NM-B cable vs THHN in conduit — same copper, different usable ampacity
Same 12 AWG copper: 20 A usable in NM-B (334.80), 25 A as THHN at 75°C terminations — but 240.4(D) caps the breaker at 20 A either way. The THHN headroom is for derating, not bigger breakers.
What this diagram shows
Comparison of the two common residential wiring methods for 12 AWG copper. NM-B cable (Romex) carries 90-degree insulated conductors but its usable ampacity is capped at the 60-degree column — 20 amps — by NEC 334.80. The same 12 AWG copper as THHN in conduit with 75-degree terminations is usable at 25 amps. Either way, NEC 240.4(D) caps the breaker for 12 AWG copper at 20 amps; the extra THHN headroom matters for derating in hot or bundled runs, not for a bigger breaker.
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