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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
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

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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Be careful when reading residential voltage with a DVM meter. My Fluke will read 121VAC at a single pole switch with the switch On, and 50vac with it off. But the scale changes from V to mv (milivolts). The scale changes dynamically and the v in milivolts on my Fluke is very small. 50-60mv is typical voltage float on ground. I have never measured a true zero with household wiring.

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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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Sounds like the power is trying to find a neutral and thats the residual you are getting. Dead leg. Does your switch interrupt the common(hot) and then feed the lights. I wired lights once interrupting the neutral instead and electricity will seek out a ground/neutral (air, back feed, etc...). Should take power from source(breaker box) to a junction box, at box take that common(hot) to the switch(single/double pole...what ever) bring that back to the common(hot) on the lights. The neutral from the breakerbox at the junction will connect to the neutral on the lights. I sent the neutral to the switch and the common to the lights once and the lights will still get power but no neutral, sure this will turn the lights on/off, but the power will at the lights that is still getting there will seek a neutral or ground. Just enough to give you a glow....even on an LED/CFL light.

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