A 600-volt, three-phase substation feeder has an established required ampacity of 1200 amperes. It uses four identical parallel sets of copper THWN-2 phase conductors, each set in a separate raceway with not more than three current-carrying conductors at 30°C ambient. The specified termination ampacity limit is 75°C; all parallel-installation conditions are satisfied, and no correction, adjustment or other condition requires a larger size. What is the minimum conductor size in each set under 2026 NEC Table 310.16?
- A250 kcmil
- B300 kcmil
- 350 kcmil
- D400 kcmil
Where to look it up
https://www.mikeholt.com/instructor2/img/product/pdf/1769017155.pdf
Why C is correct
Each of the four equal sets must provide at least 1200 / 4 = 300 A. At 75°C, 300 kcmil copper provides 285 A per set, or 1140 A total, and fails. The next size, 350 kcmil, provides 310 A per set, or 1240 A total, and meets the requirement. This answer depends on four properly installed parallel sets; a single 350 kcmil conductor does not carry 1200 A. Source: Mike Holt Enterprises, Understanding the 2026 National Electrical Code, Volume 1, public sample pages 366–367 (310.10(G)) and 379 (Table 310.16). https://www.mikeholt.com/instructor2/img/product/pdf/1769017155.pdf