update for quick response:
I am fixing the attic outlet right now. The 2nd wire has the same darkish look see https://imageshack.com/a/img924/2614/9a4Xj4.jpg (click zoom to enlarge it). Notice it is only the left wire that is darkish while the right wire is ok. Previously the old owner may either have overloaded or short the wires because it's cut in the middle of the feeder at the slab so only the outlet is working and the rest are disconnected. Now if I can still use it.. should I sand paper it or would the darkish cooper still have full conduction when connecting an outlet to it?
original post
I found one of my wires going into one room has this corroded look (it's just example in the net similar to my actual wire). Was it caused by overcurrent? But overcurrent should burn the wires and not leave it intact, isn't it?
Since I can't insert new wires into 10 meters length to the room with many unknown bends. How many amperage in percentage can it still take compared to original one (it's AWG 10 with 60C rating of 30A)? How many ampere before the insulation of the following totally break down?
Best Answer
What you call corrosion is oxidization this won't hurt the ampacity of the wire, I would want to inspect the system because this is usually caused from being overloaded or getting wet being the causes I have found in the past, a loose connection can also cause over heating but in those cases there is usually some pitting from arcing at the contact point but I don't see any evidence of that. If the insulation is still in good shape the wire should be fine at its listed capacity.
Added to cover ok comment / question: I have a thermal camera that I check panels with, when I find "Hot" temp wise wiring it is usually below 90c. I have seen number 10 wiring loaded well beyond the standard limit for residential in motor controll centers closer to 60 amps with no insulation damage on thhn. When the wire gets hot enough to deform the insulation it is hotter than 90c. At first with a severe overload the wire heats up the insulation starts to smoke and melt but it usually won't catch fire unless there is a splice and at the splice it may get hot enough to catch fire but the wire is "glowing hot" . When wiring gets this hot it usually shorts in the cable hot to neutral or ground at a bend or this is what I have found on several occasions. When the wiring is overloaded severely I think the Inter action with the thermo plastic accelerates the oxide layer as I have seen equipment that was close to 30 years old with bright shiny commer a inch or 2 back under the insulation. High humidity will also cause the oxide, I have seen this in multiple areas and work environments. I just re read your original post and it would be helpful if we could see the condition of the insulation (I was thinking that was a photo of your wire). If the insulation is deformed at all it should be replaced as a best practice, since you don't want to replace it it should be checked with a megger (high voltage resistance meter) at 500v checking between hot and ground, hot and neutral, neutral to ground (both ends disconnected) and the reading should be in the gig ohm range. You don't want to find out the hard way that your neutral to ground has shorted and someone touches the metal frame of a lamp, appliance whatever and gets a bad shock, or worse.