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I am in the process of replacing switches in my 1920's home which was renovated in 1970. While doing so I am creating notes of what wires exist and do not exist in each box. I found a light switch where the line in connection is hot, per my Klein non-contact detector. But, when I turn the light switch on neither the load or the line wire connections are showing up as hot, but the light is on. On the light switches I looked at so far, when I turn the switch on it results in both the line and load connection as hot.

Is there something wrong with the wiring?

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    Did you double check your voltage detector is still working, and the breaker is still on? Dec 15 '15 at 22:46
  • I agree Blats ,if the light is on the tester is out,,,
    – Ed Beal
    Dec 15 '15 at 22:53
  • I triple checked the voltage detector and it works 100%.
    – RB Baldwin
    Dec 16 '15 at 2:23
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This has all the smells of a switch in the neutral wire, which is a no-no as it's a potential shock hazard for a clumsy bulb changer, and forbidden by NEC 404.2(B):

(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.

The reason why the voltage detector turns off when the switch is on is because when the switch is on, the switched wire is effectively connected to the grounded conductor, putting it a few volts above earth ground. When the switch is off, though, there's no current flowing, so the switched wire "floats" at mains voltage, which is picked up by your detector.

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  • I may be confused or misunderstanding something, but if it's a neutral, the voltage detector shouldn't indicate hot when it's off. Dec 16 '15 at 11:09
  • @batsplatsterson, I think that normally the hot and neutral "meet up" at the fixture, and the switch interrupts the hot wire from ever getting to the light. In the case of a switched neutral, the hot wire flows right through the light and all the way down to the switch. I'd venture to say that the "line in connection is hot" the OP refers to is actually the load wire coming from the light, not the hot from the breaker box.
    – JPhi1618
    Dec 16 '15 at 17:41
  • @JPhi1618 I do see what you're saying. On the other hand - even under load, the NCVT doesn't light up in proximity to the neutral. Dec 16 '15 at 18:08
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did they even have an nec in 1920? some of the shit i have seen in houses from that era would certainly not fly today and would fail any inspection if used now as a wiring method. now to answering the question at hand.. unless you decided to rewire the entire building you may have to simply leave things as is because messing with old wiring is not a good idea its usually old and brital and fires start when old wiring is subjected to disturbance by someone who is not trained in how to work on it. my advice would be to consult a licensed electrician if you have concerns about the safety of such a system or about its condition.

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