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?
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?