how to size circuit breaker


Why Sizing Circuit Breakers Matters

Sizing circuit breakers is a crucial step in electrical installations. Whether you are designing a new system or ensuring the safety and efficiency of an existing one, selecting the right circuit breaker size is essential. Circuit breakers are designed to protect electrical systems and equipment from overloads and short circuits. They act as a safety mechanism, interrupting the flow of electricity when necessary to prevent damage or hazards. In this article, we will delve into the intricacies of sizing circuit breakers, exploring the key factors to consider and the steps to determine the appropriate size for your specific application.

Understanding Circuit Breaker Ratings

Circuit breakers come with various ratings that describe their capabilities and limitations. To properly size a circuit breaker, one must comprehend these ratings and how they relate to the electrical system. Here are the most common circuit breaker ratings you need to be aware of:

- Current Rating: This rating indicates the maximum current a circuit breaker can carry continuously without tripping. It is typically expressed in amperes (A) and is an important parameter when sizing circuit breakers.

- Short Circuit Current Rating (SCCR): The SCCR of a circuit breaker refers to the maximum amount of fault current that it can safely interrupt. Fault currents occur during short circuits when excessive current flows through a circuit due to a fault or malfunction. Higher SCCR ratings ensure that the circuit breaker can handle the fault current magnitude and interrupt it safely.

- Voltage Rating: The voltage rating specifies the maximum voltage the circuit breaker can safely handle. It is crucial to choose a circuit breaker with a voltage rating that matches or exceeds the system voltage to ensure proper functioning and safety.

- Interrupting Rating: Also known as the breaking capacity or interrupting capacity, this rating indicates the maximum fault current that a circuit breaker can safely interrupt. It defines the magnitude of fault current that the circuit breaker can handle without causing damage or hazards.

- Frame Size: The frame size refers to the physical size and current-carrying capacity of the circuit breaker. It determines the maximum current that the circuit breaker can handle.

Factors to Consider When Sizing Circuit Breakers

To determine the appropriate size for a circuit breaker, several factors must be taken into account. Here are the key considerations:

- Load Current: The load current is the amount of current drawn by the electrical load connected to the circuit breaker. It is crucial to accurately calculate the load current to avoid selecting an undersized or oversized circuit breaker. To determine the load current, you need to consider the power rating and operating characteristics of the connected electrical equipment.

- Temperature: Environmental factors such as temperature can affect the performance and trip characteristics of circuit breakers. In high-temperature environments, circuit breakers may trip at lower currents due to decreased heat dissipation. Conversely, in low-temperature conditions, higher than rated currents may be allowed by the circuit breaker. It is important to consider the ambient temperature and apply appropriate correction factors when sizing circuit breakers.

- Fault Current: Fault currents occur during short circuits, and the circuit breaker must be able to interrupt these currents safely. The fault current magnitude can vary depending on the system's fault level, which is influenced by factors such as transformer size, impedance, and available fault sources. Understanding the potential fault currents in your system is crucial for selecting circuit breakers with the appropriate interrupting ratings.

- System Voltage: The circuit breaker's voltage rating must be suitable for the system it is being installed in. Using a circuit breaker with a lower voltage rating than the system voltage can lead to insulation breakdown and safety hazards. Conversely, using a circuit breaker with a higher voltage rating can be unnecessary and costly. Therefore, ensure that the selected circuit breaker matches the system voltage.

- Application: Different applications may have specific requirements or regulations that dictate the size and rating of circuit breakers. For instance, industrial environments may require higher interrupting ratings due to higher fault current levels. Carefully assess the specific application and any applicable codes or standards to select the most suitable circuit breaker size.

Steps to Size Circuit Breakers

Now that we have discussed the important factors to consider, let's delve into the steps involved in sizing circuit breakers:

1. Calculate the Load Current: Begin by determining the load current, which is the sum of the expected currents drawn by all the electrical loads connected to the circuit breaker. Consider the characteristics and ratings of each device to accurately determine the load current.

2. Select the Trip Setting: The trip setting determines the current level at which the circuit breaker will disconnect the circuit. It should be set slightly above the expected load current to avoid nuisance tripping while still providing adequate protection. Consider factors such as motor starting currents and inrush currents when selecting the trip setting.

3. Consider Ambient Temperature: If the circuit breaker will operate in extreme temperatures, apply temperature correction factors to ensure proper sizing. Consult the manufacturer's documentation or relevant standards for guidance on correction factors.

4. Calculate the Fault Current: Determine the potential fault current level in the system. This can be done using available fault current calculations or by consulting electrical equipment manufacturers or utility companies. Make sure the interrupting rating of the selected circuit breaker exceeds the calculated fault current.

5. Verify Voltage Rating: Ensure that the circuit breaker voltage rating matches or exceeds the system voltage. Installing a circuit breaker with a lower voltage rating can lead to hazardous conditions.

6. Review Application Requirements: Consider any specific requirements or regulations applicable to the application. This may include industrial standards, local codes, or safety guidelines. Adjust the circuit breaker size accordingly.

By following these steps and considering the critical factors, you can accurately size circuit breakers for your electrical system, ensuring optimal performance and safe operations.


Properly sizing circuit breakers is vital for the safety, reliability, and efficiency of electrical systems. By understanding the various ratings, factors, and steps involved in sizing circuit breakers, you can make informed decisions to protect your equipment and prevent hazards. Remember to consider load currents, fault currents, system voltage, application requirements, and any environmental factors that may impact the circuit breaker's performance. By accurately sizing circuit breakers, you can ensure the smooth and safe operation of your electrical system.


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