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Tonight I removed an old switch from one of the walls in my living room. We recently bought our house, and there were two holes in the wall where the old thermostat and humidity controller were. I patched those up and then took care of this old switch that was just as ugly as the two holes in the wall. The switch was a dual button on/off toggle switch from what it looks like. I removed it to install a new flip switch and put a nice face plate on it. However I noticed that after testing the copper leads with my voltometer, I find that the circuit reads 40VAC. I have no idea what this outlet does at this point in time since the old switch was broken.

Will my 15A-120VAC switch I bought at Home Depot tonight work well as a replacement?

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Yes, a new 15A-120VAC switch will certainly work in a lower-voltage application.

Regarding the voltage across the switch leads, 40VAC is a very mysterious reading. Could the current actually be DC (for a thermostat perhaps)? I'd want to figure out what the switch was supposed to control:

  • Visually trace the wiring from the switch, if possible
  • Are the wires normal 12 or 14-gauge thickness? If so, it's probably supposed to be a light or outlet switch.
  • Can you isolate the circuit by process of elimination? Are there other lights or receptacles on the same circuit that can give you a clue about what this switch is for?
  • How many wires are available in the switch box? If there are only two, then check the voltage of each against ground. One should be 120VAC (supply) and one unenergized.
  • You can check your hypothesis that the old switch was broken by testing for continuity in the switch's ON state.

It's entirely possible that this switch is wired to a circuit that is no longer useful, or perhaps that the wires aren't actually connected to anything and what your meter is picking up is an induced electrical current due to current in adjacent cables. Without determining what this switch was originally intended for, it will be hard to explain the voltage reading, and hard to know whether the old switch was broken, or whether the wiring was disconnected.

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I can't/don't want to trace any wires since the house is old (asbestos) and I just really don't care that much. I tried the process of elimination, and have come to the conclusion that it must have controlled the old thermostat that used to be there. No other lights/outlets seem to be affected by this switch. The only reason I wanted to install a new one was for aesthetics. –  Styler Aug 21 '11 at 20:36
    
Could still be some old 40 volt lighting hidden away somewhere that you haven't found yet. –  Brian Knoblauch Aug 22 '11 at 19:43

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