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Discussion Starter · #1 ·
In the course of rewiring portions of my 1939 home I encountered an outlet box with a duplex and single outlet - no grounding of course. The single was disconnected and a lone red wire was hanging behind it. The duplex was pigtailed to three hot and three neutrals.

When I shut off the breaker and started taking the wires apart, a 3-way light at the top of our stairs came on when two of the neutrals touched each other. The volt meter showed 110 V ac. I found that the power came on and off depending on the position of the light switch. This 3-way is on this circuit, but apparently got juice from somewhere else when its breaker was opened.

Anyone have a clue what might be happening here? I was theorizing that the red wire might be a traveler, if the disconnected single outlet was previously controlled by the stair light, but don't know how to check this out.
 

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There are MWBC - multi-wire branch circuits
These are 14-3 or 12-3 wire
2 hots went to (2) reg breaker or (1) 240v breaker supplying 2 different 120v circuits with a shared neutral
Single breakers are supposed to be handle tied together
And they must be on opposite hot legs in order for the MWBC to work correctly

Usually just to outlets, but possible they ran some lights off the red hot circuit
 

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In the course of rewiring portions of my 1939 home I encountered an outlet box with a duplex and single outlet - no grounding of course. The single was disconnected and a lone red wire was hanging behind it. The duplex was pigtailed to three hot and three neutrals.

When I shut off the breaker and started taking the wires apart, a 3-way light at the top of our stairs came on when two of the neutrals touched each other. The volt meter showed 110 V ac. I found that the power came on and off depending on the position of the light switch. This 3-way is on this circuit, but apparently got juice from somewhere else when its breaker was opened.

Anyone have a clue what might be happening here? I was theorizing that the red wire might be a traveler, if the disconnected single outlet was previously controlled by the stair light, but don't know how to check this out.
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1st of all, you said you were "rewiring". So in this particular enclosure that you opened & were working on, you didin't indicate whether the wires in that enclosure were via conduit or were they non-conduit type such as NMC or "BX"?..It's hard to rewire if the answer is the later!! and besides, when you are working on an ungrounded system, you can't really tell, unless you go to a proven ground, what the real polarity of the wires is, i.e., the whites could be the hots & the blacks could be neutrals/??.??Was the red wire still hot after you shut off the duplex circuit??..if so, that's a hint that you are on a 3 wire circuit, i.e., 2 hots - 1 neutral, hots are opposite phases....so assuming your 1939 wiring is correctly polarized, what you were experiencing while opening the neutrals was an obvious "back-feed" from an active 3-wire circuit, meaning that your stair lights are powered from another circuit (other than from what you shut off) BUT using the common neutral, the likes of which might confuse a less than experienced electrician. When you flip the 3-way switches on the stair lights, you are making & breaking the bulb power but causung the "back-feed" (110v) via the bulb filaments because there is no neutral to complete the circuit, so the hot (thru the filament) runs back to your voltmeter & makes all this confusion. In conclusion, as long as everything is working correctly after your wires are all tied back together properly, you really don't have any problem of concern, you are simply experiencing the "mystery of the wires" which prevails when you start opening neutral wires that are tied to 3-wire circuits. rjhrjt
 

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As said above, the wires in some electrical boxes may be controlled by more than one circuit breaker!

And because these situations exist, it is best to turn off ALL the power to the house before working on electrical wiring.

In the case of a MWBC, this would be two hots and one neutral. You could turn off power to one of these hots, yet still be shocked by the "shared neutral" or other hot wire!

More about multiwire branch circuits (MWBC)...
http://www.google.com/search?hl=en&...=0CAYQBSgA&q=multiwire+branch+circuit&spell=1
 

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Discussion Starter · #5 ·
Thanks all for the lightning quick replies so far. The wire coming in was old style romex (braided cloth). The red wire had no power to it. Once I wired it back together (blacks together/whites together), the whole system worked just fine, including the 3-way switch. I imagine it has operated fine like this for 70 years, so unless someone suspects something dangerous, I will trust to the expertise of the 1930's pros.
 
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