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Our 1950s era house has a 2-prong switched outlet by the front door. Both plugs are turned on/off by the switch next to the front door, so it's not one of those split kind where one plug is always hot and the other is switched. The outlet has 3 wires like this:

enter image description here

The metal between wires 2 and 3 on the left is connected, so it's not like the modern outlets where you can break the tab to stop the continuity between these 2 screw terminals.

I did an experiment where I disconnected each wire one at a time and found these results:

  • Disconnected 1: Both plugs on this outlet stop working.
  • Disconnected 2: Both plugs on this outlet stop working.
  • Disconnected 3: Both plugs on this outlet work, but another outlet on a different wall stops working.

I held a non-contact voltage detector by each wire and it shows 1 and 3 with voltage. The really baffling part is that the voltage tester still shows voltage on 1 and 3 when the switch is turned off, but I've read that you can have "phantom voltage" so maybe it's showing that. The switch doesn't turn off the other outlet, just this one.

Does the following sound like a correct assessment?

  • Wire 1 is the incoming hot for this outlet.
  • Wire 2 is the neutral.
  • Wire 3 is the neutral leg coming from the outlet on the other wall.

I'm also curious how the switch is involved here to turn off the outlet. Maybe Wire 1 is being turned off despite the tester showing voltage there. I also thought perhaps the switch is interruping the neutral wire 2, but if that's the case then the other outlet would be shut off too.

  • Are there other wires in the box spliced together? – Speedy Petey Oct 1 '15 at 23:01
  • What does your voltage tester read between wire 3 and ground? – Brian Duke Oct 1 '15 at 23:05
  • Good question on the other wires being spliced. I will look in there. Regarding the voltage tester, I wasn't sure what to use as a ground because there isn't one in there. What could I use as a substitute? Run a long wire to the grounding rod outside attached to the cold water pipe I guess? – James Toomey Oct 1 '15 at 23:57
  • I guess I could test the voltage on 3 if I assume 2 is a neutral? I just wasn't sure if 2 turns out not to be a neutral and it won't give me an accurate reading. – James Toomey Oct 2 '15 at 0:39
  • Are all of the wires coming into the box separately? If they are you could be dealing with Knob and Tube wiring where the Line and Neutrals are run separately. – Brad Gilbert Oct 2 '15 at 22:47
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Wire number 1 is your switched power wire from the switch, wire number 2 is the neutral path back towards the panel, and wire number 3 is the neutral wire coming from other outlets down the line.

The reason you read 70 volts on number 3 is that something on that circuit is using power and you disrupted the path. You can be shocked if you complete the circuits. Even if nothing is plugged into the other outlet there is most likely another outlet on that neutral with something plugged in. You see this with a lot of homes with knob-and-tube wiring.

Think of it like this you turn on the faucet and fill a glass of water, you put the rim of the glass halfway in the stream, some water fills the glass the rest is going down the drain. Think of the number 3 wire as the drain and a light bulb for example as the cup. That's why you read 70 volts

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I used a continuity tester and found that

  • The right wire #1 comes from the switch. The switch's other wire is the hot, so when the switch is turned on, the power flows through the switch, then to this wire that feeds this outlet on that right wire.
  • The bottom-left wire #3 comes from the other outlet, and it's a neutral in that outlet, so basically it is the continuation of that outlet's neutral. (I'm not sure why they hooked it up this way rather than using a cap to tie it together with a pigtail to the outlet? But I guess electrically it's the same. I think I will rewire it the way I'm used to seeing.)
  • The top-left wire #2 must be the neutral.

As far as why there's voltage on #3, I'm baffled! I don't know how hacky this is but I used my jumper cables to hook to the grounding rod outside so I had a definite ground (this is a 2 prong outlet so I didn't know what else to use as ground), then tested the wires and #1 had 120V as expected with the switch on, but #3 had 70V even when it was disconnected from this outlet! I'm going to have to call an electrician to investigate this further because I have no clue why.

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  • IF there is a device connected to that other outlet, then what you are seeing is the live coming through that device. Disconnect it and you'll see it drop to 0. – Agent_L Nov 6 '15 at 10:33
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    70V is what you would expect when measuring voltage in series, which is exactly what happens when you measure a neutral coming out of one device into another ground or neutral. The lamp or other upstream device is getting 50V, and your tester measures the other 70V, for a total of 120V. Connecting electrical devices in series splits the voltage according to resistance (I believe, not an EE) and connecting them parallel uses more amps. To measure the 120V, you need to measure hot to ground, not an in-use neutral to ground. A disconnected neutral to ground would be 0V. – BMitch Dec 6 '15 at 16:31
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I had trouble following your description. Where do these wires enter the box? Is there more than one cable entering the box? For the two switched outlets in the same box, there are a couple of ways this can be wired.

A single power cable enters the switch box with the hot connected to the switch and the other switched wire becomes the hot wire to the outlet. In the outlet the switched hot connects to one side of the outlet and neutral to the other side. In the switch box, neutral from the outlet is pigtailed to neutral from the supply. If downstream devices are supported, they are wired from the live hot and neutral fed through their own cable.

If the supply cable enters the outlet box, the hot side is fed to the cable to the switch box and the neutral wire in that cable is coded as hot in both boxes and connects to the hot side of the outlet and one side of the switch. Downstream devices must be supplied by a cable from the outlet box that is pigtailed to the corresponding supply wires.

Given that new outlets are cheap, you might wish to replace the old two-prong outlets with child-safe (if you approve of children) modern outlets.

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  • thanks for answering. I will try to post a picture of it. There are 3 cables that enter the box, I believe. It's the old tar-cloth 2-wire Romex style of cable. I believe it's the way you described it first, ie, "A single power cable enters the switch box with the hot connected to the switch and the other switched wire becomes the hot wire to the outlet". The wire I labeled #2 is the neutral, I'm pretty certain, it was #3 that was really confusing me. That's coming from a separate cable. – James Toomey Oct 5 '15 at 20:25
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Based on a recent exercise I went through in dealing with phantom voltage, it is possible the 70 volts you measured on line 3 is phantom voltage. If it was a typical hot wire, it would read 120 volts (unless something else is badly wrong.) You might want to test the voltage on the outlet the #3 neutral wire comes from to confirm it is also reading 120 volts. I don't know of a good way to check if voltage is phantom. One article I found in this forum said an older volt meter may not sense phantom voltage. The other possibility is there may be a timer or dimmer on the circuit that continuously runs a low voltage through the system to keep the timer/dimmer working. If there is more than the single outlet connected to line #3 (e.g. the neutral in the other outlet goes somewhere else as well) anything upstream could be adding the phantom voltage.

btw, I didn't see it mentioned in the other responses, but if a 2-prong receptacle is wired correctly, the hot wire will go into the smaller of the two prongs (as shown on the right side of your picture) and the neutral wire goes to the larger of the two prongs. So your analysis is consistent with a correctly wired plug.

In the situation you describe, I would be worried about where the hot wire in the other outlet is coming from (the one with the neutral wire as wire #3). I hope it is coming from the box that contains the switch that works the first outlet, but if it is coming from somewhere else, there may be a risk that it is on a different fuse/circuit breaker. You might want to confirm that everything is on the same circuit. And if they are all on the same circuit, shutting off the circuit should also eliminate the 70 volts you are measuring on wire #3.

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We need to establish some terminology:

Cable - is a three-wire of black or white insulation and bare copper wrapped in an insulator. This is a fabric in older cables and plastic in newer cables.

Wire - is a copper conductor that is bare for ground wires, black for hot wires and white for neutral.

You appear to have established that one cable goes to the switch box, one connects to the power panel and the other connects to further downstream devices.

You appear to be comfortable with electricity so I will remind you to do all work with power off at the panel and a lock or note on the door. Trust no one, least of all yourself. Always test for live current before touching anything.

In normal usage the hot wire from the panel can be connected to the hot wire to the switch and to the hot wire to downstream. The white wire from the switch could be 'coded to black' with electrical tape or paint. This last wire will be connected to the hot side of your outlet.

The ground wires will all be connected together, attached to the box with a machine screw and also connected to the outlet's green screw (if you love your family and used a new outlet like I suggested).

New three-prong outlets can be tested with those little testers that plug in and light up so nicely.

The three neutral wires will all be connected and a white pigtail will connect to the neutral side of the outlet.

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  • It seems to me like they used the spare terminal for wire #3 as an alternate way to connect the neutrals #2 and #3, rather than twisting them together with a cap and then having a single white pigtail connect to the neutral side of the outlet like you said. From the electricity's point-of-view, using the spare terminal will still work, right? But it's confusing because you can't tell what is going on. Which makes me wonder if twisting the neutrals together and having a pigtail is partly done for clarity so you can tell what the heck is going on! – James Toomey Oct 7 '15 at 19:22
  • @JamesToomey -- partly for clarity and partly because it means that pulling a device out won't interrupt the neutral wire. – ThreePhaseEel Jan 6 '16 at 4:54

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