how to calculate circuit breaker size



Choosing the right circuit breaker size is crucial in ensuring the safety and optimal performance of your electrical circuits. Circuit breakers act as protective devices, preventing excessive current flow and possible damage to the electrical system. The size of a circuit breaker is evaluated based on several factors, including the maximum current the circuit can handle and the type of load it will be operating. This article will guide you through the process of calculating the appropriate circuit breaker size for your application, considering various aspects such as wire gauge, circuit length, and load types. Understanding these concepts will help you make informed decisions and maintain a safe electrical environment.

Factors to Consider when Calculating Circuit Breaker Size

Circuit breaker size calculation involves analyzing multiple factors to ensure the breaker can handle the electrical load. Let's explore the primary considerations to keep in mind when determining the appropriate circuit breaker size.

Ampacity of Conductors and Wire Gauge

The first factor to take into account is the ampacity of the conductor, which refers to its maximum current-carrying capacity. This rating is determined by factors such as the type of insulation and the size of the wire. National Electrical Code (NEC) tables provide ampacity ratings for different wire sizes and insulation types. To calculate the circuit breaker size based on ampacity, you will need to identify the wire gauge for your circuit, checking the relevant NEC tables for the corresponding allowable ampacity. It is crucial to select a circuit breaker with an amp rating equal to or higher than the ampacity rating of the conductor to avoid overloading the wire.

Considerations for Continuous and Non-Continuous Loads

Another crucial aspect to consider when sizing a circuit breaker is the type of load it will be powering. There are two main types of loads to consider: continuous and non-continuous loads.

Continuous loads are those that run for three hours or more, typically generating a consistent load level. Examples of continuous loads include electric heaters, ovens, and air conditioners. To calculate the circuit breaker size for continuous loads, multiply the continuous load current by 1.25 or consult NEC requirements for specific devices. This additional factor ensures the circuit breaker can handle the sustained load without tripping due to prolonged operation.

Non-continuous loads, on the other hand, operate intermittently, frequently drawing high currents for short durations. Common non-continuous loads include motors, refrigerators, and power tools. To calculate the circuit breaker size for non-continuous loads, you can usually follow the general guideline of selecting a breaker with a rating equal to the load current. However, it is recommended to check the equipment specifications and consult applicable codes to ensure accurate sizing.

Sizing Circuit Breakers for Motor Loads

Motor loads require special consideration due to their unique characteristics. When determining the circuit breaker size for motors, factors such as starting current, running current, and service factor come into play.

The starting current, also known as inrush current, is the peak current drawn by the motor when it is initially energized. This surge of current is significantly higher than the steady-state running current. While circuit breakers should withstand brief overload conditions, they may trip if the starting current exceeds their rating. Therefore, selecting a circuit breaker size that can handle the inrush current is crucial for motor applications.

To size the circuit breaker correctly, you need to calculate the Full Load Amps (FLA) of the motor, which corresponds to the running current. The NEC provides tables with standard FLA values for motors of different sizes and types. Once you have the FLA value, you can select a circuit breaker with a rating equal to or slightly higher than the FLA to account for the starting current without compromising safety.

Additionally, considering the service factor of the motor is necessary for accurate breaker sizing. The service factor represents the extra load a motor can handle above its rated capacity for short periods of time without sustaining permanent damage. By multiplying the FLA by the service factor, you can determine the effective current drawn by the motor and select a circuit breaker capable of handling it.

Sizing Circuit Breakers for Combination Loads

In certain applications, such as HVAC systems or panel boards, a combination of continuous and non-continuous loads is present. To calculate the circuit breaker size for such scenarios, it is essential to consider both the continuous and non-continuous load factors.

Start by determining the total continuous load current and applying the 1.25 multiplier for continuous loads, as mentioned earlier. Next, calculate the sum of the non-continuous load currents. Once both values are obtained, select a circuit breaker with a rating equal to or higher than the higher of the two currents.

Finding the Correct Wire Size

Determining the appropriate wire size is another crucial aspect of circuit breaker sizing, as the wire size must be compatible with the breaker rating. Undersized wires can overheat or lead to voltage drops, compromising the efficiency and safety of your electrical system.

When selecting the wire size, consider both the maximum current the circuit will carry and the acceptable voltage drop. Refer to the NEC tables to identify the appropriate wire gauge based on the breaker size and the length of the circuit. It is essential to choose a wire size that can handle the rated current without exceeding its ampacity rating and causing excessive voltage drop.


Choosing the correct circuit breaker size is paramount for the safe and efficient operation of your electrical circuits. By considering factors such as ampacity, load types, motor characteristics, and wire gauge, you can accurately determine the appropriate breaker size for each application. Remember to consult the NEC tables and specific equipment specifications for accurate calculations. Investing time and effort into proper circuit breaker sizing will ensure the protection of your electrical system and contribute to a reliable, hazard-free environment.


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