When considering what are the three types of wires used in distribution, we typically refer to conductors that handle phase, neutral, and ground connections within a system. However, this is not always the case for distribution switchboards. In addition to the current-carrying capacity, this choice is. Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill. These rules define when you must install a box, how large it must be, how you must install it, and how inspectors evaluate compliance.