3 Phase Motor Amperage Calculator
Calculating the Full Load Amperage (FLA) for a 3-phase electric motor is essential for sizing circuit breakers, conductors, and overload protection. This tool allows you to accurately determine the current draw based on the motor's power rating, voltage, power factor, and efficiency.
Motor Current Calculator
HP
kW
208 V
230 V
460 V
480 V
575 V
2400 V
4160 V
Full Load Amperage (FLA)
0.00 Amps
Understanding the 3-Phase Amperage Formula
The calculation for 3-phase motor current is derived from the power formula for three-phase systems. Unlike single-phase systems, we must account for the square root of 3 (approximately 1.732) because the power is distributed across three phases.
The Formula:
I = P / (V × √3 × PF × η)
- I = Current in Amperes (Amps)
- P = Power in Watts (Note: 1 HP = 746 Watts)
- V = Line-to-Line Voltage (Volts)
- √3 = Constant for 3-phase systems (1.732)
- PF = Power Factor (the ratio of real power to apparent power)
- η = Efficiency (expressed as a decimal)
Realistic Example:
Suppose you have a 15 HP motor operating at 460V with a power factor of 0.88 and an efficiency of 92%.
1. Convert HP to Watts: 15 × 746 = 11,190 W
2. Formula: 11,190 / (460 × 1.732 × 0.88 × 0.92)
3. Calculation: 11,190 / 645.15
4. Result: 17.34 Amps
Suppose you have a 15 HP motor operating at 460V with a power factor of 0.88 and an efficiency of 92%.
1. Convert HP to Watts: 15 × 746 = 11,190 W
2. Formula: 11,190 / (460 × 1.732 × 0.88 × 0.92)
3. Calculation: 11,190 / 645.15
4. Result: 17.34 Amps
Key Factors Influencing Amperage
Several variables impact the final current draw of your motor:
- Voltage Fluctuations: If the supply voltage drops, the amperage will typically increase to maintain the same power output, which can lead to overheating.
- Power Factor (PF): Inductive loads like motors cause the current to lag behind the voltage. A lower power factor means the motor requires more current to perform the same amount of work.
- Efficiency: No motor is 100% efficient due to friction, heat, and magnetic losses. Higher efficiency motors draw fewer amps for the same mechanical output.
Why Use This Calculator?
Engineers and electricians use this calculation to ensure safety and compliance with the National Electrical Code (NEC). Selecting a wire gauge that is too small for the calculated FLA can result in voltage drops and fire hazards. Similarly, setting an overload protection relay too high can lead to motor burnout during a jam or phase loss.