I was trying to replace a GFCI in a bathroom and the cable and wires are pretty black. The GFCI is also pretty burnt.

Do I need to replace the wiring and the GFCI or can I reuse the wire?

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    Sounds bad but we'd really need to see a picture to rate the level of damage to wires. Posting a picture is much easier if: First, if you haven't already, register your account by tying it to an email/password, or Google, or Facebook. Then, take a photo on your device of choice (login from there using the above credentials). – Harper - Reinstate Monica Dec 9 '20 at 18:07
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    Your best next step is to determine WHY this is happening. After that you will want to replace the wire that is burned since the insulation is now compromised. – jwh20 Dec 9 '20 at 18:20
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    Two words you never want to hear: burnt wire. Bad, bad, bad... Can we get a picture? – tnknepp Dec 9 '20 at 18:29
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    As others have said a photo is needed, it is possible the black is from arcing but we can not tell from just a description. Soot that wipes off is normally not a problem but melted wire that needs to be trimmed back and don’t use the push in connectors or back stabs use the screw terminals. Back stabs are known for failure. – Ed Beal Dec 9 '20 at 20:37
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    Can you please post photos of the situation inside the box in question? – ThreePhaseEel Dec 10 '20 at 0:43

Black is carbon; carbon is relatively conductive compared to clean vinyl or other insulator. So anything black or brown indicates a leak path. As a boy ,I miswired a 440 V box. The real electrician could not clean off the carbon well enough to use it and had to get some new equipment.

  • Big difference in 480v to 120v (even though the voltage to ground is only 277 on the 3 phase). the thermoplastic insulation on Thhn is usually easy to clean I have done it many times as a real licensed electrician. With residential nmb / Romex it is usually easier to clean lower voltage lower flash temps. – Ed Beal Dec 10 '20 at 23:50

Only way to make sure it is ok , is to insulation test the wiring with a megger.Agree with the other statements of arking/sparking being bad.

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    First most DIY folks have only heard of a megger and in this case the test is useless. Megging a conductor that the end is black in open air will pass every time, you could strip all the black off and have no insulation and pass with a megger in this case this is the reason for my downvote. A visual inspection is all that is needed. We need to provide assistance that is something a diy could do. An example of this is someone stating that a time domain reflectometer is needed to repair cable faults when a new piece of cable is only a few dollars. – Ed Beal Dec 10 '20 at 14:22
  • True , but if its arcing/sparking the current needs to travel somewhere? , whether its to neutral ,ground or another phase(unlikely).If cables are blackened we should find the cause of it , whether its caused by insulation failure(cut) or bridges in wiring or even incorrect sizing of the breaker that lead to over temp. – binaryOps20 Dec 10 '20 at 22:00
  • a loose connection is the most common cause for blackened wire in the US. Most commonly from backstabbed connections being used. I find excessive current with good connections in some cases melts the wire with out discoloration to a yellowing effect. – Ed Beal Dec 10 '20 at 23:35

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