Part II of Art. 210 provides the requirements for branch circuit ratings. The first requirement is that branch circuit conductors must have an ampacity at least as large as the load being served [291.1(A)(1)]. However, there's a twist on the ampacity requirements. It's called the 125% rule, and it applies to continuous loads. You have acontinuous load when "the maximum current is expected to continue for three hours or more” [Art. 100].
If a circuit has only noncontinuous loads, just add up the loads to get your total load (for ampacity). Before you add any continuous loads, you must multiply them by 125%. This rule applies to branch circuit overcurrent protection [210.20] as well.
The ampacity tables address only the minimum ampacities required for safeguarding personnel and property [90.1]. For operational efficiency, you also need to address voltage drop. You may address it in one of two ways: