The service entrance diagram refers to the layout and configuration of the wiring system used for this purpose. Part (1) of Section 370-16 (a) describes in detail the method of counting wires, as well as clamps, fittings, or devices (i. What happens if you put too many wires in a junction box? What does box fill mean? What is the easiest way to check wire capacity? What should you do if you are unsure about box fill? Wires in the junction box depend on the box size, wire gauge, and code rules. For example, a 4×4 inch box often. Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill. It takes the incoming power and safely distributes it to different circuits throughout your building.