Strange voltage reading at unknown switch

Hi,
I'm currently helping my girlfriend figure out what a switch in a spare bedroom actually goes to, however I ran into a problem with voltages on the switch.
Here's the setup:
1 - 12-2+G 1 - 12-3+G going into the box.
Black from 12-2 goes to top screw poll of switch Black from 12-3 goes to top push fit poll of switch Red from 12-3 goes to bottom poll of switch Whites are bonded Grounds are bonded, pigtail not connected to anything

it is a 3 way setup, I know the switch isn't a 3 way switch, so there's no real way this should work anyway. I know it doesn't go to any of the outlets in the room as they get a full 120V on top and bottom at all times.
My concern is the voltages I'm reading on the switch.
With the switch in the off position I get:
H to N: 84.8V H to G: 121.9V N to G: 27.9V
With the switch in the on position:
H to N: 0V H to G: 121.9V N to G: 121.9V
I'm confused. Anyone out there know what could possibly be going on with this switch?
Thanks.
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Hi,
I'm currently helping my girlfriend figure out what a switch in a spare bedroom actually goes to, however I ran into a problem with voltages on the switch.
Here's the setup:
1 - 12-2+G 1 - 12-3+G going into the box.
Black from 12-2 goes to top screw poll of switch Black from 12-3 goes to top push fit poll of switch Red from 12-3 goes to bottom poll of switch Whites are bonded Grounds are bonded, pigtail not connected to anything

it is a 3 way setup, I know the switch isn't a 3 way switch, so there's no real way this should work anyway. I know it doesn't go to any of the outlets in the room as they get a full 120V on top and bottom at all times.
My concern is the voltages I'm reading on the switch.
With the switch in the off position I get:
H to N: 84.8V H to G: 121.9V N to G: 27.9V
With the switch in the on position:
H to N: 0V H to G: 121.9V N to G: 121.9V
I'm confused. Anyone out there know what could possibly be going on with this switch?
Thanks.
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Max wrote:

It doesn't sound like you have a three way switch, it sounds like a regular single pole switch. The top screw terminal and the top back wire port are electrically the same, someone was just too lazy to pigtail and connect both wires to the screw terminal.
Seems most likely that it is feeding a split duplex outlet with one half switched and the other constant. The 12/2 would be the incoming power feed and the 12/3 would be the feed out the the outlet with the red the switched leg and the black the constant leg.
The voltage readings are indeed odd and likely represent a serious ground fault or wiring error at a location other than the switch box. The wiring in the switch box sounds reasonable although the ground pigtail you mention should go to a ground screw on the switch or in the box if it's metal. Not likely the source of the strange voltage readings though.
I'd start by disconnecting power, disconnecting the wiring in the switch box and then powering up and checking the lines separately. You should find the 12/2 to be the incoming power with voltages as you would expect, 120V nominal H-N and H-G and 0V N-G. I would expect everything on the 12/3 side to read 0V with it disconnected from anything in the switch box. Any readings that don't match this will give you a direction to search in.
As always be careful and don't get fried.
Pete C.
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SNIPS

Good cogent analysis.
--
Jim McLaughlin

Reply address is deliberately munged.
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It is not a 3 way switch set. The two blacks are joined together at the switch instead of being spliced together with a wire connector. Usually the black is feeding through on the 3 wire and the red is the switch leg. If you disconnected all of the wires you may find that the 2 wire is the feed and you should have approximately 120 volts between the white and black. See what doesn't work after you disconnect the black wires. There could be a ceiling fixture box that has been covered over. It is possible that there was a switched receptacle at one time and the previous owner changed it to be on all of the time. Open each outlet and look for the red wire.
How old is the house?
Stop using a voltmeter and get a pigtail socket and a light bulb and check every outlet top and bottom again.
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Digital voltmeters are often misleading. The only thing you generally need to know to troubleshoot a power system is whether or not full voltage is present between two wires. Sometimes you need to know whether or not there is continuity between two wires. A test light and a continuity tester will work much better.
The connection of the black wires is merely to feed power constantly thru that box to somewhere else and the red wire is the switched hot wire. It may go to a receptacle and the receptacle may be split so that one side is always on from the black wire while the other side is controlled by the switch. Don Young

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Max wrote:

I suspect you may be getting a false voltage reading. Modern meters are too sensitive. Put a high resistance load across those circuits and likely your odd voltages 27.9 & 84.8 will drop to zero.
--
Joseph Meehan

Dia duit
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Could have been a 3 way set up. Or the 12-3 goes to a plug. The hot side of the plug is clipped so one plug is switched (hot fed with the red lead) and other plug is always on (fed with the black). So you could plug and lamp in one and a clock radio in the other.
Check both top and bottom of each plug to see if this is the case.

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See ====> below

====> Typically these are connected together inside the switch. It was probably used as a splice.

====> Makes sense; this would likely be the "hot" lead to whatever it is/was meant to control.

====> Normal.

====> Not sure what you mean; probably they aren't using the earthing connection? Probably of no import.

====> Try disconnecting either the black and/or red and see if anything changes. Be sure you open the breaker before going in there, though.

====> What you're seeing is typical of what they call phantom voltages. Most meters have a high enough input impedance that they read voltages induced into the wiring from nearby wires, out of the air, whatever. It's not unusual and almost always not a problem. If you open the breaker to those wires you'll probably still see phantom voltages even though the breaker is open. Same for any other outlets/switches.

====> Any chance you've mixed up your terminology here? H to N should be 120V and H to G 0V. N (Neutral) is the white wire. The 120V should be appearing across the H to N wiring (black/white and/or red/white). G is Earth Ground, and is usually a bare wire though sometimes they might have green insulation. IFF the G is connected, you should also read the 120V between H and G. N and G get tied together back at the breaker box. Do NOT tie them together inside the receptacle/switch!!!!!!!! I'ts likely OK if the G isn't connected to anything, as long as no equipment is connected that requires a system ground (third wire).

====> I think nothing. You could stop by RS and pick up a cheap tester for this purpose if you want to prove they're phantom voltages you're reading. Or, you coujld rig one up yourself by using a light bulb. With a light bulb across a phantom voltage, the voltage will drop to a 0.0V reading immediately as the light is attached.

Luck figuring out what it controls!
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