Final answer:
To calculate the ampacity of each 250 kcmil THWN copper conductor in an area with an ambient temperature of 45°C, refer to the NEC Table 310.15(B)(16) for base ampacity and apply the temperature correction factor from NEC Table 310.15(B)(2)(a). The base ampacity is then multiplied by the correction factor to get the derated ampacity.
Step-by-step explanation:
The question is asking to calculate the ampacity of each of the 250 kcmil copper conductors installed in an uncovered ladder cable tray in an area with an ambient temperature of 45°C. To find the ampacity, one must refer to the National Electrical Code (NEC) Table 310.15(B)(16) for the base ampacity of 250 kcmil THWN copper conductors, and then apply correction factors for the increased ambient temperature as per NEC Table 310.15(B)(2)(a).
First, the base ampacity of a single 250 kcmil THWN copper conductor is derived from NEC Table 310.15(B)(16). We then use NEC Table 310.15(B)(2)(a) to find the appropriate temperature correction factor for 45°C. Since THWN insulation is typically rated for 90°C, you would use this column to determine the correction factor. Lastly, you multiply the base ampacity by the correction factor to get the derated ampacity for each conductor.
Since the exact values from the NEC tables are not provided in the question, a specific numerical answer cannot be provided here. The use of these tables and applying the correction factors as per the local electrical code is crucial for this calculation. However, it's important to note that when conductors are installed in groups, as in this case with three 3-conductor cables in a single layer, additional derating due to mutual heating might also apply.