Is the load continuous?
A load that runs for 3 hours or more (an EV charger, an electric water heater) is continuous, and conductors and the breaker must be sized to 125% of the load. A 50 A continuous load therefore needs conductors good for 62.5 A, which is #6 copper (65 A at 75 °C). A 50 A non-continuous load (an electric range, sized separately under its own demand rules) can use #8 copper at its 50 A ampacity.
The terminal rating caps you
Even with 90 °C wire like THHN/THWN-2, you almost always size to the 75 °C column, because breakers and lugs are rated 75 °C. So the 90 °C rating helps you with derating, but the number you compare against the load is the 75 °C ampacity: #8 Cu = 50 A, #6 Cu = 65 A.
Long runs: check voltage drop
Ampacity keeps the wire from overheating; voltage drop keeps the load working right. Over distance, a code-minimum conductor can drop too much:
- #8 Cu, 50 A, 240 V at 150 ft drops about 4.9%, past the 3% guideline.
- Bumping to #6 Cu brings that to roughly 3.1%; #4 Cu clears 3% comfortably.
The 3% / 5% figures are NEC informational notes, not hard requirements. But long feeders and EV chargers are exactly where they matter.