NEC ReferenceNEC 2023 · 2026 · Updated May 2026

NEC Article 240: Overcurrent Protection & Breaker Sizes

NEC Article 240: Overcurrent Protection covers Standard Sizes through vs Circuit Breakers. This guide explains each section with practical interpretation, common mistakes, and how these rules apply to real projects.

240.6(A) Standard Sizes

NEC 240.6(A) defines the standard overcurrent device ratings: 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. You cannot use non-standard sizes. If your load calculation results in 55A, you must use a standard breaker size (50A in this case, since the conductor must be protected at or below its ampacity).

For circuits over 600A, additional standard sizes include 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.

240.4(B) Next Size Up Rule

When a conductor ampacity does not correspond to a standard overcurrent device rating, NEC 240.4(B) permits the next higher standard rating - with two conditions: the circuit must be 800A or less, and the overcurrent device must not supply multioutlet branch circuits providing power to cord-and-plug connected portable loads.

For example, 1 AWG THHN copper has an ampacity of 130A at 75C. Since 130A is not a standard breaker size, you can use a 150A breaker under this rule. However, you could not use this rule on a branch circuit feeding multiple receptacles for portable equipment.

240.4(D) Small Conductor Protection

NEC 240.4(D) sets absolute maximum overcurrent protection for small conductors: 14 AWG copper must not exceed 15A, 12 AWG must not exceed 20A, and 10 AWG must not exceed 30A. These limits override the next-size-up rule. Even though 12 AWG THHN has an ampacity of 25A at 75C, you cannot protect it with a 25A breaker on a branch circuit - the 20A limit in 240.4(D) controls.

This is the most commonly tested NEC provision on licensing exams and the most commonly misunderstood rule by homeowners wiring their own homes.

240.21 Tap Rules

A tap conductor connects to a circuit but is not protected at its full ampacity at the point of supply. The 10-foot tap rule per 240.21(B)(1) permits unprotected conductors up to 10 feet long if the tap conductor has ampacity not less than the rating of the device at the tap end and is enclosed in a raceway.

The 25-foot tap rule per 240.21(B)(2) extends this to 25 feet with additional requirements: the tap conductor must have ampacity of at least one-third of the overcurrent device protecting the feeder. Tap rules are most commonly used for connecting subpanels to feeders and for transformer secondary conductors.

Fuses vs Circuit Breakers

Both fuses and circuit breakers provide overcurrent protection, but they operate differently. Fuses contain a metal element that melts when current exceeds the rating, permanently opening the circuit. They are typically faster-acting and more precise than breakers, especially for high-fault-current applications where the interrupting rating matters.

Circuit breakers use either thermal, magnetic, or electronic trip mechanisms and can be reset after tripping. In residential work, circuit breakers dominate because they are reusable and provide a convenient disconnecting means. In industrial and commercial settings, fuses may be required where available fault current exceeds breaker interrupting ratings.

Key NEC Tables Referenced

Frequently Asked Questions

What does NEC Article 240 cover?

Article 240 covers overcurrent protection requirements. Key sections include 240.6(A) Standard Sizes and 240.4(B) Next Size Up Rule. It applies to all electrical installations in the United States.

Where can I find the full text of Article 240?

The full NEC is published by NFPA and available at nfpa.org. Many libraries provide free access. Your local building department also has copies available for reference during permit applications.

Does Article 240 apply to existing wiring?

The NEC applies to new installations, modifications, and additions. Existing wiring that was code-compliant when installed is generally grandfathered unless a renovation triggers an upgrade requirement. Check with your local AHJ (authority having jurisdiction).

When did the current Article 240 requirements take effect?

NEC 2026 is the latest published edition (effective September 2025). NEC 2023 remains the most widely adopted edition by states. Adoption varies - check your state adoption status on our state code pages.

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
This guide provides practical interpretation of NEC requirements. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction. WireRef is a reference tool and does not replace licensed professional judgment.

NEC references verified May 2026