Circuit breaker selection formula
The art of selecting a circuit breaker!
The selection of a circuit breaker depends on several factors, including the type of circuit, the voltage and current ratings, the fault current, and the desired level of protection. Here's a general formula to help you choose the right circuit breaker:
Formula:
- Determine the maximum fault current (I_f) that the circuit can withstand. This is usually specified by the manufacturer or calculated using the short-circuit current calculation formula.
- Calculate the maximum continuous current (I_c) that the circuit will carry. This is usually specified by the manufacturer or calculated using the load calculation formula.
- Choose a circuit breaker with a rating that exceeds the maximum fault current (I_f) and the maximum continuous current (I_c).
Formula:
Circuit Breaker Rating (I_b) ≥ Maximum Fault Current (I_f) ≥ Maximum Continuous Current (I_c)
Example:
Suppose you have a 3-phase, 480V, 100A circuit with a maximum fault current of 20,000A and a maximum continuous current of 100A.
- Maximum fault current (I_f) = 20,000A
- Maximum continuous current (I_c) = 100A
- Choose a circuit breaker with a rating that exceeds both values:
Circuit Breaker Rating (I_b) ≥ 20,000A ≥ 100A
In this case, you would choose a circuit breaker with a rating of at least 20,000A or 200A (since 20,000A is greater than 200A).
Additional considerations:
- Inrush current: If the circuit has a high inrush current (e.g., from a motor or transformer), you may need to choose a circuit breaker with a higher rating to account for this.
- Arc fault protection: If you need to protect against arc faults, you may need to choose a circuit breaker with arc fault protection (e.g., AFCI or GFCI).
- Type of circuit: Different types of circuits (e.g., residential, commercial, industrial) may require different circuit breaker ratings and features.
Remember to always consult the manufacturer's specifications and follow local electrical codes and regulations when selecting a circuit breaker.