Cable circuit breaker sizing

Cable circuit breaker sizing is a critical process in electrical engineering that ensures the safe and reliable operation of electrical systems. Here's a comprehensive guide to help you size cable circuit breakers:

Why is cable circuit breaker sizing important?

Cable circuit breaker sizing is important because it ensures that the breaker can interrupt the maximum fault current that may occur in the circuit. If the breaker is undersized, it may not be able to interrupt the fault current, leading to:

  1. Fire hazards
  2. Equipment damage
  3. System downtime
  4. Potential for electrical shock or electrocution

How to size cable circuit breakers

To size a cable circuit breaker, you need to consider the following factors:

  1. Maximum fault current: The maximum fault current that may occur in the circuit, which is typically determined by the short-circuit current rating of the system.
  2. Cable size: The size of the cable that will be used in the circuit, which affects the maximum current-carrying capacity of the cable.
  3. Voltage: The voltage of the system, which affects the maximum fault current.
  4. System configuration: The configuration of the system, including the number of parallel circuits, the type of loads, and the presence of any fault current limiting devices.

Cable circuit breaker sizing formulas

There are several formulas that can be used to size cable circuit breakers, including:

  1. I2t method: This method is based on the energy that the breaker must be able to withstand during a fault. The formula is:

I2t = (Fault current)2 * (Fault duration)

Where I2t is the energy that the breaker must be able to withstand, fault current is the maximum fault current, and fault duration is the time it takes for the breaker to interrupt the fault.

  1. Short-circuit current method: This method is based on the maximum short-circuit current that may occur in the circuit. The formula is:

Isc = (Voltage) / (Impedance)

Where Isc is the maximum short-circuit current, voltage is the voltage of the system, and impedance is the impedance of the circuit.

Example calculation

Let's say we have a 3-phase, 480V system with a maximum fault current of 20,000 A. We want to size a cable circuit breaker for a 1,000 ft (305 m) long cable with a size of 3/0 AWG (19.1 mm2).

Using the I2t method, we can calculate the energy that the breaker must be able to withstand:

I2t = (20,000 A)2 * (0.1 s) = 80,000,000 A2s

Using a breaker catalog or manufacturer's data, we can find a breaker that can withstand an energy of 80,000,000 A2s. For example, a 20,000 A, 480V, 3-pole breaker with an I2t rating of 80,000,000 A2s.

Conclusion

Cable circuit breaker sizing is a critical process that requires careful consideration of several factors, including maximum fault current, cable size, voltage, and system configuration. By using formulas such as the I2t method or short-circuit current method, you can ensure that the breaker is properly sized to interrupt the maximum fault current and prevent electrical hazards.