Learn what the NEC requires for junction boxes, from box fill calculations and grounding to outdoor use and fire-rated wall installations. These rules define when you must install a box, how large it must be, how you must install it, and how inspectors evaluate compliance. Electrical safety is non-negotiable, and the National Electrical Code (NEC) sets the gold standard for safe installations in the U. One critical area is junction boxes, which protect electrical. NEC 314.