Circuit breaker rating calculation for 100a contactor

A very specific question!

To calculate the circuit breaker rating for a 100A contactor, we need to consider a few factors:

  1. Contactor rating: The contactor is rated for 100A, which means it can handle a maximum current of 100 amps.
  2. Inrush current: When the contactor closes, it can create a high inrush current, which is typically 2-5 times the rated current of the contactor. In this case, the inrush current would be 200-500 amps.
  3. Motor starting current: If the contactor is controlling a motor, the motor starting current can be significantly higher than the rated current of the contactor. The starting current can be 3-5 times the rated current of the motor.
  4. Circuit impedance: The circuit impedance, including the wire size, length, and any other components, affects the current flowing through the circuit.

To calculate the circuit breaker rating, we need to consider the maximum current that will flow through the circuit. Let's assume the motor starting current is the highest current that will flow through the circuit.

Calculation:

  1. Motor starting current: 3-5 times the rated current of the motor (assuming a 3-5 HP motor)
    • 3 x 100A = 300A (minimum)
    • 5 x 100A = 500A (maximum)
  2. Inrush current: 2-5 times the rated current of the contactor
    • 2 x 100A = 200A (minimum)
    • 5 x 100A = 500A (maximum)
  3. Maximum current: Choose the highest value between the motor starting current and inrush current
    • Maximum current = 500A (motor starting current)

To be safe, it's recommended to add a 20-30% margin to the maximum current to account for any unexpected increases in current.

Circuit breaker rating:

Based on the calculation, a circuit breaker with a rating of at least 600A or 650A would be suitable for this application. However, it's always recommended to consult the contactor and circuit breaker manufacturers' specifications and guidelines to ensure compatibility and safety.

In practice, it's common to use a circuit breaker with a rating slightly higher than the calculated value to account for any uncertainties and to provide a margin of safety.