Determine the number and type of conductors entering and exiting the box. Include all devices, such as switches, receptacles, or splices, that will be housed within the junction box. Selecting the appropriate junction box size prevents overcrowding, overheating, and potential hazards. Now, don't let “calculations” scare you. We'll break it down into manageable steps. It's more like simple arithmetic than rocket science, I promise. To safely determine how many wires are in a junction box, you must calculate the box fill capacity according to. The sizing junction boxes calculator works by evaluating the configuration of the electrical setup, including the number of raceways, conductor sizes, and types of pulls (straight, angle, or U pulls).