AC Motor FLA Calculator
Horsepower (HP)
Kilowatts (kW)
Three Phase (3Ø)
Single Phase (1Ø)
Please enter valid numeric values for all fields.
Full Load Amperage (FLA)
0.00 A
What is Full Load Amperage (FLA)?
Full Load Amperage (FLA), sometimes referred to as Full Load Current (FLC), represents the amount of current (in Amperes) an electric motor is designed to draw from the power source when operating at its full rated output power and rated voltage. Understanding FLA is critical for electrical engineers and technicians when sizing wires, selecting circuit breakers, and designing thermal overload protection.
The FLA value is usually found on the motor's nameplate. However, during the design phase or when the nameplate is illegible, it must be calculated using the motor's power rating, efficiency, voltage, and power factor.
How to Calculate FLA
The calculation for FLA depends on whether the motor is Single Phase or Three Phase. The core concept involves converting the mechanical power output (Horsepower or Kilowatts) into electrical power input (Watts), and then dividing by the voltage and relevant electrical factors.
1. Single Phase Formula
For single-phase AC motors, the formula is:
FLA (A) = Power (W) / (Voltage × PF × Efficiency)
2. Three Phase Formula
For three-phase AC motors, we must account for the square root of 3 (approximately 1.732):
FLA (A) = Power (W) / (Voltage × 1.732 × PF × Efficiency)
Variables Explained
- Power (P): The motor's rating. If given in Horsepower (HP), convert to Watts (1 HP ≈ 746 Watts). If in kW, multiply by 1000.
- Voltage (V): The line-to-line voltage applied to the motor.
- Power Factor (PF): The ratio of real power used to do work and the apparent power supplied to the circuit. Usually between 0.80 and 0.95 for industrial motors.
- Efficiency (η): The ratio of mechanical power output to electrical power input. No motor is 100% efficient; energy is lost as heat.
Why is FLA Important?
Calculating the correct Full Load Amperage is the foundation of safe electrical installation:
- Wire Sizing: Conductors must be sized to carry at least 125% of the motor's FLA (per NEC standards) to prevent overheating.
- Overload Protection: Thermal overloads are set based on the FLA to protect the motor windings from burning out during a stall or mechanical overload.
- Breaker Sizing: Circuit breakers must handle the inrush current (often 6x the FLA) without nuisance tripping while still providing short-circuit protection.