In the National Electrical Code, ampacity is a thermal limit: how much current a conductor can carry before its insulation runs hotter than its rating. NEC Table 310.16 tabulates allowable ampacities for copper and aluminum building wire from 14 AWG to 500 kcmil in three insulation columns — 60 °C (TW), 75 °C (THWN), and 90 °C (THHN/XHHW-2) — for not more than three current-carrying conductors in raceway, cable, or direct burial at 30 °C ambient. Ampacity is distinct from the conductor’s physical constants (area, diameter, DC resistance) in NEC Chapter 9, Table 8, which drive voltage-drop and fault calculations rather than heat limits.
The table value is a starting point, not a final answer. Ambient temperatures other than 30 °C are corrected by sqrt((Tc − Ta)/(Tc − 30)) per 310.15(B)(1); bundling more than three current-carrying conductors triggers the adjustment factors of 310.15(C)(1), from 0.8 for 4–6 conductors down to 0.35 for 41 or more; continuous loads must be sized at 125% per 210.19(A) and 215.2(A); and 110.14(C) caps the usable ampacity at the termination temperature rating, which is why the 90 °C column mostly serves as the starting point for derating.
Two practical wrinkles round out the picture. Aluminum of the same size carries roughly 20 to 25 percent less current than copper — 1/0 aluminum at 75 °C is 120 A versus 150 A for copper — so aluminum feeders typically run about two sizes larger. And for small conductors the breaker, not the table, governs: NEC 240.4(D) caps overcurrent protection at 15, 20, and 30 A for 14, 12, and 10 AWG copper regardless of the higher table values.