NEC Standard Circuit Breaker Sizes: A Complete Chart

Correctly identifying nec standard breaker sizes is a fundamental skill for any licensed electrician. These ratings, dictated by the National Electrical Code (NEC), are not arbitrary; they are the foundation of safe and reliable overcurrent protection. According to NEC Article 240, specifically section 240.6(A), the code lists a set of standard ampere ratings beginning at 15 A for fuses and inverse-time circuit breakers. The commonly used values in the table include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, and 600 amperes. Larger recognized equipment frame or switchgear ratings that are used for industrial switchgear and large breakers include 700 A, 800 A, 1000 A, 1200 A and higher (for example 1600 A, 2000 A, 2500 A, 3000 A, 4000 A, 5000 A, and 6000 A) depending on listed equipment and application. Understanding these standard breaker amperage ratings is the first step in proper nec circuit breaker sizing and ensuring compliance for all installations, from simple residential circuits to complex commercial feeders. These specific circuit breaker sizes form the basis for protecting conductors and equipment from dangerous overloads and short circuits when the chosen OCPD is compatible with the conductor ampacity, equipment listing, and the installation conditions required by the NEC.

Understanding NEC Article 240 and Overcurrent Protection

For any professional electrician, NEC Article 240 is the authoritative guide for overcurrent protection. Its primary purpose is to establish the rules for selecting and installing overcurrent protection devices (OCPD), such as circuit breakers and fuses. The core principle is simple: protect conductors from overheating due to excessive current, which could lead to insulation damage and fire hazards. This applies to both branch circuit protection, which safeguards individual circuits serving outlets and lighting, and feeder overcurrent protection, which protects the larger conductors supplying power to panelboards.

The code ensures that the OCPD is the weak link in the circuit—designed to trip or blow before the conductor’s conductor ampacity is exceeded. This is why adhering to the listed standard circuit breaker sizes nec regulations is not just about compliance, but about fundamental electrical safety.

The Official Chart: Standard Breaker Amperage Ratings per NEC 240.6(A)

When questions arise about approved nec breaker sizes, the definitive answer is found in NEC 240.6(A). This section provides the master list of the standard sizes of circuit breakers and fuses. It’s crucial to commit these values to memory or have them readily accessible on the job site. While there have been updates over the years, you can learn more about how the 2023 NEC changes standard circuit breaker amp ratings in our detailed course lessons.

The NEC 240.6(A) master list begins at 15 A and includes the common ratings shown below; larger frame and switchgear ratings are also recognized for listed equipment:

• Common standard ratings (typical branch and feeder sizes): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600
• Recognized larger equipment/switchgear ratings (listed devices, used for large switchboards and gear): 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000

The code provides allowances for specialized applications and listed devices beyond the main table. For example, some small, listed overcurrent devices or equipment connections used in limited or power-limited circuits may use smaller ratings; consult the code and equipment listing for those applications. For more detail on limited special permissions and classroom explanation, see how 10-amp branch circuits are permitted in the 2023 NEC.

What About Non-Standard Sizes? The “Next Size Up” Rule

A common scenario in the field involves a calculated load that doesn’t align perfectly with one of the nec standard breaker sizes. For this, electricians turn to NEC provisions such as 240.4(B) and related paragraphs. If the conductor ampacity doesn’t correspond to a standard OCPD size, the NEC allows use of the next higher standard size in certain circumstances, but specific limitations and conditions apply, including conductor protection provisions and limitations for various ampacity ranges and device types — consult NEC 240.4 and the equipment listing. Always confirm the exact clause applicable to your installation before applying the rule.

Key Factors in NEC Circuit Breaker Sizing Calculations

Properly performing a load calculation for breaker sizing is more complex than just matching a breaker to a wire. Several factors must be considered to ensure a safe and compliant installation. These breaker sizing calculations are a critical part of your job.

Continuous vs. Noncontinuous Loads

The NEC defines a continuous load as one where the maximum current is expected to continue for three hours or more. For these situations, specific rules for continuous load breaker sizing apply. The circuit breaker and the branch-circuit conductors must be sized to carry 125% of the continuous load (that is, multiply the continuous load by 1.25). For example, a 16 A continuous load requires circuits sized so that both the overcurrent device and the conductors serving that load are capable of carrying 20 A (16 A x 1.25 = 20 A). In practice you must verify conductor ampacity, termination temperature limitations, and applicable NEC provisions (see Article 220 and Article 310) before choosing the final breaker and conductor sizes.

Step-by-Step: A Basic Breaker Sizing Calculation

Let’s walk through a basic calculation for a continuous load, which is a common task in both residential and commercial work.

1. Determine the Load: Identify the total amperage of the equipment on the circuit. Let’s assume you have a fixed heating unit that draws 22 amperes and is considered a continuous load.
2. Calculate the Minimum OCPD Size: Apply the 125% rule for the continuous load. 22 A x 1.25 = 27.5 A. The circuit breaker and conductors must be sized so that both can carry this value.
3. Select the Standard Breaker Size: Refer to the list of standard breaker amperage ratings from NEC 240.6(A). Since 27.5 A is not a standard size, you must select the next higher standard size, which is a 30 A breaker.
4. Verify Conductor Protection: This is the most critical step. You must now select a conductor whose ampacity is equal to or greater than the rating of the OCPD (30 A in this case), according to the correct temperature column in NEC Table 310.16 and taking into account any ampacity adjustment or correction factors. Ensuring the correct wire size for breaker amperage is paramount for safety.

Perfecting these calculations and having NEC tables at your fingertips is essential. Get quick access to critical code info with our NEC reference guides.

Advanced Considerations for OCPDs

Beyond standard sizing, professional electricians must be aware of other critical OCPD characteristics and applications.

Ampere Interrupting Rating (AIC)

The ampere interrupting rating (AIC) (also called interrupting capacity) is the maximum fault current at a stated voltage that a breaker or fuse is identified to interrupt safely without failure; the AIC must be equal to or greater than the available fault current at the installation point. In commercial and industrial settings, calculating available fault current is a required step to ensure the selected OCPD has adequate interrupting capacity.

Specialized Breakers and Applications

A standard thermal-magnetic breaker is an inverse time circuit breaker, meaning the higher the overcurrent, the faster it trips. There are also many specialized applications, such as three-phase breaker sizing, which requires careful load balancing across phases. Furthermore, with the rise of distributed generation like solar PV, understanding how the 2023 NEC simplifies overcurrent protection in interconnected systems is becoming increasingly important for modern electrical work.

What About Standard Fuse Ampere Ratings?

It’s important to note that the list in NEC 240.6(A) also applies to fuses for the same standardized ampere ratings. The standard fuse ampere ratings generally align with the breaker ratings to provide a consistent framework for overcurrent protection. Always consult the code and the specific device listing for any device-specific differences.

Key Takeaways for Licensed Electricians

Navigating the requirements for OCPDs can be complex. Keep these key points in mind:

• The Source of Truth: NEC 240.6(A) is the definitive list for nec standard breaker sizes. Always refer to the current edition of NFPA 70 and the code text for the exact values and any clarifications.
• Calculations are Key: A proper load calculation for breaker sizing is non-negotiable. You must account for continuous and noncontinuous loads and follow the applicable articles of the NEC.
• The 125% Rule: For continuous loads, the OCPD and conductor must be sized for 125% of the load (and you must confirm conductor ampacity and terminal ratings).
• Protect the Conductor: The primary job of an OCPD is to protect the wire. Always verify that your chosen breaker does not exceed the conductor’s ampacity and that all installation conditions in Article 310 are met.
• AIC Matters: Don’t overlook the ampere interrupting rating (AIC). It is a critical safety rating, especially in commercial and industrial environments when available fault currents can be high.

Frequently Asked Questions (FAQ)

What are the nec standard breaker sizes according to the National Electrical Code?

The nec standard breaker sizes are listed in NEC 240.6(A). The common ratings start at 15 A and include the typical values up through 600 A for branch and feeder applications, with larger recognized equipment/switchgear frame ratings (700 A and above) used for listed switchgear and large breakers.

How do you perform nec circuit breaker sizing for a continuous load?

For nec circuit breaker sizing involving a continuous load (a load running for 3 hours or more), you must multiply the load’s amperage by 125% and then select the next standard-size device that meets the calculated requirement, while verifying conductor ampacity, terminal ratings, and other NEC requirements.

Where in the NEC can I find the standard sizes of circuit breakers?

You can find the official list of the standard sizes of circuit breakers in NEC Article 240, specifically in section NEC 240.6(A), titled “Standard Ampere Ratings.” This table is the primary reference for both fuses and inverse-time circuit breakers; listed equipment and manufacturer documentation are required for larger-frame devices.

Does wire size for breaker amperage always follow a simple rule?

No, the relationship between wire size for breaker amperage is governed by several factors beyond a simple chart. While general pairings exist (for example, 12 AWG copper commonly used with a 20 A breaker in typical residential branch circuits), you must always consider the conductor’s insulation type, terminal temperature ratings (60°C, 75°C, or 90°C where permitted), ambient temperature, number of current-carrying conductors in a raceway (ampacity adjustments), and any specific equipment termination limitations as detailed in NEC Article 310.