Q. What is the maximum allowable ampacity of 12 AWG THHN insulated conductors installed in a strut-type channel raceway that is 1½ inches by 1½ inches? There are a total 13 conductors in the raceway — 12 of which are considered current carrying. The terminals at the overcurrent protective devices for these conductors are rated at 75°C. It is unknown what the terminal ratings are at the load end of the conductors. The actual loads supplied by these conductors are not known.
Explanation: This example did not meet the three requirements of 384.22 (i.e. the cross-sectional area of the strut-type channel raceway did not exceed four square inches). Therefore, the derating factors of Table 310.15(B)(2)(a) shall apply.
From Table 310.15(B)(2)(a), 10 to 20 current-carrying conductors require you to apply a de-rating factor of 50% to the 30A rated 12 AWG THHN conductors (30A x 0.50 = 15A). As noted in the question, the actual load is not known in this example. Section 110.14(C) does not come into play here because a check of the 60°C column (for the unmarked terminals) reveals a 12 AWG THHN conductor is rated 25A. Our calculation has determined the maximum allowable ampacity for this application to be 15A. Overcurrent protection for this example is set at 15A (the adjusted allowable ampacity).
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 firstname.lastname@example.org.