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While replacing a light switch, I shut off the circuit breaker for it and checked the voltage to be sure it was not hot. I noticed that the hot wire read 10VAC, which was odd. It was pretty consistent. Just on a hunch, I went and shut off the circuit breaker for another circuit nearby (the GFCI circuit for the bathroom outlets) and the 10 VAC went away.

I checked the neutral wire resistance to ground, and it was very low, which is correct.

Any ideas on what to look at for this?

  • Were you using a digital voltmeter? How'd I guess? Those electronic devices have extremely high impedance and sensitivity and can pick up capacitive coupling from adjacent wires. It's not real power. – Harper Mar 29 '18 at 2:41
  • That makes total sense and what I assumed was happening. Any way to verify that is indeed the cause? Use a resistor to dissipate? – theguitarman Mar 29 '18 at 2:55
  • That would do it, 22kohm should suffice. – Harper Mar 29 '18 at 3:32
  • @theguitarman buy a $5 analog meter to test. They work terrific just for this - or invest in a Lo-Z meter. – Ken Mar 29 '18 at 10:45
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The induced voltage you were reading is from the electronic lines of force crossing the non energised conductor creating a voltage (this is how transformers work or inductive coupling) any meter that has a high impedance will detect the phantom voltage, most meters are high impedance or have over 1 mega ohm /volt internal resistance this prevents the meter from loading the circuit and affecting the reading. There are also low Z or low impedance meters that have an internal resistance that is low enough to eliminate the phantom voltage without affecting the measurement. These low Z meters are not sufficient for electronics trouble shooting in my opinion due to the circuit loading. Low z meters are fine for power measurements. you have no problems to worry about. Note if you turn on the light switch after replacing you may see the voltage drop to nothing depending on the type of lamp or load on that circuit.

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