Protection of Conductors

Overcurrent ProtectionNEC 2023CEC Equivalent: CEC Rule 14-104

At its core, 240.4 answers a basic question: how big can the breaker or fuse be for this wire? The fundamental answer is that the overcurrent device must not exceed the conductor's ampacity. But real-world sizing is not always a perfect match, so the code provides flexibility. If you have a conductor with an ampacity that falls between standard breaker sizes (say the wire is rated for 65 amps), subsection 240.4(B) lets you use the next standard size up (70 amps) for devices rated 800 amps or less. Above 800 amps, you must round down per 240.4(C) — the breaker must be equal to or less than the conductor ampacity. Subsection 240.4(D) sets hard limits for small conductors: no more than 15 amps for 14 AWG, 20 amps for 12 AWG, and 30 amps for 10 AWG. There are also exceptions for tap conductors and for situations where tripping the breaker would create a hazard, like shutting down a fire pump.

When You Need This

Selecting a breaker size for any new branch circuit or feeder installation
Confirming that existing wiring is properly protected after a panel swap or load increase
Sizing overcurrent protection for a tap conductor run — understanding the exceptions is critical
Justifying a next-size-up breaker selection to an inspector
Taking an electrical licensing exam — 240.4 questions show up consistently

Key Points

1General rule: overcurrent device must not exceed the conductor ampacity

2240.4(B): For devices 800 A or less, you may use the next standard size up if the ampacity falls between standard sizes

3240.4(C): For devices over 800 A, the conductor ampacity must be equal to or greater than the device rating — no rounding up

4240.4(D): Small conductors have fixed maximum OCPD limits — 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A

5240.4(A): Overload protection is not required where tripping would create a hazard (like fire pumps), but short-circuit protection is still required

6Tap conductors have special rules under 240.4(E) and Sections 240.21(B) and (C)

Common Mistakes

Rounding up to the next standard breaker size on circuits over 800 amps — above 800 A you must round down

Putting a 20-amp breaker on 14 AWG wire — 240.4(D) caps 14 AWG at 15 amps regardless of the wire's listed ampacity

Assuming the next-size-up rule always applies to small conductors — 240.4(D) overrides 240.4(B) for 14, 12, and 10 AWG

Forgetting about tap conductor rules and protecting a short tap run with an oversized device from the upstream circuit

Exam Tip

Memorize the three small-conductor limits from 240.4(D): 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A. Then remember that for everything else 800 A or less, you can round up to the next standard size. Over 800 A, you round down.

Frequently Asked Questions

The maximum OCPD for 12 AWG copper is capped at 20 amps by 240.4(D), regardless of the insulation temperature rating. The 30-amp 90-degree ampacity is useful for derating calculations but not for setting the breaker size.

Under 240.4(B), if a conductor has an ampacity that falls between standard OCPD sizes listed in 240.6(A), you can use the next higher standard size — but only for devices rated 800 A or less and only when the conductor is not part of a multi-outlet branch circuit supplying receptacles for cord-and-plug connected loads.

Inline Tools

Circuit Breaker Calculator

Find the correct OCPD size for your circuit

Related Code Sections

240.4(D)Small Conductor Protection 240.6Standard Ampere Ratings 310.16Ampacity of Insulated Conductors

This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.