How to calculate circuit breaker capacity

Calculating the capacity of a circuit breaker involves considering several factors, including the maximum current that the circuit is designed to carry, the voltage of the circuit, and the type of load being protected. Here's a step-by-step guide to help you calculate the capacity of a circuit breaker:

  1. Determine the maximum current: Calculate the maximum current that the circuit is designed to carry, taking into account the maximum load current, motor starting currents, and any other factors that may affect the current draw.
  2. Determine the voltage: Identify the voltage of the circuit, which is typically the nominal voltage of the electrical system.
  3. Choose the type of load: Identify the type of load being protected, such as resistive, inductive, or capacitive. This will help determine the type of circuit breaker required.
  4. Select the circuit breaker type: Choose the type of circuit breaker required based on the load type and voltage. Common types of circuit breakers include:
    • Thermal-magnetic breakers: suitable for most residential and commercial applications
    • Molded case breakers: suitable for industrial and commercial applications
    • Air-break breakers: suitable for high-voltage applications
  5. Calculate the interrupting capacity: Calculate the interrupting capacity of the circuit breaker, which is the maximum current it can interrupt without causing damage. This is typically expressed in amperes (A) or kiloamperes (kA).
  6. Calculate the short-circuit current: Calculate the short-circuit current, which is the maximum current that can flow through the circuit in the event of a fault. This is typically expressed in amperes (A) or kiloamperes (kA).
  7. Choose the circuit breaker rating: Select a circuit breaker with a rating that exceeds the maximum current and short-circuit current calculated in steps 5 and 6. The rating should also take into account any additional factors, such as:
    • Overcurrent protection: the ability of the circuit breaker to interrupt currents above the rated current
    • Short-circuit protection: the ability of the circuit breaker to interrupt short-circuit currents
    • Arc fault protection: the ability of the circuit breaker to detect and interrupt arc faults

Here's a simple example to illustrate the calculation:

Example:

Step 1: Calculate the maximum current

Maximum current = 30 A

Step 2: Determine the voltage

Voltage = 120 V

Step 3: Choose the type of load

Load type = resistive

Step 4: Select the circuit breaker type

Circuit breaker type = thermal-magnetic

Step 5: Calculate the interrupting capacity

Interrupting capacity = 100 A (based on the manufacturer's specifications)

Step 6: Calculate the short-circuit current

Short-circuit current = 200 A (based on the system's short-circuit impedance)

Step 7: Choose the circuit breaker rating

Circuit breaker rating = 200 A (exceeds the maximum current and short-circuit current)

In this example, a 200 A thermal-magnetic circuit breaker would be suitable for the application.

Remember to always consult the manufacturer's specifications and follow local electrical codes and regulations when selecting a circuit breaker.