What is the appropriate size for the main power supply line of the distribution box

The main power line of a distribution box is sized according to the panel's amperage rating, typically ranging from 100 A in residential panels up to 600 A or more in commercial installations.OverviewThe main power line, also called the service entrance conductor, connects the utility supply to the distribution panel or load center. Its size is determined by the panel's rated amperage, the voltage level, and the type of conductor insulation. For residential systems in the US, common main panel ratings are 100 A, 150 A, 200 A, or 400 A, while commercial panels can reach 600 A or higher .Residential ExampleA 100–200 A panel typically uses #4 AWG to 2/0 AWG copper or #2 AWG to 250 kcmil aluminum conductors, depending on the exact amperage and local electrical codes .Voltage is usually 120/240 V single-phase, with three wires entering the panel: two hot wires and one neutral.Commercial and Industrial ExampleThree-phase panels (common in commercial settings) operate at 208/120 V or 480/277 V.Main feeders can be larger conductors, often 250 kcmil to 600 kcmil copper or aluminum, to handle 400–600 A or more .The main line may be routed overhead or underground, connecting to a feeder breaker at the substation and then branching through the main busbar inside the panel .Key ConsiderationsAmperage Rating: The main line must safely carry the maximum load of the panel without overheating.Voltage Drop: Longer runs require larger conductors to minimize voltage drop.Material: Copper is more conductive than aluminum, so aluminum conductors are typically larger for the same amperage.Protection: The main breaker or fuse protects the conductor from overcurrent.SummaryThe size of the main power line in a distribution box is not fixed; it depends on the panel rating, voltage, and installation type. Residential panels usually use #4 AWG to 2/0 AWG copper, while commercial panels may require 250–600 kcmil conductors. Proper sizing ensures safe operation, compliance with electrical codes, and reliable power distribution .