We recently covered branch circuit requirements (Art. 210). Feeder requirements (Art. 215) are similar, but simpler. Why is Art. 210 five times as long as Art. 215? The devil is in the details, and branch circuits have more of them.
Branch-circuits have much load variability to account for, which is why the load calculations for sizing branch circuit conductors and overcurrent protection devices can be tedious. But the load on a feeder is utilization equipment (see Art. 100 definition) and a transformer, or just a transformer [215(B)(1)].
An example helps illustrate the difference in complexity between feeder calculations and branch circuit requirements. Consider a typical industrial configuration. A panel at the service has (in this example) several breakers ranging from 400A to 1,200A. Feeders from each one distribute 480V power to step-down transformers in the plant. Suppose one transformer supplies 120/208V to a branch circuit panel. It's this transformer (sized for that panel) that constitutes the load on that feeder.
Article 220 provides the calculation requirements for branch circuits and feeders. Before doing calculations, apply the requirements of Art. 210 and Art. 215. When you apply Art. 215, design for system performance rather than mere code compliance. For example, different feeders can share a neutral [215.4(A)]. But can that neutral actually handle the unbalanced current that will be imposed upon it?