1) Generally, the incoming line of power distribution box adopts five wire system, i. three phase lines a, B and C (generally yellow, green and red), one zero line (light blue) and one ground line (yellow with green stripes). However, the key to a safe and reliable system lies in proper installation. This guide helps you determine the correct dimensions based on wire fill capacity, device requirements, and installation environment, ensuring a safe and efficient electrical system. Home Blog Best Practices Electrical Box Dimensions: Standard Sizes, Types & Selec. Whether you are installing outlets, switches, lighting. The distribution box is the central hub of the home circuit and the general control of our daily power consumption.