⚡ HP to Amps Calculator

Our HP to Amps Calculator is a digital tool designed for engineers, electricians, and technicians to determine the electrical current a motor will draw based on its mechanical power output.

This conversion is vital for selecting the correct wire sizes, circuit breakers, and overload protection devices, ensuring both safety and efficiency in electrical installations .

While motors are rated by their output horsepower (hp), electrical circuits are sized based on the input current, in amperes (amps). Our calculator seamlessly bridges this gap, performing accurate conversions for Direct Current (DC), Single-Phase Alternating Current (AC), and Three-Phase AC systems, all within a single, intuitive interface.

How to Use Our Calculator

1. Select Current Type: Choose the motor’s electrical system: DC, AC-Single-phase, or AC-Three-phase.
2. Specify Voltage Type (for Three-Phase): If applicable, choose between Line-to-Line or Line-to-Neutral voltage.
3. Enter Power: Input the motor’s power. You uniquely allow input in multiple units: Electrical Horsepower (hp(E)), Mechanical Horsepower (hp(M)), Watts (W), or Kilowatts (kW).
4. Input Voltage: Provide the system’s operating voltage.
5. Provide Power Factor (for AC systems): Enter this value, typically between 0.8 and 0.98 for AC motors.
6. Set Efficiency: Input the motor’s efficiency, usually found on its nameplate and represented as a percentage.
7. Read the Result: The calculator instantly displays the amperage in your preferred unit (A, mA, kA).

How It Works

These formulas show how our calculator intelligently accounts for real-world motor losses (efficiency) and the phase difference between voltage and current in AC systems (power factor) to provide accurate, real-world amperage values.

Practical Applications

Knowing how to convert HP to Amps is essential for:

• Preventing Electrical Hazards: Correctly sizing wires and breakers prevents overheating and potential fire risks .
• Ensuring System Performance: An undersized motor can overload and fail, while an oversized one operates inefficiently, wasting energy and money .
• Complying with Codes: Electrical installations must adhere to standards like the NEC, which require calculations based on motor full-load current .

Example:
Let’s find the current for a 1 hp, single-phase AC motor running on 200 V, with 90% efficiency and a power factor of 0.92 .

1. Convert HP to Watts: 1 hp × 746 = 746 W.
2. Calculate the Denominator: 200 V × 0.92 (PF) × 0.9 (η) = 165.6 V.
3. Compute Amperage: 746 W / 165.6 V ≈ 4.505 A.

National Electrical Code (NEC) and Nameplate Conventions

When working with motors in the United States, it’s crucial to understand that the Full Load Amperage (FLA) calculated by our tool may differ from the values used for sizing circuit protection in the National Electrical Code (NEC) . The NEC provides standardized tables of full-load currents to ensure consistent and safe wiring and protection design.

• North American vs. European Motor Ratings: Be aware that North American motors are rated by their output horsepower, meaning the formulas must account for efficiency to find input current. In contrast, European motors are often rated in output watts, simplifying the calculation as the 746W conversion factor is not needed.
• Practical Implication: When sizing conductors, breakers, and fuses for a motor, you must use the NEC tables, not just the calculated or nameplate FLA. For instance, the NEC might list the full-load current for a 3-phase, 25 HP, 460V motor as 34 amps, which is used to determine wire size and breaker rating.

Charts and Rules of Thumb

For quick field estimations, electricians often use rules of thumb. Our calculator provides precise results, but these approximations can be useful for verification .

For more precise planning, here are reference values for common motor sizes at 90% efficiency. Note how amperage decreases as voltage increases .

Table: Common Single-Phase Motor Amperages (at 90% Efficiency)

Table: Common Three-Phase Motor Amperages (at 90% Efficiency)