Suppose you have an outbuilding that doesn’t have its own service. Instead, it’s served by a branch circuit or feeder (on the load side of a service disconnect). This is a common arrangement. For example, an industrial facility may have a guard shack or pump house supplied this way.
Outside Branch Circuits and Feeders, Part 5
One of the rules for this arrangement is only one branch circuit or feeder can supply that building [225.30].