Primary distribution box: three-phase power supply, ground wire and zero wire are introduced from the transformer. Level III distribution box: control cabinet of electrical. The terms primary, secondary, and tertiary distribution boxes are relative. From the transformer's low-voltage side (0. A feeder usually begins with a feeder breaker at the distribution substation. These boxes feature bottom entry and exit cables, front-opening doors, and main busbars connected with copper strips for optimal contact. 4kV to. Understanding the fundamental distinction between Primary and Secondary distribution in electrical systems is pivotal for designing efficient and reliable electrical distribution systems tailored to specific needs across various domains.