Electric Motor HP Calculator
Calculate horsepower output for single-phase, three-phase, and DC electric motors from nameplate values. Results update instantly as you type.
DC motors have no power factor. HP = V × I × Eff / 746
This electric motor HP calculator converts electrical inputs — voltage, current, efficiency, and power factor — into mechanical horsepower for single-phase, three-phase, and DC motors. It's built for electricians, engineers, and technicians sizing motors or verifying that a motor's draw matches its nameplate rating.
Electric Motor HP Formulas
Eff = motor efficiency (decimal, e.g. 0.90) · PF = power factor (decimal, e.g. 0.85)
All other variables same as single-phase formula
The constant 746 is the number of watts per horsepower (1 HP = 746 W). Efficiency converts input electrical power to mechanical shaft power—a motor rated at 90% efficiency loses 10% as heat. Power factor (AC only) accounts for the phase difference between voltage and current waveforms in inductive loads.
Single-Phase, Three-Phase, and DC
Single-phase motors run on standard residential power and are common up to a few horsepower. Three-phase motors dominate industrial settings: the √3 factor reflects power delivered across three conductors, so they draw less current and run more smoothly and efficiently than single-phase motors of the same output. DC motors have no power factor because there's no phase shift in a DC circuit. Across all three, efficiency and (for AC) power factor determine how much electrical input is needed to produce a given shaft horsepower.
Standard Motor HP, kW, and Full-Load Amps (three-phase)
| HP | kW | FLA @ 230V | FLA @ 460V |
|---|---|---|---|
| 1 | 0.75 | 3.6 | 1.8 |
| 5 | 3.7 | 15.2 | 7.6 |
| 10 | 7.5 | 28 | 14 |
| 25 | 18.6 | 68 | 34 |
| 50 | 37.3 | 130 | 65 |
| 100 | 74.6 | 248 | 124 |
FLA values approximate NEC Table 430.250. Use the code tables for actual conductor and overcurrent sizing, and size feeders at 125% of full-load current for continuous duty.
Worked Example — Three-Phase Motor
Nameplate Values & Typical Motor Efficiencies
The motor nameplate contains everything needed for this calculation. Efficiency ratings have improved significantly—modern NEMA Premium® motors can achieve 95–96% efficiency at higher HP ratings. Typical ranges:
| Motor HP | Standard Eff. | NEMA Premium Eff. | Typical PF |
|---|---|---|---|
| 1 HP | 77% | 85.5% | 0.72–0.80 |
| 5 HP | 85% | 89.5% | 0.82–0.86 |
| 10 HP | 88% | 91.7% | 0.84–0.88 |
| 25 HP | 90% | 93.6% | 0.86–0.89 |
| 50 HP | 91% | 94.5% | 0.87–0.90 |
| 100 HP | 92% | 95.4% | 0.88–0.91 |
This calculator uses nameplate values for full-load conditions. Actual HP output varies with load — a motor runs below its rated HP when lightly loaded. Always use nameplate FLA, efficiency, and PF for accurate sizing. For motor protection, size overload relays at 115–125% of FLA per NEC 430.52.
Service Factor, and Why 1 HP Can Legally Deliver 1.15 HP
Most industrial motors carry a service factor (SF) on the nameplate — commonly 1.15 for open drip-proof motors per NEMA MG-1. A 10 HP motor with SF 1.15 can run continuously at 11.5 HP at rated voltage and frequency, at the cost of extra heat and reduced insulation life. Treat the SF band as headroom for brief overloads and voltage sag, not as free capacity to size into.
- SF 1.0 — no overload margin (common on totally enclosed and inverter-duty motors run on VFDs).
- SF 1.15 — the usual industrial default; amps at SF load are listed on the nameplate as "SFA".
- Derate when ambient exceeds 40 °C or altitude exceeds 3,300 ft — the SF margin disappears first.
When your calculated load lands between standard sizes (e.g. 8.2 HP), the correct choice is the next size up (10 HP) rather than a 7.5 HP motor living permanently inside its service factor. Motor standards and letter codes are defined in NEMA MG-1; starting-current code letters and efficiency classes (IE/NEMA Premium) also appear on the nameplate alongside FLA.
Frequently Asked Questions
Full Load Amps (FLA) is the maximum current a motor draws when running at its rated horsepower under full load. It is printed on the motor nameplate and used to size circuit breakers, fuses, and wire conductors. Running a motor consistently above FLA causes overheating and premature failure. Always use nameplate FLA, not calculated values, for protective device sizing.
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps). A motor with PF = 0.85 uses only 85% of the power drawn from the supply for actual work — the remaining 15% is reactive power from the inductive windings. Low power factor increases utility bills and requires larger wiring. Capacitor banks can improve PF. DC motors have no power factor because the current is not alternating.
Single-phase motors are common in homes and light commercial settings (typically under 7.5 HP). They require only two power conductors but produce pulsing torque. Three-phase motors are standard in industrial environments — they deliver smoother torque, run more efficiently, are self-starting, and are available up to thousands of HP. For the same HP rating, three-phase motors draw less current and are smaller and lighter than single-phase equivalents.
A motor nameplate lists: HP (rated output), V (supply voltage — match exactly), A/FLA (full-load amps), RPM (rated speed at full load), Hz (frequency, 60Hz in North America), PH (phases: 1 or 3), Eff% (efficiency percentage), and PF (power factor). Dual-voltage motors show two voltages (e.g., 230/460V) and corresponding amps (e.g., 20/10A) — use the value matching your supply voltage.
It depends on voltage and phase, but a 5 HP three-phase 230V motor draws roughly 13–15 amps. Use the calculator with your exact voltage, efficiency, and power factor.