Q. What is the rule for adjusting the ampacity of conductors based on the number of conductors in the raceway?
A. Where the number of current-carrying conductors in a raceway or cable exceeds three — or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in. — the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.
For example, if you wanted to know the ampacity of 10 THHN, where four current-carrying conductors are installed in a raceway or cable in a dry location, you would calculate it as follows:
Adjusted Ampacity = Table 310.16 Ampacity x Bundled Ampacity Adjustment Factor from Table 310.15(B)(2)(a)
Table 310.16 ampacity for 10 THHN is 40A at 90ºC. The bundled adjustment factor for four current-carrying conductors is 0.80.
Therefore, the adjusted ampacity would be:
Adjusted Ampacity = 40A x 0.80 = 32A
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.