Code Q&A

Jan. 10, 2008

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?

Mike Holt

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.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.