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After unplugging the 8,000 btu window a/c that had been there for a couple of months, the outlet is noticeably black/scorched. I haven't inspected it yet but I'm disturbed. Not sure if I did something wrong.

The home is older and there are no ground wires but should be grounded through BX cables. It has 100 Amp service and (besides the dryer, stove) some of the breakers are 15 Amp and some are 20. I replaced a lot of receptacles and tried to do it correctly. I used spec grade, either Leviton or Legrand. No backstabs, and everything should have been securely attached. I used an outlet tester and each one showed that it was wired correctly and was grounded.

I know some of the Levitons were self grounding through contact with the metal box. On others I attached a ground wire to the metal box myself. I DID use 20 Amp receptacles in most locations pretty indiscriminately, without confirming whether it was actually on a 15 or a 20 Amp circuit. I didn't think that was a big issue. If the outlets were a bit overbuilt, I figured it would only be dangerous if I overtaxed the circuit. Not sure whether I did. I don't THINK the A/C's plug is only for a 20 amp outlet but I'll have to check.

What went wrong?

Checked circuit, it is 20 Amp. Checked a/c, it says it draws 10 Amps. Opened the outlet and the hot wire was burned all the way through about three inches from the receptacle. The hot and neutral were both still securely attached to the screws. The only thing I noticed was it looked like the BX cable wasn't securely clamped to the box and may have slipped out. I reclamped the metal cable to the metal box and installed a brand new receptacle, again spec grade. Everything looks fine and works fine and the tester says it's wired correctly and grounded. But I still don't know what happened. Anyone? I mean if the cable came unattached that means it wasn't grounded. But my understanding is that a ground is like a seat belt. Not having one doesn't cause an accident, it just means you're not protected in case of one. So what could have caused the hot wire to burn right through?

Sorry I wrote the above before I saw there were answers to my question, and haven't finished reading them. As far as the 20 Amp receptacles, I accept that I was mistaken in thinking perhaps the "guts" were better, but I never would have plugged anything into it that drew more than 15 Amps anyway. As it turned out it was on a 20 Amp circuit (and the wires in the box are clearly 12 gauge) and the a/c only drew 10 Amps max so that shouldn't have been the problem. Unless maybe someone was running a vacuum cleaner on the same circuit while the a/c was on, but that would have tripped the breaker. Thank you for the answers (not fully read or digested yet, though).

Only two of the breakers are 15 Amp. All the rest are 20. Receptacle actually says "spec grade" on it. No extension cord. I will inspect the a/c plug. Could the BX cable coming free from the clamp have caused arcing? Can testing the receptacle with a multimeter determine if damage has occurred? What would I be looking for, a drop in voltage?

... there are no ground wires but should be grounded through BX cables... each one showed that it was wired correctly and was grounded. I know some of the Levitons were self grounding through contact with the metal box. On others I attached a ground wire to the metal box myself.

Grounding has nothing to do with this.

I DID use 20 Amp receptacles in most locations pretty indiscriminately, without confirming whether it was actually on a 15 or a 20 Amp circuit. I didn't think that was a big issue. If the outlets were a bit overbuilt, I figured it would only be dangerous if I overtaxed the circuit.

Huge mistake. This would allow a 20A appliance to be put on a 15A circuit. This should be obvious, no?

I gather you may have overthought the difference in outlets, forgetting about this one, and focusing on "surely these 20A receptacles must be tougher/better/higher grade, right?" Actually, they're not. The guts are identical. The only difference is keying/gating on the outer plastic, designed to keep you from plugging a NEMA 5-20 plug into a 15A outlet.

What caused this?

• My postmortem would start at the A/C plug. I'd be looking for scorching on the pins of the plug, indicating a defective or damaged plug, or poor insertion. Given your level of attention to mains electrical I assume you're smarter than this, and you didn't mention it, but a consumer grade extension cord would also have this effect.


• Then, I'd pull out the socket and have a look at the wire connections. Screw terminals are better in every respect, but one: they can be mis-torqued, usually under-torqued. NEC 2014 now requires actual torque screwdrivers to set torques, because it's a huge problem even for pro electricians.

This for sure, this was definitely arcing, and could've been a real mess. With 20/20 hindsight, a better investment would have been AFCI breakers.

There are a few things that might be causing your problem.

First in just looking up 8000 Btu AC's in general, they pull between 5.8 - 6.2 amps. That should work on any normal 15A circuit if there is no other equipment attached to that circuit.

Second every new AC I looked at required a grounded circuit to feed their unit.

Third Damage as you described is usually caused by over heating at a junction or splice. When you say spec grade I am not sure what you mean. From a commercial stand point a spec grade receptacle usually means it is a Hubble 5252 (15A) or a 5352 (20A) or an equal by another manufacturer. You can judge if you used one of these receptacle they run between $10.00 to $15.00 each. Since this AC can run longer than two hours the needs to be considered as a continuous duty circuit.

Using the above points then in order for your AC to be installed properly you must run a grounded separate circuit for your AC. You should used a heavy duty receptacle that can take a continuous duty load. It can be either a 15A or 20A circuit.

Also the power supply cord on the ac is most certainly damaged also so you need to have that replaced also.

One final point. Overheating creates high resistance and overheating. High resistance causes unusually large voltage drops. Large voltage drops causes equipment to perform inefficiently. Electrical equipment does not handle low voltage very well and this causes shorter life for your compressor and other electrical machinery inside the AC, not to mention larger power usage.

In conclusion, when all else fails, remove everything and replace with correct wiring materials and methods. I will promise you it is worth it and it will work properly for years to come.

Hope this helps and good luck.

I'm going to suggest that this is the heart of your problem:

I DID use 20 Amp receptacles in most locations pretty indiscriminately, without confirming whether it was actually on a 15 or a 20 Amp circuit. I didn't think that was a big issue. If the outlets were a bit overbuilt, I figured it would only be dangerous if I overtaxed the circuit.

The scorching on the outlet that you describe is a prime symptom of an overloaded circuit. Be thankful that you're seeing the signs here as opposed to it all happening in a hidden location and starting a fire without warning.

At this point it's NOT safe to use this outlet at all until the wiring is inspected from the panel to the wall outlet since additional damage may have occurred anywhere along its length.

Do yourself a HUGE favor and get a licensed electrician to come in and inspect and correct any mis-wired outlets you have. Yes, it DOES matter if you use a 15A vs. 20A since the wiring requirements are different for a 15A vs. a 20A circuit. Yes, pulling 20A from a 15A rated wire is very dangerous.

On top of the interventions indicated by others, I would investigate the AC unit for a ground fault. It's the most likely explanation as to why there'd be current on the ground to cause arcing in the first place.

My 2 cents (but I'm not a certified electrician): This usually happens when there is a poor contact somewhere. Perhaps the plug had a poor contact with the receptacle, or perhaps there was a poor contact between the cable in the wall and the receptacle. In older houses, that can happen over time if there's enough humidity and the copper/aluminium slowly tarnishes. That in turn causes high resistance at the point of contact, and at high loads the whole thing heats up. I've had this happen to me both in receptacles and in junction boxes.