Corrected Ampacity = Table 310.16 Ampacity × Ambient Temperature Correction Factor. For a dry location, use the 90°C column for THHN. Ambient Temperature Correction Factor for 100°F = 0.91 for THHN.

Three years later, the 2014 NEC cleared up this confusion. It added the note at the bottom of Table 310.15(B)(3)(a) to make it clear that you count spare conductors for ampacity adjustment. By ...