Wire ampacity is capped by the lowest-rated termination

Use the ampacity column of the lowest-rated termination (75°C for most gear, 60°C for NM-B) — the 90°C column is only for derating.

For 6 AWG copper, NEC Table 310.16 lists three temperature columns: 90 degrees C is 75 A (insulation rating, derating only), 75 degrees C is 65 A (most breakers and lugs — the number to use), and 60 degrees C is 55 A (where NM-B Romex is capped). You use the column of the lowest-rated termination. Separately, 240.4(D) caps the breaker: 14 AWG to 15 A, 12 AWG to 20 A, 10 AWG to 30 A. — Source: NEC Table 310.16; 110.14(C); 240.4(D)

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For 6 AWG copper, NEC Table 310.16 lists three temperature columns: 90 degrees C is 75 A (insulation rating, derating only), 75 degrees C is 65 A (most breakers and lugs — the number to use), and 60 degrees C is 55 A (where NM-B Romex is capped). You use the column of the lowest-rated termination. Separately, 240.4(D) caps the breaker: 14 AWG to 15 A, 12 AWG to 20 A, 10 AWG to 30 A.

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