This might be better asked over on electrical engineering, but it concerns household voltage and I know we have a some smart electricians on this site, so maybe one of you might be able to shed some light.

One of my sump pumps and my A/C condenser fan both broke this evening. It seemed an odd coincidence and we'd had a few mild surges from a thunderstorm passing through, so I wanted to check the voltage at an outlet to see what was coming through from the grid.

I picked a random outlet in the kitchen and stuck the probes in, getting a rather boring 123v between the poles. So I guess the motor issues really were just coincidental. However, I noticed as I was removing the probes that I still get a reading with only one connected. If I leave the neutral connected and hold the other probe in the air, I get a reading of ~2v. If I do the same with only the hot terminal connected, the reading is almost 20v.

Can my outlets really just dump that much voltage into (literally) thin air, or is something else going on here? Is the air highly charged from the storm (wouldn't we feel it?) What could cause these readings?

Thermostat is reporting 68% relative humidity if that makes any difference.

  • 1
    A.C. voltage can be read by sensitive volt meters even with only one side is connected; I.e incomplete circuit or open circuit. The reason is the other end of the meter which is floating in thin air actually capacitively coupled to the ground. By this way the meter reads voltage as the resultant of line voltage minus voltage drop across the stray capacitive reactance which might be in orders of mega-ohms. Commented Sep 27, 2018 at 2:37
  • Interesting! Would I get a lower reading on a dry day then?
    – CactusCake
    Commented Sep 27, 2018 at 2:42
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    Stray capacitance will vary according to humidity and temperature. So you might be right. Commented Sep 27, 2018 at 2:47
  • You may also get different readings depending on how much of your skin is touching the insulation on the meter's probes and wires. If you stick one probe in the outlet and leave the other on the floor, I bet you'll get a lower reading than if you're holding the disconnected probe in the air. (Human bodies are better at capacitative coupling than most flooring materials.)
    – mrog
    Commented Sep 27, 2018 at 22:41

2 Answers 2


Volt meters are rated in mega ohms per volt. Most meters today are at least 1M ohm with many being 10M this so the meter won't load the circuit and provide an inaccurate reading. Since the impedance of the meter is so high it can detect voltage with almost no potential or what we call phantom voltage. There are low Z or low impedance meters out there that put enough of a load that phantom voltages won't be read, but in electronics trouble shooting low Z meters affect the circuit that's why most meters are high impedance.


Sensitive voltmeters, including most cheapies, will read stray voltages where no voltage of importance is present. It is picking up stray EMF garbage from wires, radio, or you.

Treat it like an idiot savant, write the numbers down and play them on a lottery ticket. That is probably the best use of such readings.


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