white-white 240 volts?????


I'm redoing the utility room in our 40 year old house, and I stupidly removed the socket from the box to put in a GFCI without marking the original wires. Stupidly because there has been some previous "work" done by possibly-incompetent folks, so I'm not sure what to make of this. I've got a blue wire and two white wires coming into the box, and the voltage between the two whites is 120, the voltage between one of the whites and the blue is 120, and the voltage between the other of the whites and the blue is 240. How is this possible? Or is there something obvious I'm missing? The wires themselves seem to be original wiring for the house, but I'm not sure what they might be connected to at the other end.
Any ideas what the blue wire was supposed to be originally for?
TIA!
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Measure each using electrical ground ( hoping you have one!) wire to ground reading zero is neutral and wire to ground reading 120 is the hot. It would be difficult without tracing wires to tell the difference if they used a white wire for ground though.
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It sounds like a multi-wire branch circuit, or Edison circuit. Two hot legs of different potential sharing a common neutral. I'm assuming that if your outlet was 120 volts, one of the wires was not connected. If all three wires were connected, the brass tab was removed on the hot side of the receptacle, separating the upper from the lower receptacle. Each receptacle would then be on separate circuits. You should remark the white wire, which is hot, with black tape of marking pen.
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Curiosity question:
If this is an Edison circuit which had each receptacle on a separate circuit, how would the OP install the GFCI?
You can't split the circuits on a GFCI can you?
Would you just wire nut (and mark) one of the hot wires and push it back into the box?
P.S. This leads to another issue:
Would an Edison circuit mean that either the breakers for these 2 circuits were ganged together (which should have been a clue) or the OP was playing in a box with a live circuit, assuming he only shut off one breaker?
I only mention that because I have an Edison circuit in my house and the breakers were not ganged when I moved in. The receptacle I was working on was not split, so when it went dead I though I was OK. As I was working on the circuit and removing wires, I noticed a stair landing light begin to glow dimly. That was my first clue that something was amiss - and my first introuduction to Edison circuits.
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DerbyDad03 wrote:

That sounds good to me. You could use the other circuit on a different receptacle or extend either or both circuits to elsewhere.
I would want to find out where the circuits landed in the panel before I went further. And whether there is a ground at the box.

The NEC has required breakers for a multiwire circuit where both circuits connect to the same device ("yoke") to have a common disconnect for, I think, at least 10 years. Could be a 2 pole breaker or listed handle tie.
The 2008 NEC requires all [new] multiwire branch circuits to have a common disconnect.
--
bud--

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Curiosity question:
If this is an Edison circuit which had each receptacle on a separate circuit, how would the OP install the GFCI?
You can't split the circuits on a GFCI can you?
Would you just wire nut (and mark) one of the hot wires and push it back into the box?
P.S. This leads to another issue:
Would an Edison circuit mean that either the breakers for these 2 circuits were ganged together (which should have been a clue) or the OP was playing in a box with a live circuit, assuming he only shut off one breaker?
I only mention that because I have an Edison circuit in my house and the breakers were not ganged when I moved in. The receptacle I was working on was not split, so when it went dead I though I was OK. As I was working on the circuit and removing wires, I noticed a stair landing light begin to glow dimly. That was my first clue that something was amiss - and my first introuduction to Edison circuits.
See Bud's comments for the answers to your questions.
Regarding your first experience with an Edison circuit, often the three wires were run to an outlet box, then two wires would split off from there to another set of outlets. In the "main" junction box one of the hot legs would go to that receptacle, the other hot leg would be spliced to the hot leg going to the other circuit's outlets, but often the neutrals would be connected to the receptacle. This is probably the situation that you encountered. You killed the circuit to the outlet you were working on, however the second circuit, which shares the neutral is still alive, and when you lift the neutral from the receptacle, you lose you return path, and create a dangerous situation. For this reason,and others, the code doesn't allow the neutral of a multi wire branch circuit to be dependant upon a device. Unfortunately, this situation is all to common.
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