Under the small-conductor rule, what is the maximum standard overcurrent device permitted to protect 12 AWG copper conductors, regardless of the conductor's insulation temperature rating?
Not quite. The answer is B: 20 A
The small-conductor rule in 240.4(D) caps 12 AWG copper at a 20 A overcurrent device, even though the conductor's own ampacity reads higher at 75°C (25 A) and 90°C (30 A) in Table 310.16. The two figures answer different questions: ampacity is what the wire can carry, while the 240.4(D) limit is the largest breaker or fuse allowed to protect it. The same rule caps 14 AWG at 15 A and 10 AWG at 30 A.
Reference: NEC 2023 240.4(D) (Small Conductors — Overcurrent Protection Limits)
Checked against multiple published sources
A Mike Holt 2023 NEC calculations text lists 12 AWG copper at 20 A under 240.4(D)(6), and a Southwire spec sheet states the 15/20/30 A limits for 14/12/10 AWG copper under 240.4(D). The subsection digit comes from the Mike Holt text only, so the citation stays at 240.4(D). Neither source is the code text.