Final answer:
To determine the correct size of THW conductors, the current load is divided between the parallel conductors to find the amperage each one must carry. Using the NEC ampacity tables, it's found that a 1/0 conductor size would be adequate for a 143-ampere load per conductor.
Step-by-step explanation:
When determining the size of THW conductors required to meet the NEC requirements, we must consider the current that the conductors will carry and the temperature rating of the insulation. As the calculated load is 286 amperes, and considering the THW has a maximum temperature rating of 75°C, we can refer to the NEC ampacity tables to determine the correct conductor size.
To accommodate the 286 amperes, we need to size each phase conductor before they are paralleled. In this situation, the current for each phase conductor would be 286 amperes / 2 (since they are paralleled) = 143 amperes. Checking NEC Table 310.15(B)(16) would allow us to find the right size THW conductor that can carry at least 143 amperes.
For example, if a 1/0 conductor can carry up to 150 amperes at 75°C, it would be adequate for the load. Larger sizes like 2/0 or 3/0 could also be used if the ampacity is higher, but not necessary if 1/0 meets the requirement. Therefore, in this hypothetical scenario, the right size would be 1/0 THW conductors.