Code reference · NEC 240.4(D); Table 310.16
NEC 240.4(D): the small-conductor overcurrent caps
240.4(D) caps overcurrent protection for small copper conductors regardless of their table ampacity: 14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A (aluminum: 12 AWG → 15 A, 10 AWG → 25 A).
That's why 12 AWG THHN, whose 90°C table value is 30 A, still lives on a 20 A breaker. The higher table columns exist for derating headroom, not bigger breakers.
The same functional caps exist in the Canadian code (CEC Rule 14-104(2)) — the two codes agree here.
Run the numbers yourself
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