16 sets limits on how many conductors and devices can be safely enclosed in a junction box. Overcrowding can lead to overheating, arcing, or even code violations. The National Electrical Code (NEC) governs electrical junction box rules. 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. One critical area is junction boxes, which protect electrical connections from fire hazards, moisture, and damage. This count includes each conductor. Nonmetallic boxes are marked with their cubic inch capacity.