Amps to HP Calculator
Convert amps to horsepower for DC, single-phase, and three-phase motors. Enter the current draw, voltage, efficiency, and power factor to estimate motor output — or flip the direction to convert HP back to amps.
Typical: 0.85–0.95 (default 0.90)
Typical: 0.80–0.90 (default 0.85)
Note: For sizing conductors and overcurrent protection, the NEC requires using the full-load current values from Tables 430.248 and 430.250 — not calculated or nameplate values (NEC 430.6(A)). Use this calculator for estimates and load planning.
How to Convert Amps to Horsepower
To convert amps to horsepower, calculate the motor's output power in watts and divide by 746(one horsepower equals 746 watts). The output power is the electrical input — amps times volts (times 1.732 for three-phase) — reduced by the motor's efficiency (η) and, for AC motors, the power factor (PF).
DC: HP = Amps × Volts × η ÷ 746
Single-phase AC: HP = Amps × Volts × η × PF ÷ 746
Three-phase AC: HP = Amps × Volts × 1.732 × η × PF ÷ 746
Worked example: a single-phase motor drawing 30 amps on a 230V circuit with 90% efficiency and a 0.85 power factor:
HP = 30 × 230 × 0.90 × 0.85 ÷ 746
HP = 5,278.5 ÷ 746 ≈ 7.1 HP
This estimate is only accurate when the motor is running at or near full load — a lightly loaded motor draws less current, so the same formula would understate its rated horsepower. Keep in mind that for conductor and breaker sizing, NEC 430.6(A) requires the full-load current values from Tables 430.248 and 430.250 (shown below), not measured or nameplate amps.
Amps to HP Reference Chart (NEC Full-Load Currents)
Use these NEC full-load currents to match a measured amp draw to a standard motor horsepower. Three-phase motors per NEC Table 430.250:
| 230V Three-Phase | 460V Three-Phase | Horsepower |
|---|---|---|
| 3.6 A | 1.8 A | 1 HP |
| 6.8 A | 3.4 A | 2 HP |
| 9.6 A | 4.8 A | 3 HP |
| 15.2 A | 7.6 A | 5 HP |
| 22 A | 11 A | 7.5 HP |
| 28 A | 14 A | 10 HP |
| 42 A | 21 A | 15 HP |
| 54 A | 27 A | 20 HP |
| 68 A | 34 A | 25 HP |
| 80 A | 40 A | 30 HP |
| 104 A | 52 A | 40 HP |
| 130 A | 65 A | 50 HP |
Single-phase motors per NEC Table 430.248:
| 115V Single-Phase | 230V Single-Phase | Horsepower |
|---|---|---|
| 16 A | 8 A | 1 HP |
| 24 A | 12 A | 2 HP |
| 34 A | 17 A | 3 HP |
| 56 A | 28 A | 5 HP |
| 80 A | 40 A | 7.5 HP |
| 100 A | 50 A | 10 HP |
Important:NEC 430.6(A)(1) requires these table values — not nameplate or measured current — when sizing branch-circuit conductors, short-circuit protection, and disconnects for a motor of a given horsepower.
Frequently Asked Questions
How do you convert amps to horsepower?
Multiply amps by volts, efficiency, and power factor to get output watts, then divide by 746. For single-phase AC: HP = Amps × Volts × Efficiency × Power Factor ÷ 746. For three-phase AC: HP = Amps × Volts × 1.732 × Efficiency × Power Factor ÷ 746. For DC motors, omit the power factor: HP = Amps × Volts × Efficiency ÷ 746.
How many horsepower is 30 amps?
It depends on the voltage and phase. At 230V single-phase with 90% efficiency and a 0.85 power factor, 30 amps is about 7.1 HP (30 × 230 × 0.90 × 0.85 ÷ 746 = 7.08). At 460V three-phase with the same efficiency and power factor, 30 amps is about 24.5 HP. Amps alone cannot tell you horsepower — you must know the voltage, phase, efficiency, and power factor.
Why is my calculated horsepower lower than the motor's rated HP?
Rated horsepower is mechanical output at full load, while your measured amps reflect electrical input reduced by efficiency and power factor. If the motor is lightly loaded, it draws less current than its full-load value, so the calculation gives less than nameplate HP. Also, measured current includes reactive current that does no work — that is why the power factor term is in the formula.
What efficiency and power factor should I use for the conversion?
If the motor nameplate lists efficiency and power factor, use those values. Otherwise, 0.85–0.95 efficiency and 0.80–0.90 power factor are typical for standard induction motors at full load; this calculator defaults to 0.90 efficiency and 0.85 power factor. Small motors and lightly loaded motors sit at the low end of both ranges, while large premium-efficiency motors sit at the high end.
Can I determine a motor's horsepower from measured amps?
You can estimate it, but only if the motor is running at or near full load. Measure the current with a clamp meter, then apply the formula with the supply voltage and the nameplate efficiency and power factor. For code work, the NEC works the other way: 430.6(A) requires using the full-load currents in Tables 430.248 and 430.250 for a given horsepower when sizing conductors and overcurrent protection — not field measurements.
Why is 746 watts one horsepower?
One mechanical horsepower is defined as 550 foot-pounds of work per second, which converts to 745.7 watts — rounded to 746 W in electrical work. That is why the amps-to-HP formula divides the input power (amps × volts × efficiency × power factor) by 746 to express the motor's mechanical output in horsepower.
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