A feeder circuit typically supplies power to several branch circuits, each serving its own motor. Each branch circuit has its own protective devices, contacts, overload relays, disconnect devices, etc. How should you size the feeder conductors?
Each branch circuit must have an ampacity equal to 125% of the full-load current for the motor in that circuit. The feeder ampacity must be large enough to feed all motors operating simultaneously, and be capable of starting the largest motor with all of the others running fully loaded at the same time. This requires an ampacity equal to the sum of all full-load currents (taken from Table 430-148 through 430-l50, etc.) plus one-fourth (25%) of the current for the largest motor on that feeder. It is not necessary that the feeder ampacity be equal to the sum of all branch circuit ampacities. This is because only one motor will probably be started at a time.
If it is known that some of these motors on the feeder would never be running at the same time as the others, it is permissible to factor this noncoincident diversity into the design. However, this is usually not done.
