Q. Section 110.14(C) requires the use of the 60°C column for 100A or less and the 75°C column for above 100A for conductor sizing. What is the value of THHN rated 90°C if the ampacity cannot be used for conductor sizing?

A. The advantage of 90°C conductor insulation only appears when the conductor ampacity must be adjusted because of conductor bundling or elevated ambient temperatures as per 310.15. When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].

For example: What is the adjusted ampacity of nine current carrying 12 AWG THHN conductors in a raceway?

Per Table 310.16, the ampacity of 12 AWG THHN is 30A
Adjustment factor [Table 310.15(B)(2)(a)] = 0.70
Adjusted ampacity = 30A x 0.70 = 21A
Therefore, this conductor is still suitable to be used on a 20A circuit.