When correcting conductor ampacity for elevated ambient temperature, use the correction factor [310.15(B)(2)(a)] for THHN/THWN conductors, based on the: 90°C rating of the conductor in a dry location.

Step 1: Find amperage for eight current-carrying conductors per Table 310-16. No. 12 THHN cu. = 30A. Step 2: Find derating factor per Table B-310-11 in Appendix B. 16 conductors requires a 70% ...