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240.4(D)

Small Conductor Protection

Overcurrent ProtectionNEC 2023CEC Equivalent: CEC Rule 14-104

Even though 12 AWG THHN copper has a rated ampacity of 30 amps at 90 degrees C, the NEC does not let you put a 30-amp breaker on it in most applications. Section 240.4(D) overrides the ampacity tables and sets firm maximums: 14 AWG copper gets a 15-amp max, 12 AWG copper gets a 20-amp max, and 10 AWG copper gets a 30-amp max. For aluminum, it is 12 AWG at 15 amps and 10 AWG at 25 amps. These limits exist because small conductors are the most widely used wires in building construction and are the most likely to be abused, overloaded, or installed in conditions where they could overheat. The code takes a conservative approach to protect the countless miles of small wire running through walls and ceilings. There are a few narrow exceptions — air-conditioning equipment and certain motor circuits have their own rules — but for general-purpose wiring, these limits are absolute.

When You Need This

  • Wiring any residential branch circuit — receptacles, lighting, or small appliance circuits
  • Deciding what breaker to install for a new 12 AWG circuit in a commercial tenant space
  • Explaining to a homeowner why you cannot put a 20-amp breaker on their 14 AWG lighting circuit
  • Answering exam questions that test whether you know the small-conductor OCPD caps
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Key Points

114 AWG copper: maximum 15-amp overcurrent protection
212 AWG copper: maximum 20-amp overcurrent protection
310 AWG copper: maximum 30-amp overcurrent protection
412 AWG aluminum: maximum 15-amp overcurrent protection
510 AWG aluminum: maximum 25-amp overcurrent protection
6These limits override the next-size-up rule from 240.4(B)
7Exceptions exist for specific applications like motor circuits and A/C equipment where Articles 430 and 440 apply

Common Mistakes

Arguing that 14 AWG THHN is rated for 25 amps so it should get a 20-amp breaker — 240.4(D) says 15 amps maximum

Putting a 30-amp breaker on a 10 AWG circuit and then using it for receptacles — the wire allows 30 A, but the receptacle rating may not

Assuming aluminum small conductors have the same limits as copper — they do not (12 AWG aluminum is capped at 15 A, not 20 A)

Forgetting this rule entirely during an exam and using the raw ampacity tables to answer small-conductor questions

Exam Tip

This is a straight memorization item. Burn these into your brain: 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A. If the exam gives you a scenario with 12 AWG wire and asks for the maximum OCPD, the answer is 20 A — period — unless the question specifically involves motor or A/C equipment exceptions.

Frequently Asked Questions

The 30-amp value is the wire's thermal limit at 90 degrees C. But 240.4(D) sets a conservative overcurrent protection maximum to account for the real-world conditions small conductors encounter — connections, splices, and installation methods that could create hot spots. The 30-amp ampacity is still useful as a starting point for derating math.

Yes, but they are narrow. Motor circuits sized per Article 430, A/C equipment circuits per Article 440, and certain transformer secondary conductors have their own protection rules that may allow different OCPD sizing. For general-purpose branch circuits, 15 A is the absolute max for 14 AWG.

Inline Tools

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Related Code Sections

240.4Protection of Conductors240.6Standard Ampere Ratings310.16Ampacity of Insulated Conductors

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