I received a service call about a 'bad breaker'. I opened the panel cause I heard an arcing noise. Part of a Square D Neutral bar was beginning to melt. Long story short. There were 3 wires under the terminal screw, 2 Grounded Conductors and a Ground. The panel is the main. What and where does the NEC state that it has to be one wire per terminal in the panel, or is that part of the UL Listing? Also what is everyones feelings about more than one wire under a terminal.