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

  1. Enter the motor's rated power and select unit (kW/HP).
  2. Input the system voltage (V) and select the phase.
  3. Enter the motor's power factor (e.g., 0.8 for induction motors).

Select Application Type

  1. Choose the application type from the dropdown menu.
  2. This selection determines the contactor's utilization category (e.g., AC1, AC3) and its making capacity multiplier.
  3. Refer to the "Contactor Sizing Fundamentals" section for details on categories.

Calculate & Interpret Results

  1. Click "Calculate" to get the Full Load Current (FLC).
  2. View the Recommended Contactor Size (100% FLC) and the Making/Breaking Capacity.
  3. 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.

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