Motor Rated Current Calculator
Three Phase AC
Single Phase AC
DC Motor
kW
HP
Watts
Usually between 0.8 and 0.95
Calculation Results
Rated Current (FLA):
0 A
Input Power (Pin):
0 kW
Apparent Power (S):
0 kVA
How to Calculate Rated Current of a Motor
Calculating the rated current, often referred to as Full Load Amps (FLA), is essential for selecting the correct circuit breakers, cables, and overload protection for electric motor installations. The calculation depends heavily on the type of motor system (DC, Single Phase AC, or Three Phase AC) and factors like efficiency and power factor.
Calculation Formulae
The core concept is derived from the electrical power formula. However, because motors are not 100% efficient, we must account for the input power required to produce the rated output mechanical power.
1. Three Phase AC Motor Formula
This is the most common industrial motor type. The current is calculated as:
I = P / (√3 × V × PF × η)
Where:
- I = Rated Current (Amps)
- P = Motor Output Power (Watts)
- V = Rated Line-to-Line Voltage (Volts)
- PF = Power Factor (0 to 1)
- η = Efficiency (decimal, 0 to 1)
- √3 = Approximately 1.732
2. Single Phase AC Motor Formula
Used for smaller appliances and residential applications.
I = P / (V × PF × η)
3. DC Motor Formula
For Direct Current motors, Power Factor is not applicable.
I = P / (V × η)
Understanding the Variables
- Motor Power (P): Usually found on the nameplate. It represents the mechanical output power. Our calculator converts HP to Watts (1 HP ≈ 746 Watts) automatically.
- Efficiency (η): No motor converts 100% of electrical energy into mechanical energy. Losses occur due to heat and friction. If unknown, 85-90% is a standard estimation for modern motors.
- Power Factor (PF): Represents the phase difference between voltage and current in AC circuits. Inductive loads like motors typically have a lagging power factor between 0.80 and 0.95.
Example Calculation
Imagine a 15 kW Three-Phase motor connected to a 400V supply. The nameplate states a Power Factor of 0.85 and an efficiency of 90%.
- Convert efficiency to decimal: 90% = 0.90.
- Convert power to Watts: 15 kW = 15,000 W.
- Apply formula: I = 15000 / (1.732 × 400 × 0.85 × 0.90)
- Calculate denominator: 1.732 × 400 × 0.85 × 0.90 ≈ 530.1
- Final Result: I = 15000 / 530.1 ≈ 28.3 Amps