At the bottom are some photos to help aid in my description.
I have 3 circuits leaving my main panel, traveling ~5 feet behind drywall in NB-B cable to a surface mounted junction box where the NM-B splices to THHN wires. The THHN runs through EMT.
- The first circuit (110V 20A) ends at a receptacle
- The raceway continues with the other two circuits where the second circuit (220V 15A) ends at a receptacle
- The raceway continues until the third circuit ends at a third receptacle.
I just finished wiring all of this and things were working well. I flipped the breakers to kill power so I could safely put the cover plates on. I switched on the two 220 circuits and all was well, but when I on the 110 there was a loud pop on its breaker.
I opened the first junction box (where the NM-B splices to THHN) to find the hot wire had rubbed against the screw tab on the box, cutting the insulation and exposing a small bit of copper causing a ground fault (pictured). The neutral wire is burned (pictured)... At the receptacle end, all appears normal.
The other two circuits seem unaffected - other than one wire nut appears burned (pictured), but I think this was just because it was touching the burned neutral wire.
The junction box was pretty full, but was within box fill calculations. My ground was connected properly.
I am curious where to go from here? It seems like I should at least fully replace the THHN of the 110 circuit? Or can I just cut out the burned area? I likely should replace the NM-B in the wall? Do I need to replace any portion of the other circuits? Why was there such a loud pop and burned neutral - I would have guessed the short would have traveled down on my ground and tripped the breaker pretty uneventfully?