NEC 240.4(D): Fourteen Is Fifteen, Twelve Is Twenty, Ten Is Thirty
Every electrician knows 12 gauge means a 20-amp breaker. The exam knows you know it, so it hands you Table 310.16 showing 12 AWG at 25 or 30 amps and waits for you to trust the table over the rule. Here is the rule, the three numbers, and the exception that flips it on motor and HVAC questions.
Last reviewed September 2026
The three numbers
- 14 AWG copper: 15-amp device maximum.
- 12 AWG copper: 20-amp device maximum, even though Table 310.16 lists 25 A at 75 C and 30 A at 90 C.
- 10 AWG copper: 30-amp device maximum, even though the table lists 35 A at 75 C and 40 A at 90 C.
- Aluminum and copper-clad aluminum: 12 AWG at 15 A, 10 AWG at 25 A.
Ampacity and device rating are two different things
Table 310.16 tells you what a conductor can carry under stated conditions. Section 240.4(D) tells you the largest device you may put in front of the three smallest common sizes. They are not in conflict; they answer different questions. Derating math for a hot attic or a bundled raceway starts from the table, including the 90 C column where the insulation allows it. The breaker you install still stops at the 240.4(D) number.
The exception the exam tests: 240.4(G)
Section 240.4(G) lists the circuits that follow their own article for overcurrent protection, and two of them show up constantly: motor circuits under Article 430 and air-conditioning and refrigeration equipment under Article 440. On those, the device is sized to start the load, not to the conductor, so a 10 AWG copper conductor on a 40-amp breaker is a correct condenser circuit and a correct motor circuit. On a receptacle circuit the same conductor stops at 30.
Worked example
A 10 AWG copper THHN conductor supplies receptacles in a small shop. The terminations are rated 75 C. What is the maximum overcurrent device?
- Table 310.16 at 75 C says 35 A. That is the ampacity.
- 240.4(D) caps the device for 10 AWG copper at 30 A.
- Receptacle circuit, so no 240.4(G) exception applies.
- Answer: 30 amps. The 35 in the choices is the trap.
Where candidates lose the point
- Sizing the breaker from the 75 C or 90 C column. Those are ampacities, not device limits.
- Applying 240.4(D) to a motor or condenser circuit. 240.4(G) sends those to Articles 430 and 440.
- Treating 240.4(D) as an ampacity for derating. Derating starts from Table 310.16, not from 15/20/30.
- Forgetting aluminum has its own pair: 12 AWG at 15 A, 10 AWG at 25 A.
- Reaching for the next-size-up rule in 240.4(B). It does not override the small-conductor ceiling.
Would you have taken the 35-amp bait?
The free 15-minute diagnostic pulls from the same overcurrent and conductor rules the exam tests hardest, and shows you exactly which ones you would miss.