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I am trying to put in a switch and a light it controls in a storage building and this is how I wired it.

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When I checked the wiring it registered 120V when the switch was on, and right around 50V when the switch was off. Why would power still be getting to the light (I'm guessing there is a problem with the switch) and why would it only be 50V? Oh, and where everything branches off at the beginning I have a junction box there where all of the wires are joined there.

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If you measure AC current instead of voltage (only with the switch off, in the 50V situation!!!) how much is it? It may be useful to have an idea of how much current can this light receptacle deliver when it is supposedly off. This might not be a big deal. Suppose the hypothesis is correct: all wiring is good, except that the switch leaks. Since your multimeter has about 10 million ohms impedance, and is dropping 50V, the leaky switch in series with it has about 14 million ohms. Try disconnecting the switch wiring to see whether the problem persists. If not, it confirms the switch. –  Kaz Mar 31 '13 at 1:50
    
I want to repeat, do not measure the current of a 120V voltage source. It should be safe to measure the current of the receptacle in the 50V situation; the current is limited somewhere, which is why there is a voltage drop from 120. –  Kaz Mar 31 '13 at 1:51
    
Check the neutral to ground voltage, it does happen that the wire colors get reversed and the "neutral" wire is actually carrying power. –  bcworkz Mar 31 '13 at 17:47
    
What are you measuring in respect to? Is this 50V line to ground, line to neutral, line to a pile of dirt, line to your finger? –  Tester101 Apr 1 '13 at 11:28
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Are there any "lighted" switches or older dimmer switches. These can have small leakage currents that might account for the 50v –  HerrBag Apr 2 '13 at 2:01

2 Answers 2

The continuity test on the switch, described in comments, confirms that it is bad, since it shows continuity in both positions. Replace it.

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I'm seeing the similar phenomenon, 75V to ground. It would make a CFL bulb slightly flick (ghosting). I first thought there was something wrong with the wiring. But after double check several times, I realized it must be the leakage current causing this. I couldn't quite measure the voltage drop with a resistive load (regular bulb) in place at off state. I bet it will be close to zero. It does light up an electrical test pen, kind of make a uninformed person scared.

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