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The requirements for sizing conductors of 480V, 277V, 298V, 120V branch circuits are based on the idea that you are sizing conductors to carry the load without overheating. In fact, the first sentence in Section 210.19 is:
“Branch-circuit conductors shall have an ampacity not less than the maximum load to be served.”
This seems like plain old common sense, but actually doing it can be tricky. That’s why Section 210.19 provides some rules on how to get there.
Note that there’s a difference between sizing conductors to meet NEC requirements (so they won’t melt) and sizing them for “optimum performance” (an engineering decision that usually means exceeding NEC minimums).
The difference between “optimum” and “maximum” performance is simple. If you wanted maximum performance, all of your conductors would be the biggest ones available. “Optimum” balances cost and performance, to arrive at a conductor size that provides a reasonable return on the additional investment (e.g., larger terminations, larger raceway, installation time, etc.).
The steps for determining ampacity for the conductors of a given circuit are [210.198(A)(1)]:
Source: Mark Lamendola | Mindconnection