Convert volts and amps to motor horsepower for DC, single-phase or three-phase power, with typical efficiency and power factor, or solve for amps or volts.
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Volts To Horsepower Formula
Voltage alone does not determine horsepower. Select shaft horsepower, supply current or supply voltage, then enter the other operating-point values. Choose DC, single-phase AC or balanced three-phase AC; for three-phase select line-to-line or line-to-neutral RMS voltage and use line current. Enter efficiency as a percentage and AC power factor as a decimal. Select Calculate; edits mark the previous result as outdated. Values are saved in this browser; Reset clears this calculator’s saved data. The following is the single-phase formula using the engineering approximation 1 mechanical hp ≈ 746 W.
Variables:
- HP is the power output (horsepower)
- V is the voltage supply (volts)
- I is the current (amperes)
- E is the efficiency of the motor (decimal)
- PF is the power factor (decimal)
To calculate the horsepower, multiply the voltage supply by the current, then multiply the result by the efficiency of the motor and the power factor. Divide the result by 746 (since one horsepower is equivalent to approximately 746 watts).
| Voltage (V) | Horsepower (HP) |
|---|---|
| 12 | 0.123 |
| 24 | 0.246 |
| 48 | 0.492 |
| 60 | 0.615 |
| 90 | 0.923 |
| 110 | 1.128 |
| 120 | 1.231 |
| 208 | 2.133 |
| 220 | 2.256 |
| 230 | 2.359 |
| 240 | 2.461 |
| 277 | 2.841 |
| 300 | 3.076 |
| 380 | 3.897 |
| 400 | 4.102 |
| 415 | 4.256 |
| 440 | 4.512 |
| 460 | 4.717 |
| 480 | 4.922 |
| 600 | 6.153 |
| * Rounded to 3 decimals. HP = V x A x n x pf / 746. Assumes I=10A, n=90%, pf=0.85. | |
Three-Phase and DC Motor Formulas
The single-phase formula above applies to standard household and light commercial motors. For industrial three-phase motors, the formula includes the square root of 3 (approximately 1.732) to account for the phase relationship between the three power lines:
Where V_LL is the line-to-line voltage (e.g. 208V, 230V, 460V, or 575V). For balanced three-phase operation using line-to-neutral RMS voltage and line current, use a factor of3. Use consistent measurements; unbalanced operation requires a fuller power measurement.
For DC motors, there is no power factor or phase angle to consider, so the formula simplifies to:
DC motors are common in battery-powered vehicles, traction drives, and variable-speed industrial applications where precise torque control is required.
Why 746 Watts Equals One Horsepower
The 746 constant in the formula traces back to James Watt’s experiments in the 1780s measuring the sustained output of draft horses turning a mill wheel. Watt determined that a strong horse could perform roughly 33,000 foot-pounds of work per minute. Converting that figure into SI units yields 745.69987 watts, which rounds to the standard 746 W/hp used in electrical engineering today. This mechanical origin is why the conversion factor is not a clean number and why it appears in every electrical-to-mechanical power calculation.
Understanding Efficiency and Power Factor
Motor efficiency (E) is mechanical shaft output divided by real electrical input. Losses include winding resistance, core losses, friction and windage. Efficiency depends on the motor, temperature and operating load; its peak is not fixed at a universal load percentage. The reference table below lists nominal NEMA Premium efficiencies for a specific motor class, not a guarantee for every motor or operating point. See ABB/Baldor’s NEMA Premium table and the DOE motor-system handbook.
Power factor (PF) is real electrical power divided by apparent power in an AC circuit. A power factor of 1.0 does not imply100% motor efficiency: electrical real power still includes motor losses. Induction motors typically operate between 0.80 and 0.95 power factor at full load. At light loads, the power factor drops sharply because the magnetizing current stays roughly constant while the working current decreases. Low power factor increases the apparent current drawn from the supply, oversizes conductors and transformers, and may trigger utility power factor penalties. Capacitor banks or variable frequency drives are common corrections.
Typical Motor Efficiency and Power Factor by Size
| Motor Size (HP) | NEMA Premium Nominal Full-Load Efficiency (%) | Illustrative Power Factor |
|---|---|---|
| 1 | 85.5 | 0.75 – 0.82 |
| 3 | 89.5 | 0.80 – 0.86 |
| 5 | 89.5 | 0.82 – 0.87 |
| 10 | 91.7 | 0.84 – 0.89 |
| 25 | 93.6 | 0.86 – 0.91 |
| 50 | 94.5 | 0.88 – 0.92 |
| 100 | 95.4 | 0.89 – 0.93 |
| 200 | 95.8 | 0.90 – 0.95 |
| * Nominal efficiency values: NEMA MG 1:2014 Table 12-12, 60 Hz, 4-pole open motors, reproduced in ABB/Baldor PM318. These nominal values are not an individual motor’s minimum guaranteed efficiency. Power-factor ranges are illustrative, not requirements of that table; use the actual motor data. | ||
NEC Full-Load Ampere Reference
The National Electrical Code (NEC) Tables 430.248 and 430.250 provide standard full-load current values used for conductor sizing, overcurrent protection, and branch circuit design. These values assume standard-speed, normal-torque motors. Table full-load current generally serves conductor, switch and branch short-circuit/ground-fault sizing, while nameplate current generally serves separate motor overload protection, subject to exceptions and the locally adopted edition. These reference values do not replace installation design requirements. See Schneider Electric’s motor-protection guidance.
| HP | 115V | 200V | 208V | 230V |
|---|---|---|---|---|
| 1/6 | 4.4 | 2.5 | 2.4 | 2.2 |
| 1/4 | 5.8 | 3.3 | 3.2 | 2.9 |
| 1/3 | 7.2 | 4.1 | 4.0 | 3.6 |
| 1/2 | 9.8 | 5.6 | 5.4 | 4.9 |
| 3/4 | 13.8 | 7.9 | 7.6 | 6.9 |
| 1 | 16.0 | 9.2 | 8.8 | 8.0 |
| 1.5 | 20.0 | 11.5 | 11.0 | 10.0 |
| 2 | 24.0 | 13.8 | 13.2 | 12.0 |
| 3 | 34.0 | 19.6 | 18.7 | 17.0 |
| 5 | 56.0 | 32.2 | 30.8 | 28.0 |
| 7.5 | 80.0 | 46.0 | 44.0 | 40.0 |
| 10 | 100.0 | 57.5 | 55.0 | 50.0 |
| * Reference values reproduced from NEC Table 430.248; apply the locally adopted edition and its exceptions. Table FLC and nameplate current serve different sizing/protection purposes. | ||||
| HP | 115V | 200V | 208V | 230V | 460V | 575V |
|---|---|---|---|---|---|---|
| 1/2 | 4.4 | 2.5 | 2.4 | 2.2 | 1.1 | 0.9 |
| 3/4 | 6.4 | 3.7 | 3.5 | 3.2 | 1.6 | 1.3 |
| 1 | 8.4 | 4.8 | 4.6 | 4.2 | 2.1 | 1.7 |
| 1.5 | 12.0 | 6.9 | 6.6 | 6.0 | 3.0 | 2.4 |
| 2 | 13.6 | 7.8 | 7.5 | 6.8 | 3.4 | 2.7 |
| 3 | — | 11.0 | 10.6 | 9.6 | 4.8 | 3.9 |
| 5 | — | 17.5 | 16.7 | 15.2 | 7.6 | 6.1 |
| 7.5 | — | 25.3 | 24.2 | 22.0 | 11.0 | 9.0 |
| 10 | — | 32.2 | 30.8 | 28.0 | 14.0 | 11.0 |
| 15 | — | 48.3 | 46.2 | 42.0 | 21.0 | 17.0 |
| 20 | — | 62.1 | 59.4 | 54.0 | 27.0 | 22.0 |
| 25 | — | 78.2 | 74.8 | 68.0 | 34.0 | 27.0 |
| 30 | — | 92.0 | 88.0 | 80.0 | 40.0 | 32.0 |
| 40 | — | 120.0 | 114.0 | 104.0 | 52.0 | 41.0 |
| 50 | — | 150.0 | 143.0 | 130.0 | 65.0 | 52.0 |
| * Values from NEC Table 430.250 for squirrel-cage induction motors. | ||||||
Where This Conversion Applies
Converting volts to horsepower is necessary whenever you need to determine or verify the mechanical output of an electric motor from its electrical supply characteristics. HVAC technicians use it to confirm that a replacement blower or compressor motor matches the original equipment rating. Electricians size branch circuits, contactors, and overload relays based on the expected horsepower and corresponding full-load amps. Industrial maintenance teams use it to audit whether existing motors are appropriately loaded or oversized, because efficiency and power factor can fall at light load; the actual motor’s load curves determine the effect. In automotive and marine applications, this conversion helps compare electric drive systems with traditional combustion ratings, particularly as EV motor specifications are often listed in kilowatts while legacy benchmarks remain in horsepower.
Common Voltage Standards by Application
Residential circuits in North America supply 120V single-phase for small appliances and 240V single-phase for larger loads like well pumps and air compressors. Light commercial buildings commonly use 208V three-phase derived from a wye transformer. Common nominal industrial supplies include480V or600V three-phase;460V and575V commonly denote corresponding motor utilization ratings. Higher voltage can reduce current for a given power, subject to the complete installation design. Motors above 200 HP in heavy industry may use medium-voltage supplies of 2,300V or 4,160V. Each voltage level changes the current draw for a given horsepower rating, which directly affects wire gauge selection, breaker sizing, and overall installation cost. The NEC tables above reflect these standard voltage tiers.
