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?