What is the ampacity of each current-carrying conductor in a 120/208-volt system using two "A" phase conductors, two "B" phase conductors, and two neutrals with 10 AWG XHHW copper?

To arrive at the correct ampacity for 10 AWG XHHW copper conductors in a 120/208-volt system, it's essential to understand key factors influencing ampacity ratings. The National Electrical Code (NEC) provides guidelines in Table 310.16, which lists the ampacity for various sizes and types of conductors based on their insulation type and temperature ratings.

For 10 AWG XHHW (cross-linked high heat water-resistant) copper conductors, we typically refer to the appropriate thermal rating, which for XHHW is usually rated for 90°C. For this insulation type, the ampacity listed is typically 40 amps under standard conditions.

However, in practical applications, we often need to consider additional factors such as the number of conductors bundled together, any ambient temperature adjustments, or the specific use conditions that may require derating the ampacity. If there are six conductors present (as in this case), NEC Rule 310.15(B)(3)(a) would require derating.

At a minimum, if there is a total of six conductors, such as two conductors from phase A, two from phase B, and two neutrals, the ampacity may need to be