Contactor Size Calculator
Determine the right contactor for your motor applications
Contactor Size Calculator
Compliant with IEC 60947 Standards
Results:
Full Load Current (FLC): 0.00 A
Recommended Contactor Size (AC-X): 0.00 A
Making/Breaking Capacity: 0.00 A
Note: For practical selection, choose the next standard contactor size equal to or greater than the recommended rating.
How to Use the Calculator
Motor & System Details
- Enter the motor's rated power and select unit (kW/HP).
- Input the system voltage (V) and select the phase.
- Enter the motor's power factor (e.g., 0.8 for induction motors).
Select Application Type
- Choose the application type from the dropdown menu.
- This selection determines the contactor's utilization category (e.g., AC1, AC3) and its making capacity multiplier.
- Refer to the "Contactor Sizing Fundamentals" section for details on categories.
Calculate & Interpret Results
- Click "Calculate" to get the Full Load Current (FLC).
- View the Recommended Contactor Size (100% FLC) and the Making/Breaking Capacity.
- Select a contactor with an appropriate AC-X rating and making capacity.
Contactor Sizing Fundamentals
What is a Contactor?
A contactor is an electrically controlled switch used for switching a power circuit. It is typically used for controlling electric motors, lighting, heating, and other electrical loads. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current.
Utilization Categories & Making Capacity
Contactor selection is based on IEC 60947-4-1 utilization categories, which define the type of load and operating conditions. Each category has a "Making Capacity" multiplier, indicating how many times the rated current the contactor can safely make (close on) without damage.
- AC-1: Non-inductive or slightly inductive loads. Making Cap: 1.5x FLC.
- AC-3: Squirrel-cage motors: starting, switching off running motors. Making Cap: 10x FLC (to handle inrush current).
- AC-4: Squirrel-cage motors: rapid start/stop, plugging, inching. Making Cap: 12x FLC (for severe duty).
- The "Recommended Contactor Size" is typically 100% of the Full Load Current (FLC). The "Making/Breaking Capacity" is calculated by multiplying the FLC by the "Making Cap" value from the selected application category [^iec_60947_making_cap].
Key Formulas for Full Load Current (FLC):
Single Phase AC: FLC (A) = (Motor Power (kW) × 1000) / (Voltage (V) × Power Factor)
Three Phase AC: FLC (A) = (Motor Power (kW) × 1000) / (√3 × Voltage (V) × Power Factor)
Where √3 ≈ 1.732
Making/Breaking Capacity:
Capacity (A) = Making Cap Multiplier × FLC (A)
Calculation Formulas
Single Phase AC Motor:
FLC = (P_kW × 1000) / (V × PF)
Three Phase AC Motor:
FLC = (P_kW × 1000) / (√3 × V × PF)
Making/Breaking Capacity:
Capacity = Making Cap × FLC
Where:
- P_kW = Motor Power in Kilowatts
- V = Line Voltage in Volts
- PF = Power Factor
- √3 = 1.732 (for three-phase systems)
- Making Cap = Multiplier from Application Type chart
Example Calculation
Three Phase AC Motor, Squirrel Cage (AC3):
Motor Power: 15 kW
Voltage: 400 V
Power Factor: 0.85
Application Type: Squirrel Cage Motor (AC3)
FLC = (15 × 1000) / (1.732 × 400 × 0.85)
FLC = 15000 / 588.88 ≈ 25.47 A
Recommended Contactor Size (AC3): 25.47 A
Making Cap for AC3: 10
Making/Breaking Capacity = 10 × 25.47 A = 254.7 A
For practical selection, a 32A AC-3 contactor with a making capacity of at least 254.7A would be chosen.