This afternoon, my wife and I noticed a peculiar smell coming from the garage. We localized it to a faulty 3-wire 240 V outlet. Something got really hot resulting in part of the rubber melting on the socket:


Upon further examination, it looks like the socket itself has issues. Here's a view of it with the cover on:

Socket With Cover

Here's a view with the cover off (and yes, it's completely powered off at the breaker box :D)

black/corrosive contacts?

Obviously the dust is a concern. I plan to clean that out. What concerns me more though is that the black wire appears to be frayed and turned black. Like it was overheating perhaps?

Here's another view:

Another view with wire off

I'm prepared to hire an electrician to come out but I'd like to know more about what's going on here so I don't get swindled and spend exactly one billion dollars fixing something that isn't related.

If it helps to clarify, the house is about 50 years old and I'm pretty sure the original romex is still going to this socket.


  • What could cause this issue?
  • What are the next steps for troubleshooting/repairing?
  • Would the entire wiring need to be replaced or could this just be a faulty socket?
  • 1
    You should also replace the damaged device (dryer?) cord.
    – Tester101
    Feb 17 '12 at 11:51

I'm not in the US, so I used Google to deduce which prong is doing what in this outlet. I'll assume that the left and the right wires are phase wires and the middle wire is the ground.

Note that although the left wire looks melted the right one looks just fine although they usually carry equal currents. So this is not melting due to overload. The most likely scenario is that the left terminal got rusty and its surface resistance raised and so it overheated and this causes minor melting on the left wire.

The signs of burn on the outlet face are quite typical and likely don't indicate a wiring problem. Outlet contacts tend to get loose over time and once that happens they no longer maintain tight contact with the plug contacts. Also when a high load is disconnected an arc can start and this makes the contacts burn and cover with carbon and further increases the resistance. This is more likely to lower quality outlets, but happens just about everywhere.

So the bottom line is the outlet has just worn out and needs careful replacement. Remove the old outlet, clean the exposed wires, carefully connect the new outlet making sure all screws on the terminals are tight. You might want to test the new outlet - plug some high load and let it run for an hour, then trip the breaker, open the outlet cover and check whether there're any signs of warming up.

  • Thanks @sharptooth I followed your advice and it's working great. Plus, I learned a lot in the process.
    – Mike B
    Feb 28 '12 at 23:31
  • 2
    Just a note; exactly how the power from a 220V outlet is used is up to the appliance. A heavy-duty power tool like a table saw will combine the two phases to get 220V, but your clothes dryer will run the heater on one 110V phase and the tumbler motor on the other. So, you could expect one side to draw more than the other (the tumbler motor runs at about the same speed whether the heat setting is high, low or off), which may overload that one side causing this kind of damage.
    – KeithS
    Apr 11 '12 at 21:14

It looks like there is a little rust on the screw for the black wire. This would have increased the resistance for that leg.

More resistance means there is more heat generated.

The heat would have caused expansion, which could increase the resistance further.

An electrician would, at the very least, cut off the damaged insulation, and replace the outlet.

An electrician may have to replace the whole line as there is no ground.

Since you have to replace the outlet, and the plug; I would recommend using a 4 pin outlet.
( 2 hots, a neutral, and ground )

  • Good point; new electrical code requires 220V outlets to have four prongs (phase 1, phase 2, neutral, ground). So if you bring in a licensed electrician, he'll have to run that ground wire which pretty much means he has to re'fish that line (and if it's stapled to the studs like it should be he'll need to cut holes).
    – KeithS
    Apr 11 '12 at 21:20
  • @KeithS -- Nope, current Code still allows NEMA 10s as replacements where an EGC is not present; see the exception to 250.140. Mar 10 '15 at 2:10

What it looks like to me is that the insulation looks over-heated and the insulation is bubbling and failing. Also the insulation is cracking. Bubbling is from overheating, like too many amps being pulled through the romex, and the breaker did not trip, which it should because it is thermally protected. Thru the picture it is hard t tell but if you see the insulation bubbling and dis-figured then maybe the best thing would be to get an electrician to look at it and determine if the wire is ok and he just has to shorten the wires to the socket, or replace the romex.

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