Horsepower to Amps Converter
Calculate motor current from horsepower for DC and AC circuits with precision
Quick Conversions
How to Convert Horsepower to Amps
Converting horsepower to amps requires knowing the voltage, motor efficiency, and circuit type. Electric horsepower represents the power output of a motor, equal to 746 watts. The conversion formula varies depending on whether you’re working with DC or AC circuits.
I (A) = P (HP) × 746 / (V × η)
I (A) = P (HP) × 746 / (V × η × PF)
I (A) = P (HP) × 746 / (√3 × V × η × PF)
Where: I = current in amperes, P = power in horsepower, V = voltage in volts, η = efficiency (as decimal), PF = power factor
Conversion Steps
- Convert HP to Watts: Multiply the horsepower value by 746 to get watts. For example, 1 HP × 746 = 746 watts.
- Account for Efficiency: Divide the wattage by the motor efficiency (expressed as a decimal). If efficiency is 90%, use 0.9. This accounts for energy losses in the motor.
- Apply Circuit-Specific Formula: For DC circuits, divide by voltage. For AC circuits, also divide by the power factor. For three-phase systems, divide by √3 (approximately 1.732) times voltage.
- Calculate Current: The result gives you the current draw in amperes. Round to appropriate decimal places based on your application requirements.
Common Motor Current Values
| Horsepower | 120V DC (90%) | 240V AC 1φ (90%) | 480V AC 3φ (90%) |
|---|---|---|---|
| 0.25 HP | 1.73 A | 0.86 A | 0.60 A |
| 0.5 HP | 3.45 A | 1.73 A | 1.19 A |
| 0.75 HP | 5.18 A | 2.59 A | 1.79 A |
| 1 HP | 6.91 A | 3.45 A | 2.39 A |
| 1.5 HP | 10.36 A | 5.18 A | 3.58 A |
| 2 HP | 13.82 A | 6.91 A | 4.77 A |
| 3 HP | 20.72 A | 10.36 A | 7.16 A |
| 5 HP | 34.54 A | 17.27 A | 11.93 A |
| 7.5 HP | 51.81 A | 25.91 A | 17.90 A |
| 10 HP | 69.07 A | 34.54 A | 23.87 A |
Note: All AC values calculated with 0.9 power factor. Actual current may vary based on specific motor characteristics.
Practical Examples
Example 1: DC Motor Current Calculation
A 3 HP DC motor operates at 120 volts with 90% efficiency. Calculate the current draw:
Solution: I = (3 × 746) / (120 × 0.9) = 2,238 / 108 = 20.72 A
This motor will draw approximately 20.72 amperes during operation.
Example 2: Single-Phase AC Motor
A 2 HP single-phase motor runs on 230V with 90% efficiency and 0.92 power factor:
Solution: I = (2 × 746) / (230 × 0.9 × 0.92) = 1,492 / 190.44 = 7.83 A
The current requirement is 7.83 amperes for this configuration.
Example 3: Three-Phase Industrial Motor
A 25 HP three-phase motor operates at 480V line-to-line, 90% efficiency, 0.9 power factor:
Solution: I = (25 × 746) / (1.732 × 480 × 0.9 × 0.9) = 18,650 / 672.05 = 27.75 A
This industrial motor draws 27.75 amperes per phase.
Related Conversions
Amps to Horsepower
Calculate motor power output from current draw by reversing the formula. Multiply amps by voltage and efficiency, then divide by 746.
Watts to Horsepower
Convert watts to horsepower by dividing by 746. One horsepower equals exactly 746 watts of electrical power.
kW to Horsepower
Multiply kilowatts by 1.341 to convert to horsepower. This conversion factor accounts for the difference between metric and imperial power units.
Amps to Watts
Calculate watts by multiplying amps times volts for DC circuits. For AC circuits, also multiply by power factor.
Motor Specifications Guide
When selecting wire size and circuit breakers for motors, current ratings are critical. The calculated amperage represents the full-load current, but motors require higher starting current.
Key Considerations
- Starting Current: Motors typically draw 4-8 times their full-load current during startup. Circuit protection must accommodate this surge.
- Continuous Rating: Wire and breakers should be rated for 125% of the full-load current per National Electrical Code guidelines.
- Voltage Drop: Long wire runs reduce effective voltage. Calculate voltage drop to maintain motor performance.
- Temperature Rating: Higher ambient temperatures reduce wire current-carrying capacity. Consult derating tables.