Upper incoming line, lower outgoing line, main circuit on the left, control circuit on the right, horizontal and vertical. The concealed laying is mostly through the pipe and hidden in the building wall or. Learn how to wire a distribution box step by step! This video shows real on-site footage of electrical installation, demonstrating safe and standardized wiring methods used by professionals. Follow this guide for a clear and safe connection process: Before starting, always ensure the main power is turned off to avoid electrical shock. It has three categories: residential, commercial and industrial electrical distribution boxes, all of which play important roles in their respective electrical.