Final answer:
The allowable ampacity of 13 - #12 THHN conductors in a raceway at 40° C, considering adjustment and correction factors from the NEC, is approximately 19.11 amperes per conductor.
Step-by-step explanation:
To determine the allowable ampacity of 13 - #12 THHN conductors in a raceway, we must consult the National Electrical Code (NEC) which provides ampacity tables that adjust for conductor size, insulation type, and ambient temperature. Since the question specifies that the ambient temperature is 40° C, we will need to apply a correction factor to adjust the standard ampacity for increased temperature. The standard ampacity for #12 THHN wire is 30 amperes in free air or in a raceway, based on Table 310.15(B)(16) of the 2017 NEC (this may vary in other editions of the NEC code, so always refer to the code applicable at the time of installation).
When more than three current-carrying conductors are bundled together in a raceway or cable, the NEC requires applying an adjustment factor from Table 310.15(B)(3)(a). For 13 conductors, this adjustment factor is 70%. Thus, 30 amperes times the adjustment factor of 0.70 equals 21 amperes. Furthermore, due to the increased ambient temperature of 40° C, another correction factor must be applied, which for THHN insulation is typically around 0.91 for 40° C from Table 310.15(B)(2)(a). Therefore, the adjusted ampacity is 21 amperes times 0.91, resulting in approximately 19.11 amperes. This is the allowable ampacity for each of the 13 conductors at these specific conditions.
In practice, it's always advised to consult the latest NEC code book and local amendments before making any final calculations, as this example serves for educational and illustrative purposes only.