Direct Answer: The primary NEC codes governing junction boxes are found in Article 314, with key requirements including proper sizing (314. These rules define when you must install a box, how large it must be, how you must install it, and how inspectors evaluate compliance. This guide breaks down the actual rules inspectors check — with calculations and. This guide explains the key NEC junction box requirements, including box fill, splice rules, accessibility, grounding, outdoor use, common violations, and how to choose the right metal junction box for your application.