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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? Area of exposed wire Simple circuit diagram Burned neutral wires Close-up of other area of burned neutral wire that isn't as obvious in previous pic

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I think the neutrals just have some soot from the flash on them. The damaged part of the black wire should be cut off and the wire reconnected to the other black. You might have damaged it when you screwed the cover on, screwed the screw into it. Direct shorts on a breaker are never uneventful and pops are common. Examine the wires closely for any damage but I think they'll be OK. The breakers are meant to protect the wiring.

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  • thanks for the reply. The wire was damaged from the screw tab on the box. After rubbing my finger against it, I can't believe how sharp the edge is. Its the top left tab - you can see where the arc was on the 3rd and 4th pics. Because of the loud pop, I figured the breaker was fried. Also, when the breaker flipped, it only went about half way to off - enough to kill power, but not all the way over - this also made me think the breaker was bad. But after your comment, I went and tested it. All still works as normal. You think I don't need to change anything on the neutral wires? – dave_karl Apr 25 '20 at 16:30
  • I think your neutrals look OK. FYI, when a breaker trips, it only goes half way to off. You have to then go all the way to off, then to on. – JACK Apr 25 '20 at 17:07

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