.1 |
| 10 | 27.1 | 24.5 | 12.3 |
| 25 | 67.8 | 61.3 | 30.7 |
| 50 | 135.6 | 122.6 | 61.3 |
These are estimates — use NEC Table 430.250 full-load currents for actual conductor and overcurrent-protection sizing, and consult a licensed electrician for installations.
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Calculated Amps vs. NEC Full-Load Amps (FLA)
The formula on this page gives the theoretical current for your exact efficiency and power factor. But for sizing conductors and overloads, the National Electrical Code requires using the standardized full-load current tables (NEC 430.248 for single-phase, 430.250 for three-phase) — not the nameplate or a calculation. The NEC values are deliberately conservative, so they usually run higher than what you'll calculate:
| Motor HP | 1-φ 115 V | 1-φ 230 V | 3-φ 230 V | 3-φ 460 V |
|---|---|---|---|---|
| 1 | 16 A | 8 A | 4.2 A | 2.1 A |
| 2 | 24 A | 12 A | 6.8 A | 3.4 A |
| 3 | 34 A | 17 A | 9.6 A | 4.8 A |
| 5 | 56 A | 28 A | 15.2 A | 7.6 A |
| 7.5 | 80 A | 40 A | 22 A | 11 A |
| 10 | 100 A | 50 A | 28 A | 14 A |
| 15 | — | — | 42 A | 21 A |
| 25 | — | — | 68 A | 34 A |
Values from NEC Tables 430.248 / 430.250 (induction motors, typical published editions) — always verify against the current code edition adopted in your jurisdiction.
Why the three numbers disagree
- Calculated amps — your exact motor at its stated efficiency/power factor. Best for energy and load studies.
- Nameplate FLA — what the manufacturer measured at rated load. Use for overload (heater) selection.
- NEC table FLA — standardized worst-case per HP/voltage. Required for wire, conduit and breaker sizing.
A typical mistake is sizing wire from a calculation that assumed a 95%-efficient motor, then installing an older 82% unit — the NEC tables exist precisely to absorb that spread. This work should be done or checked by a licensed electrician.
Frequently Asked Questions
Power factor is the ratio of working power (true power) to apparent power in an AC circuit. It reflects how effectively the electrical power is being converted into useful work.
For three-phase: Amps = (HP × 746) ÷ (Volts × √3 × power factor × efficiency). For single-phase, drop the √3. Enter your values to get the exact current.
Motors aren't 100% efficient and draw reactive current, so power factor and efficiency convert the ideal power into the real current the motor pulls from the supply.
Roughly 4–5 amps, depending on the motor's efficiency and power factor. The calculator gives a precise figure for your specific motor.
No. DC uses Amps = (HP × 746) ÷ (Volts × efficiency) with no power factor or √3 term, since DC has no phase or reactive component.