Q. What is the allowable ampacity of any one of the current-carrying 12 AWG THHN insulated conductors installed in a cellular concrete floor raceway with 29 other current-carrying 12 AWG THHN conductors and five equipment grounding conductors? Disregard the terminals in your calculations.

  1. 30A
  2. 20A
  3. 13.5A
  4. 11.25A

Answer: C

As per Table 310.16, 12 THHN is a 30A conductor at 30°C (86°F). As per 310.15(B)(2)(a) and Table 310.15(B)(2)(a), the adjustment factor for 30 current-carrying conductors is 45%. Therefore, the calculation is as follows: 30A x 0.45 = 13.5A.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.