how to size a circuit breaker


Sizing a Circuit Breaker for Electrical Safety

Imagine a world without circuit breakers, where electrical systems ran amok, untamed and unprotected. It's undoubtedly a scary thought. Circuit breakers are an essential component in any electrical system, serving as the guardians of electrical safety. These devices are responsible for interrupting the flow of current when an overload or fault occurs, preventing potential hazards like fires, electrocution, and damage to appliances and equipment. To ensure that your circuit breaker is up to the task, it is crucial to size it appropriately for the specific electrical load it will be protecting. In this article, we will guide you through the process of sizing a circuit breaker, providing you with the knowledge to maintain a safe and efficient electrical system.

Understanding Circuit Breakers

Before diving into the intricacies of circuit breaker sizing, let's first gain a basic understanding of how these devices work. A circuit breaker is designed to protect electrical circuits from excessive current. It consists of a switch-like mechanism that automatically trips and interrupts the flow of electricity when it detects a fault or an overload. By doing so, the circuit breaker safeguards the circuit and its components.

Circuit breakers come in various types and sizes, catering to different electrical systems and loads. The size of a circuit breaker refers to its current rating, measured in amperes (A). The current rating indicates the maximum current the circuit breaker can safely carry without tripping. However, it is important to note that the circuit breaker's current rating does not determine the amount of electricity consumed; rather, it denotes the current-carrying capacity of the device.

Factors to Consider when Sizing a Circuit Breaker

Sizing a circuit breaker involves considering several factors to ensure it effectively and safely handles the electrical load. Here are the key considerations to keep in mind:

The Type of Circuit

Different types of circuits require different sizing approaches. The two main types of circuits are:

1. Branch Circuits

Branch circuits supply power to individual outlets, fixtures, or appliances. When sizing a circuit breaker for a branch circuit, the maximum load is typically determined by the rating of the circuit wiring, as specified by the National Electrical Code (NEC). The circuit breaker should have a rating equal to or slightly higher than the branch circuit's ampacity.

2. Feeder Circuits

Feeder circuits, on the other hand, distribute power from the main electrical panel to subpanels or larger loads. Unlike branch circuits, the NEC does not specify a maximum load for feeder circuits. Instead, the sizing of a circuit breaker for a feeder circuit is primarily determined by the load characteristics and electrical codes specific to the installation.

The Electrical Load

Determining the electrical load is a crucial step in sizing a circuit breaker. The load refers to the total amount of power consumed by the circuit, typically measured in watts (W) or volt-amperes (VA). It is essential to calculate the load accurately to select an appropriately sized circuit breaker.

1. Continuous Load vs. Non-continuous Load

Electrical loads can be classified as either continuous or non-continuous. Continuous loads are those expected to persist for three hours or more, while non-continuous loads are intermittent or short-term in nature. When sizing a circuit breaker, different rules apply to these two types of loads.

For continuous loads, the circuit breaker should be sized at 125% of the total continuous load. This accounts for the sustained demand placed on the circuit. Non-continuous loads, on the other hand, only require the circuit breaker to be sized at 100% of the total load since they do not place a sustained demand on the circuit.

2. Diversity Factor

The concept of diversity factor plays a crucial role in determining the circuit breaker size for interconnected loads. Diversity factor refers to the likelihood that all loads will operate simultaneously at their full capacity. Since it is uncommon for all loads to reach their maximum power consumption simultaneously, applying a diversity factor allows for a more realistic and efficient circuit breaker sizing.

The diversity factor is typically calculated as a percentage and is dependent on the specific situation. For instance, in residential applications, the diversity factor for general lighting and receptacle loads is often assumed to be 100% due to the low likelihood of full load operation. However, for commercial or industrial applications, a lower diversity factor may be considered due to the higher chance of simultaneous heavy load usage.

Short-Circuit Current Rating (SCCR)

In addition to considering the electrical load, it is essential to assess the short-circuit current rating (SCCR) when sizing a circuit breaker. The SCCR indicates the maximum level of current the circuit breaker can safely interrupt without suffering damage or compromising safety. To ensure proper sizing, the circuit breaker's SCCR should be equal to or greater than the available short-circuit current at the point of installation.

The calculation of SCCR involves understanding the available fault current and determining the appropriate interrupting rating for the circuit breaker. Factors such as the system configuration, source impedance, and protective devices within the circuit should be taken into account when evaluating the SCCR.

Temperature and Ambient Conditions

The operating temperature and ambient conditions surrounding the circuit breaker can influence its performance and lifespan. The temperature rating of a circuit breaker outlines the environmental conditions in which it can operate efficiently and safely. It is crucial to choose a circuit breaker with a temperature rating suitable for the installation environment to avoid premature failure or compromised operational integrity.

The Importance of Professional Assistance

Sizing a circuit breaker requires a thorough understanding of electrical systems, load calculations, and safety regulations. While this article provides a general guide, it is imperative to consult with a qualified electrician or electrical engineer to ensure accurate circuit breaker sizing for your specific application. Their expertise and knowledge will help navigate the complexities involved, ensuring the safety and efficiency of your electrical system.


Sizing a circuit breaker is a critical step in maintaining electrical safety and protecting your electrical system from potential hazards. By considering factors such as the type of circuit, electrical load, short-circuit current rating, and ambient conditions, you can select the appropriate circuit breaker for your specific application. Remember that accurately sizing a circuit breaker requires a deep understanding of electrical principles and regulations, so professional assistance is highly recommended. By taking the time and effort to size your circuit breaker correctly, you can ensure the safe and reliable operation of your electrical system for years to come.


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