An electrical sub panel, also known as a sub distribution board or sub circuit breaker panel, is a smaller secondary panel connected to the main electrical panel in a building. It serves as an extension of the main electrical panel to distribute power to different areas or circuits. Primary: The main distribution panel, supplies power from the transformer. Secondary: Intermediate panel, routes power to buildings or zones. A feeder usually begins with a feeder breaker at the distribution substation. At this. Let's make a hypothesis: a newly built residential area introduces a 10kV incoming line and builds a distribution room. The outgoing line from the low-voltage end of the transformer is 0. Each component plays a specific role.