Whether conductors are supplying power to branch circuits, feeders or services, the ampacity must not be less than the load. The load can be an actual load, or it can be calculated in accordance with the requirements in Article 220 of the National Electrical Code (NEC).
Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC), Jim will help you solve it. Questions can be sent to firstname.lastname@example.org. Answers are based on the 2014 NEC. Type SE cable support