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Two power sources for one receptacle!

Danno | Posted in General Discussion on July 29, 2006 10:40am

I’m not an electrician, so i may not have the terms right, but Marc and I were working on a house this past week that is very old–even has balloon framing. (I was sweating when we replaced a header over a first floor window in a two story building by using two 2×4 deadmen! Pinched the saw in the first kerf, which told me there was still a load on the wall!) Marc told me it had new electrical, but when we took off some of the plaster and a couple laths, I saw knob and tube in the wall–he said it was unused, but when we got all the plaster and lath down, it turns out the K & T is indeed still in use!

Anyway, we plug a vacuum into an outlet and turn it on and I throw breakers until it turns off (breaker box was new). So Marc is taking apart the receptacle and gets shocked. Gets out his tester–there is still voltage! On another circuit, he held the tester while I threw breakers and said I got it, then yelled no, the tester must have slipped. I kept going and did turn off the right breaker. Later, we’re in the dark basement, and he’s wiring a junction box for two of the wires (modern 12/2) (including the one that shocked him earlier and which I thought he had found the right breaker for and had it off). I’m holding a flashlight that is supplying less light than a match when the screw driver slips and pow, flash (actually flash, poof) and another breaker trips.

I’m thinking with K & T, someone mixed the black (power) and the white (return–“grounded”–neutral?) on one or more circuits. On the old wiring and receptacles there are no bare copper grounding wires, so could that back-feed and give this weirdness? I hate knob and tube!!

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  1. renosteinke | Jul 29, 2006 11:08pm | #1

    I doubt that the ground wires have anything to do with this problem.

    I sounds like both of you are involved in a more involved project than is proper... you ought to get a more seasoned helper. But, I wander...

    The Knob and tube works the same way any other wire does... add electricity, and you get zapped.
    Rule #1 is.... turn it off! Don't assume it's off, or that it's not used, etc. Since the place has two types of wiring, it is safe to say that the system has been worked on before, possibly by someone who didn't know what he was doing. Make sure!

    The next step is to find out just where BOTH ends of the wires go. It may be a circuit, it may be a switch leg, it may dead-end in a closed-off area.
    If it is no longer used, the proper approach is to remove it all the way back to a junction box.
    If it is in use, then you'll want to re-route it, using proper methods.

    Electricians have various tools that, together with their experience, let them sort this stuff out a lot quicker, with a lot less demolition, than you can. I suggest getting one out there to look at it.

  2. BarryO | Jul 29, 2006 11:15pm | #2

    Some DIY and "handyman" work can result in two branches hooked up to one receptacle.  If they're on the same 120V leg, it'll appear to work just fine.  'sounds like the time to call in a pro and let them sort it out.

    1. User avater
      BillHartmann | Jul 29, 2006 11:30pm | #3

      Is also possible that it was done properly with the tabs broken off the receptacle. Then either only the one 1/2 was verify off or at some time the receptacel was replaced and someone did not break of fthe tabs.

      1. Danno | Jul 30, 2006 12:15am | #4

        Thanks to all who replied. Probably do need pro help, but it's not my job, I am the assistant. I told Marc I had to stop myself suggesting honeycomb black and white tile floor to the homeowner because it would fit in with her decorating scheme--old fashioned brushed nickel pendant light fixtures and so on. He gave me a dirty look--he hates doing small tile. Glad I kept quiet!

  3. mikeingp | Jul 30, 2006 12:43am | #5

    Electrician or not, you need to add one of these testers that can test through insulation, such as this Fluke to your tool box (they cost $10 or $15). You say you had a tester, but I'm thinking it was probably a volt meter requiring bare wires since you say it "might have slipped". The tester shown will light up if anywhere close to a live wire. My uncle gave me one (what a guy!) and I don't even look at an electical repair without it.

    1. Danno | Jul 30, 2006 03:32pm | #6

      I'll look into getting one of those. The one Marc had did need bare wires. He has one that clips on, but didn't have it with him. I have a pretty good one (I think it also clips but needs bare wires) but it was at home.

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