Code reference · NEC Table 310.16; 110.14(C); 310.15; 240.4(B); 240.6(A)
8 AWG copper wire ampacity (NEC 310.16)
Under NEC Table 310.16 (up to three current-carrying conductors in raceway, 30°C ambient), 8 AWG copper is rated 40 A with 60°C insulation (TW), 50 A at 75°C (THWN), and 55 A at 90°C (THHN/XHHW-2). For most fixed wiring the 75°C column governs, because breaker and lug terminations are rarely rated above 75°C (NEC 110.14(C)).
Breaker pairing: with 75°C terminations the conductor is good for 50 A, which matches the standard 50 A device rating exactly (240.6(A)). That assumes the calculated load itself doesn't exceed the conductor ampacity.
Real installs derate. At 45°C ambient (attic run) the engine-computed usable ampacity is 47.9 A (90°C base 55 A × 0.87 ambient correction, then capped at the 50 A termination limit where applicable — NEC 310.15(B)(1), 110.14(C)). Bundle six current-carrying conductors in one raceway at 30°C and it becomes 44 A (× 0.8 per 310.15(C)(1)).
Run the numbers yourself
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More references: 14 AWG copper wire ampacity (NEC 310.16) · 12 AWG copper wire ampacity (NEC 310.16) · 10 AWG copper wire ampacity (NEC 310.16) · 6 AWG copper wire ampacity (NEC 310.16) · 4 AWG copper wire ampacity (NEC 310.16) · 3 AWG copper wire ampacity (NEC 310.16) · all 97 references