how to size a circuit breaker for a motor


How to Size a Circuit Breaker for a Motor


When it comes to ensuring the smooth operation of motors, one crucial factor to consider is choosing the right circuit breaker size. Circuit breakers act as a safety mechanism, protecting electrical circuits and the motors they power from excessive currents. Sizing the circuit breaker accurately is vital to prevent overheating, electrical damage, and potential hazards. In this article, we will explore the key considerations and steps involved in determining the appropriate circuit breaker size for a motor.

Factors to Consider When Sizing a Circuit Breaker:

1. Motor Type and Design: The type and design of the motor play a vital role in determining the appropriate circuit breaker size. Different motor types, such as induction motors, synchronous motors, and direct current (DC) motors, have varying starting and operational characteristics. Understanding the motor's specifications and requirements is crucial in the sizing process.

2. Motor Full Load Current (FLC): The Full Load Current (FLC) is the maximum current required by the motor during normal operation at full load. It is essential to have accurate FLC information to size the circuit breaker correctly. This information can usually be found in the motor's documentation or specification sheets. If not readily available, it may require measuring the motor's current under full load conditions using a clamp meter.

3. Starting Current: During motor startup, the current drawn by the motor can be significantly higher than the Full Load Current. This starting current is known as the inrush current and plays a crucial role in sizing the circuit breaker. The inrush current depends on factors such as the motor type, voltage, load, and starting method. It is typically several times higher than the Full Load Current and must be considered when determining the circuit breaker size.

4. Ambient Temperature: The ambient temperature, or the temperature of the surrounding environment, affects the motor's performance and, consequently, the circuit breaker's sizing. Motors operating in high-temperature environments will require larger circuit breakers to compensate for the reduced heat dissipation. It is important to factor in the ambient temperature as specified by the motor's manufacturer or governing standards when selecting the circuit breaker.

5. Circuit Breaker Type and Characteristics: The type of circuit breaker chosen will impact its sizing requirements. There are different types of circuit breakers, such as thermal-magnetic, magnetic only, and electronic trip units. Each type has its own characteristics and performance limitations. Understanding the specific type of circuit breaker and its trip characteristics is necessary to ensure it can adequately protect the motor.

The Sizing Process:

Sizing a circuit breaker for a motor involves several steps to ensure optimal performance and safety. Following a systematic approach, as outlined below, will help in making an informed decision regarding the circuit breaker size:

Step 1: Gather Motor Information: Begin by collecting all relevant information about the motor. This includes the motor type, design, Full Load Current (FLC), starting method, ambient temperature, and any additional specifications provided by the motor's manufacturer. Having accurate and up-to-date information is crucial for a successful sizing process.

Step 2: Determine Starting Current: Once the motor information is available, determine the starting current. The starting current is typically given as a factor multiplied by the Full Load Current (FLC). The multiplication factor depends on the motor type, voltage, and starting method. It is essential to refer to motor manufacturer documentation or relevant standards for accurate starting current values.

Step 3: Account for Ambient Temperature: Adjust the starting current calculated in the previous step based on the ambient temperature. Higher ambient temperatures reduce the motor's ability to dissipate heat, leading to a higher starting current. Refer to manufacturer guidelines or governing standards to determine the appropriate correction factor for the ambient temperature.

Step 4: Calculate the Required Circuit Breaker Size: To calculate the circuit breaker size, add the Full Load Current (FLC) and the adjusted starting current. The resulting value represents the minimum size of the circuit breaker required. Select the nearest standard circuit breaker size equal to or larger than the calculated value. It is essential to choose a circuit breaker with a suitable ampere rating to prevent nuisance tripping and ensure proper motor protection.

Step 5: Consult Regulations and Standards: It is crucial to consult local regulations and electrical standards applicable to your location when sizing a circuit breaker for a motor. These regulations may provide specific guidelines or requirements that must be followed to ensure compliance and safety.


Choosing the correct circuit breaker size for a motor is vital in ensuring its reliable and safe operation. Various factors, such as motor type and design, Full Load Current (FLC), starting current, ambient temperature, and circuit breaker characteristics, must be taken into account during the sizing process. By following a systematic approach and considering all relevant information, one can select the appropriate circuit breaker size that provides optimal protection and performance for the motor. Remember to consult motor manufacturer documentation, relevant standards, and local regulations to ensure compliance with safety requirements. Properly sizing the circuit breaker will help protect the motor from excessive currents and enhance its longevity and efficiency.


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