⚡ HP to Amps Calculator
Convert horsepower to amperes
Line voltage for the motor
Motor efficiency (typically 80-95%)
Power factor (typically 0.8-0.95)
How to Use This Calculator
Enter Horsepower
Input the motor's rated horsepower (HP). This is the mechanical power output of the motor.
Enter Voltage
Input the line voltage the motor operates at, in Volts (e.g., 120V, 230V, 460V for single-phase, or 208V, 230V, 480V for three-phase).
Select Phase Type
Choose whether the motor is single-phase or three-phase. This affects the current calculation formula.
Enter Efficiency and Power Factor
Input motor efficiency as a percentage (typically 80-95%) and power factor as a decimal (typically 0.8-0.95). If not known, use defaults: 85% efficiency and 0.85 power factor.
Calculate
Click the "Calculate Amperes" button to get the full-load current in Amperes that the motor will draw from the electrical supply.
Formula
Single Phase Motor
I = P / (V × PF)
where P = (HP × 746) / Efficiency
Three Phase Motor
I = P / (√3 × V × PF)
where P = (HP × 746) / Efficiency
Where:
- I = Current (Amperes, A)
- P = Electrical Input Power (Watts, W)
- HP = Horsepower
- V = Voltage (Volts, V)
- PF = Power Factor (0 to 1)
- Efficiency = Motor Efficiency (0 to 1, or 0% to 100%)
Example Calculation:
For a 5 HP, 230V, single-phase motor with 85% efficiency and 0.85 power factor:
P = (5 × 746) / 0.85 = 4388 W
I = 4388 / (230 × 0.85) = 22.4 A
Note: 1 HP = 746 Watts. This calculates full-load current. Starting current is typically 5-7 times higher.
About HP to Amps Calculator
The HP to Amps Calculator converts motor horsepower to full-load current in amperes, accounting for motor efficiency and power factor. This is essential for sizing circuit breakers, wires, and other electrical components when installing electric motors. The calculation differs for single-phase and three-phase motors.
When to Use This Calculator
- Motor Installation: Determine wire size and circuit breaker rating for motor circuits
- Electrical Design: Size electrical systems and components for motor loads
- Load Calculations: Calculate total electrical load in installations with motors
- Equipment Selection: Select appropriate electrical components for motor applications
- Code Compliance: Ensure installations meet electrical code requirements
Why Use Our Calculator?
- ✅ Dual Phase Types: Supports both single-phase and three-phase calculations
- ✅ Accurate Results: Accounts for efficiency and power factor for realistic current values
- ✅ Electrical Design: Essential for proper motor circuit sizing
- ✅ Free Tool: No registration or payment required
- ✅ Safety: Helps ensure proper wire and breaker sizing for safety
Common Applications
Industrial Motor Installation: Calculate full-load current when installing motors in industrial facilities, ensuring proper wire gauge selection and circuit breaker sizing. Understanding the current draw helps prevent overload conditions and ensures code compliance with NEC (National Electrical Code) requirements.
HVAC Systems: Determine electrical requirements for compressor motors, fan motors, and pump motors in heating, ventilation, and air conditioning systems, allowing proper electrical service sizing and preventing overloads on building electrical systems.
Motor Control Centers: Size contactors, overload relays, and other motor control components based on full-load current. This ensures reliable motor operation and protection, preventing motor damage from overload conditions.
Tips for Best Results
- Use motor nameplate values for voltage, efficiency, and power factor when available
- Full-load current is for continuous operation at rated HP
- Starting current (locked-rotor current) is 5-7 times full-load current
- For safety, size breakers at 125% of full-load current for continuous duty
- Three-phase motors are more efficient and draw less current than single-phase for same HP
Frequently Asked Questions
What is the difference between single-phase and three-phase?
Single-phase motors use one alternating voltage waveform, while three-phase motors use three synchronized waveforms. Three-phase motors are more efficient, produce more power for the same size, and draw less current. The formula includes √3 for three-phase to account for the relationship between line voltage and phase voltage.
Why do I need efficiency and power factor?
Efficiency accounts for power losses in the motor (converting electrical to mechanical power). Power factor accounts for reactive power in AC circuits. Together, they determine the actual electrical input power needed to produce the rated mechanical horsepower output.
Is this the same as starting current?
No, this calculates full-load current (steady-state running current). Starting current (locked-rotor current) is typically 5-7 times higher and only lasts a few seconds during motor startup. Circuit breakers are sized for full-load current, while overload protection must handle starting current.
How do I size the circuit breaker?
For continuous duty motors, size breakers at 125% of full-load current (per NEC). For example, if full-load current is 20 A, use a 25 A breaker. This provides protection while allowing for normal operation and starting current.
What if I don't know the efficiency and power factor?
Use typical values: 85% efficiency and 0.85 power factor for most motors. High-efficiency motors may have 90-95% efficiency. Power factor is typically 0.8-0.95. Check motor nameplate for actual values. Using defaults gives a reasonable estimate.
Can I use this for DC motors?
This calculator is for AC motors. For DC motors, use I = (HP × 746) / (V × Efficiency), without power factor (since DC has no reactive power). DC motors typically have simpler current calculations because there's no power factor consideration.