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I just did an electrical survey of my house receptacles and fixtures. All voltages are around 119.5V except one. On the light that is controlled by 2 switches (ie on 2 different levels of the house) the voltages are 42V when the light is off, and 119V when on. The other 3 way lights (ie same set up on 2 other levels in the house) are 0V when off and 119V when on.

Any idea what is the issue?

Thanks in advance.

  • What else is on the "problem" circuit? (receptacles, appliances, lights). If anything else (besides the 3-way light) is "on", what does the voltage read at the switch? – manassehkatz Nov 25 '18 at 17:57
  • Nothing but the one light. – Fed Nov 25 '18 at 18:07
  • Is the breaker "normal" or is it connected to another circuit (MWBC is the official term)? Are the switches "normal" or is one a dimmer or timer? – manassehkatz Nov 25 '18 at 18:19
  • The breaker is 15A on the main box, also supplying 2 other 3way lights. The switches are all normal. No illumination, dimming, or anything else. – Fed Nov 25 '18 at 18:21
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    So it is not "nothing but the one light" - it is also supplying 2 other 3-way lights. Does it make a difference whether any of the other 3-way lights are on or off as far as getting 42V vs. 119V on the problem 3-way light? – manassehkatz Nov 25 '18 at 18:24
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It's another case of "phantom voltage".

When a wire that is disconnected/floating runs alongside any other wire for a distance, it picks up (via capacitive coupling) some voltage, which can read on a sensitive enough voltmeter such as a DVM (including most cheapies). There is no current behind this voltage, it's more like the "son of voltage" because it will disappear at the slightest demand to do useful work. So an analog meter, the kind whose needle movement is powered by the voltage it is measuring, would not pick it up.

When a 3-way switch is on, the unused traveler "floats" unconnected, as you can imagine in the below drawing if both switches were up or down. That dead traveler will have phantom voltage since it parallels the live traveler.

Try switching that 3-way switch to the OFF position, as illustrated, so one traveler is connected to supply and the other is connected to the bulb filament and then to neutral. The voltage will "magically" go away.

enter image description here

Colors: black=always-hot yellow=travelers red=switched-hot white=neutral

Also, while you have the boxes open, get a 5-pack of colored tape and tag the 3-way travelers, I like yellow. Make both the same color since there is no need to distinguish them. Travelers are on the brass screws, or the screws not marked "common".

Other things to look for, while you're in boxes, is the "tab" on receptacles between the screws (on some, it may be broken off, very important when changing receptacles), and watching out for troublesome backstab connections and converting those to screws.

  • Makes sense, but why would only one, of the 3 similar circuits in the house, have a phantom voltage? The others are 0V when off. Also, I don’t understand what you mean about the voltages going away. The setup you describe is when the phantom voltage appears (ie the light is not on). – Fed Nov 26 '18 at 9:49
  • @Fed The amount of current in the parallel wire/cableprobably has a lot to do with it. – Harper Nov 26 '18 at 16:24

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