What is the minimum size ungrounded THWN conductors required for a 480-volt, 3-phase feeder with a continuous load of 85 amperes?

In determining the minimum size of ungrounded THWN conductors for a specific load condition, it’s essential to consider both the current carrying capacity of the conductors and the continuous load requirements.

For a continuous load, the NEC (National Electrical Code) stipulates that conductors must be sized to carry 125% of the continuous load. In this scenario, with a continuous load of 85 amperes, the calculation would be:

85 amperes x 1.25 = 106.25 amperes.

Referring to the NEC ampacity tables for THWN conductors, you would look for the smallest conductor size that can handle at least 106.25 amperes. According to the tables, a 250 kcmil copper conductor is rated for 130 amperes at an ambient temperature of 30°C, which is more than sufficient to meet the requirement for a continuous load of 106.25 amperes.

Choosing a smaller gauge, such as 2/0, 3/0, or 4/0, would not adequately support the continuous load based on their respective ampacity limits. Therefore, the 250 kcmil conductor is the most suitable option for ensuring safety and compliance with the electrical code