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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?

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    Photos of your damage might help illustrate the issue. – Criggie Sep 29 at 21:23
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... 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.

  • Thank you. I added some edits. The circuit did turn out to be 20 Amp, 12 gauge wire. Receptacle has been replaced. Screws were still securely attached but BX cable came loose from the clamp in the box. Unsure if this could have been the cause of the problem? Unsure whether it's safe now? I know it's not good practice to have 20 Amp receptacles on my two circuits that are 15 Amp, but I knew I would never be plugging anything high draw into them. Spec grade 20 Amp were actually less than 15 Amp so I bought a bunch of them. Still, I will replace. Thanks again for taking the time to answer. – Jim H Sep 28 at 23:32
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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.

  • Thank you. I added some edits to my original question. As a RETIRED master electrician it's possible that you've mellowed a little (thank you for that). Electrical can be complicated and certainly dangerous if done incorrectly. If the BX cable came loose from the clamp inside the box (which happened), could that have caused arcing? It's been fixed but do I need to call an electrician because of likely damage elsewhere in the wiring? Can I test whether there's damage myself using a multimeter? Thanks again very much for taking the time. – Jim H Sep 28 at 22:46
  • @JimH - You are welcome. Depending on how sensitive your multimeter is you can turn the power out and measure the resistance (ohms) of the conductors. Then using Table 8 of the NEC, you need to find the average resistance of the conductor (probably a 14 or 12 under the coated copper column). the resistance is for ohms/kFT. This is not exact since you have to estimate the length and the average ambient temperature, but you looking for a large difference between what it should be and what your meter is reading. – Retired Master Electrician Sep 29 at 13:39
  • @JimH - What I just described above does take some skill. So if everything is out of wack don't panic. Since you have decided to call a service man out let him do the heavy lifting. Also you just might go through all of the circuit and look for any discoloration or overheating. Check all splice and connections and clean all of your conductors "like a bright penny" and make sure they all are mechanically and electrically secure. Replace and repair as needed. Good luck. – Retired Master Electrician Sep 29 at 13:45
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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.

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    Turn off all the 15A breakers. If any NEMA 5-20 receptacles have lost power, change them to 5-15. – Harper - Reinstate Monica Sep 28 at 16:59
  • Be aware that loading a 15 amp circuit with 20 amp will NOT cause it to just become 33% hotter. It will become 78% hotter (more current causes more voltage drop, so heating of a constant-resistance conductor is current squared). – rackandboneman Sep 29 at 18:44
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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.

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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.

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