Unusual wiring for receptacle

[xanadu245]

Starting with A little history to help fill in a few details. The house was built in 1956 and a fair bit of the wiring is likely still original which was MC cable with a hot and neutral which had the plastic/rubber coating and a fabric sheath (kind of hard to tell which was black and which was white as both looked black with one being just a little lighter).
I went to replace a receptacle as the plastic at the bottom was breaking as it seemed to be old and brittle. I thought it would be a simple replacement that I've done lots of times. I used a "circuit detective" to figure out what breaker it was and shut off that breaker in preparation for the swap. It was a 20amp single pole breaker. When I got the cover off and started removing the receptacle, I found it was 15amp. OK, I have several options handy with a regular 15 or 20 amp receptacle as well as a 20amp GFCI receptacle to replace it with, so not a major issue. It's in a 2-gang metal box with a duplex receptacle on one side and light switch with indicator light for the basement lite on the other side. After turning off the breaker, I found that the light switch is still live.

Here's where the "Unusual wiring" comes in. When I got the receptacle out, being that the MC Cable didn't have a 3rd ground conductor like more modern MC and Romex cable), I initially figured that someone had just replaced a 2 prong receptacle with a 3 prong and didn't connect a ground, which I don't like to see, but have seen done. When I got it further out and twisted it around more to get a better look, I found that both of the wires (black and lighter black (see note at the beginning)) were connected to the brass screws and nothing appeared to be connected to the silver screws. I also found that someone had put a small piece of wire from the ground screw to one of the silver screws. I've never seen anything like that before. With the 2 wires disconnected from the old receptacle and separated, I turned the breaker back on temporarily to test the wires with a multimeter. As the box was bonded, I tested from each wire to the box and found that both were showing about 90-100v. Not entirely unexpected with the lower voltage as the MC cable isn't a true ground, but I had expected only one of them to be hot, not both. Before starting the whole process, I had also tested with a plug in tester to see if it showed an open neutral or any other issues and it showed "correct". It had also tested at 118v between neutral and hot with the multimeter. As the only things that have been plugged into this receptacle were a nightlight and occasionally a crock-pot (both 2 prong), I was just going to put in the GFCI receptacle, label it as "No Equipment Ground", and call it a day, but having both wires connected to the brass screws and nothing but a short jumper wire from the silver screw to the ground screw threw me off and had me concerned. It's definitely not a 240v receptacle. I've attached some pics to help, hopefully. I'm not an electrician, but I have done quite a bit of wiring for outlets and lighting to add new circuits or make repairs. I could have just duplicated what was already there with a new outlet, but doesn't seem right, even though that's how it was when we bought the house and it's been fine for 2+ years. Sorry if this is a bit of a head scratcher.

 

[Drick]

Don't put this back together as is, it is dangerous. There is no neutral and the outlet is using the MC cable jacket as the neutral. Yes it worked, but that wasn't a good thing. If you look at the top screw hole on the junction box you will notice it is blackened. You will also notice the brownish yellow color of the top of the outlet. This was caused by the outlet using the top screw as part of the return path for your circuit and it was arching and overheating in the process. That happened because the short black jumper wire that is connecting the neutral side of your outlet to the ground screw also connects it to metal yoke you use to fasten the outlet to the wall. Get rid of the jumper wire! I have no idea where your neutral wire is but you need to figure that out (or hire an electrician to figure it out for you). Also, you should not have two live hot wires connected to your outlet. If they are connected to two different circuit breakers they theoretically could allow you to draw double the rated current through a single receptacle. Are you certain they are both live? Inexpensive voltage testers can give false readings when you don't have a good ground reference which you do seem to be lacking. If they are in fact both live cap off one of the two hot wires and just don't connect it to the outlet.

 

[Sponsor]

 

[Reach4]

"I also found that someone had put a small piece of wire from the ground screw to one of the silver screws. I've never seen anything like that before. With the 2 wires disconnected from the old receptacle and separated, I turned the breaker back on temporarily to test the wires with a multimeter. As the box was bonded, I tested from each wire to the box and found that both were showing about 90-100v. Not entirely unexpected with the lower voltage as the MC cable isn't a true ground, but I had expected only one of them to be hot, not both. "

Your indicated voltage with respect to the not-a-ground is meaningless. I would put the GFCI in with the label.

If you are skeptical but curious, run one test lead to a real ground. But to measure to a floating ground with a high-impedance meter is not meaningful.

 

[Fitter30]

If you did pull one of the two hots off the reciprocal your going to get voltage on both the tab between the two screws is still in tack. Bet only one is hot feeding something else. With using the bx as a neutral a disaster is waiting to happen. If the neutral was lost at the panel, light was switched on the bx would become hot and you could become the neutral getting between the bx and panel.

 

[Robert Gift]

Would it be possible to insert a solid core ground wire into the metal jacket and push ito the receptacle?
In my little old house I passed a test to obtain a homeowner's electrical permit and replaced all of the wiring.

 

[Fitter30]

Would it be possible to insert a solid core ground wire onto the metal jacket and push ito the receptacle?
In my little old house I passed a test to obtain a homeowner's electrical permit and replaced all of the wiring.

No there's no room for another wire between the cloth and rubber insulation.

 

[xanadu245]

Thank you for the recommendations. I have an electrician coming to help me troubleshoot it further and make it safe. In the mean time, I've left the circuit breaker off.

 

[xanadu245]

Update: I took a deeper look and discovered that the two wires that had been connected to the hot side were in fact both hot and allowing passthrough via the receptacle. There also appeared to be a traveler wire that wasn't used for that circuit. After talking, my wife and I decided it wasn't really vital to have a receptacle there, I opted to just eliminate it.

 

[Fitter30]

Looks like aluminum wiring? If it is need special receptacle ,wire nuts and switches.

 

[xanadu245]

Looks like aluminum wiring? If it is need special receptacle ,wire nuts and switches.

no, definitely copper, at least at this outlet and any of the others I've looked at. It's a 2-gang box. As there are 2 separate circuits coming into the box, I opted to just remove the outlet and leave the light switch for the basement light. As it's apparently not a common 2-gang cover plate I ordered one with a regular receptacle on one side and a blank on the other.