Journeyman Electrician Practice Exam

What is the ampacity of 500 kcmil copper conductors with THWN insulation, given eight current-carrying conductors in a raceway at 125°F?

1. 150.2 amperes

2. 178.2 amperes

3. 200.0 amperes

4. 220.0 amperes

The correct answer is: 178.2 amperes

To find the ampacity of 500 kcmil copper conductors with THWN insulation, it's essential to refer to the National Electrical Code (NEC) Table 310.15(B)(16), which provides ampacity values based on various factors, including conductor size, material, insulation type, and condition of use. For copper conductors with THWN insulation, the base ampacity listed for 500 kcmil at an ambient temperature of 90°C (194°F) is typically around 380 amperes. However, in this situation, there are several important factors to consider that influence the final ampacity: 1. **Number of Conductors**: When multiple conductors are installed together, like the eight current-carrying conductors in this example, derating rules apply. According to NEC guidelines, you must derate the ampacity based on the number of conductors in the raceway. For eight conductors, the ampacity is typically reduced according to the derating factors outlined in the NEC. 2. **Temperature Adjustment**: The temperature rating of THWN insulation allows for operating conditions at higher temperatures, but the actual ampacity needs to be adjusted for the higher operating temperature of 125°F. This adjustment is