NEC Table 310.16 Ampacity Chart

How to read Table 310.16

Find your conductor material and size, then read the column that matches the insulation temperature rating for the location, wet or dry. THHN, the everyday raceway conductor, reads from the 90°C column. The table assumes not more than three current-carrying conductors in a raceway or cable and a 30°C (86°F) ambient. Change either condition and the printed number no longer applies as-is.

The column you can actually use

The insulation column and the column you may load are different questions. Terminations govern. Unless the equipment is marked otherwise, circuits rated 100 A or less are sized from the 60°C column, and larger circuits from the 75°C column · 110.14(C). The 90°C column still earns its keep: it is the starting point for derating, so a THHN conductor survives correction factors that would sink a TW conductor of the same size.

Corrections that change the number

For ambient temperatures other than 30°C, multiply by the correction factor · 310.15(B)(1). For more than three current-carrying conductors in a raceway or cable, apply the adjustment factor · 310.15(C)(1). After all of that, small conductors hit a hard stop: 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper, with the aluminum caps shown above · 240.4(D). The free wire size calculator runs the whole chain and shows which limit governed.