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Electrical - 3 way switch

Discussion in 'Home Improvement' started by Sundog, Feb 11, 2023.

  1. Feb 11, 2023 at 11:10 AM
    #1
    Sundog

    Sundog [OP] Zoom Zoom

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    Helping the in-laws get their house ready for sale, and this I think I know what is going on but want to double check.

    So am replacing a 3-way dimmer. Removed both switches so bare wires as shown below.

    Box 1 has the black HOT that reads 122.6 V. But 2 brown traveler wires are reading 27/28 V.

    Am I remember correctly that this is happening because induced voltage (with very minute current) is from the hot wire being next to the travelers, coupled with the use of a digital (vs analog) multimeter?




    upload_2023-2-11_12-53-0.jpg
     
  2. Feb 11, 2023 at 11:45 AM
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    1lowlife

    1lowlife Toxic prick and pavement princess..

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    What was the dimmer switch dimming?
    An overhead light or speed control for a ceiling fan?

    What was the second switch in the double box used for?

    There should only be one dimmer switch, was it in the single or double box?

    [​IMG]

    It's odd to get the low voltage unless the lighting was some kind of low-voltage track lighting..
     
    Last edited: Feb 11, 2023
  3. Feb 11, 2023 at 12:04 PM
    #3
    Sundog

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    The dimmer switch was dimming 2 overhead lights. No ceiling fan.

    The dimmer switch is in box 1 (single) with the HOT from the panel.

    Box 2 houses the a switch that connects to the dimmer in box 1 (and the light). The second switch in box 2 is a double switch - controlling 1. two overhead spot lights and 2. the upper outlets of the rooms duplex outlets.
     
  4. Feb 11, 2023 at 12:58 PM
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    NWPirate

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    Are you positive the travelers don't junction somewhere between switches?
    An open neutral causes weird low readings like that as well, double check those connections.
    With 120v you shouldn't ever see induction, that's typical for higher voltages.
     
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  5. Feb 11, 2023 at 1:52 PM
    #5
    1lowlife

    1lowlife Toxic prick and pavement princess..

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    What kind of overhead lights?
    Are they still connected?
    Incandescent, LED, flouresent?
    Any possibility they have some kind of capacitor or ballast in them that is giving you that low voltage reading?

    I'm not an electrician, I'm just curious as to what is going on..
     
  6. Feb 11, 2023 at 4:00 PM
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    Sundog

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    @NWPirate
    Are you positive the travelers don't junction somewhere between switches? There isn't anything else for them to junction to. No other 3 way switches even close. The only other one on the floor (also w/brown traveler wires) I powered off at the breaker (not the same breaker), these read in mV, the room I was working in with the HOT on still read at 27/28.
    An open neutral causes weird low readings like that as well, double check those connections. This wouldn't matter with no connections made. The two travelers are stand alone at this point and showing the 27/28V. The line going to the light fixtures and thus connected to the neutral post fixture was reading in mV.
    With 120v you shouldn't ever see induction, that's typical for higher voltages. Maybe I have the wrong term.

    @1lowlife
    What kind of overhead lights? Old style pot/sunken in square light fixtures
    Are they still connected? No, they are the pink wire in the double box and unconnected
    Incandescent, LED, flouresent? LED, but everything is completely disconnected.
    Any possibility they have some kind of capacitor or ballast in them that is giving you that low voltage reading? Everything is completely disconnected

    I did some more digging and found multiple references to this:
    If you're measuring with a typical digital multi-meter (DMM), you're reading so-called 'phantom' voltage, which is from capacitive coupling between conductors. The DMM has such a high input impedance that there is real voltage with virtually no current behind it. Put a wiggy or other solenoid type voltage tester on it, and it'll read nothing. In other words, even a very tiny load would be enough to reduce the measured voltage to zero, but the DMM doesn't even do that since the impedance is extremely high (for this kind of work).

    Also:
    It's capacitively picking up some stray voltage from the hot wire that parallels it. Your DMM has a high-impedance input so there is no place for that induced voltage to go. If you were to place a load on the wire, the voltage would go to near 0V as you would expect.

    Bottom line: It's normal and this is a case where the sensitivity of the instrument you're using (i.e. the DMM) is fooling you.

    Also:
    A digital multimeter will measure voltage without loading the circuit under test. In electronic work were the available current is often small that is an asset. In electric work however it is a better idea to use a solenoid tester of the "Wigington" type. The Square D "Wiggy" http://164.109.40.74/cgi-bin/huggycg...&Ref=6610VT1is is the simplest and usually least expensive version of this type of tester. The virtue of solenoid testers is that they place a small load on the circuit under test. On 120 volt circuits they will draw approximately seven milliamperes of current which coincidentally is just enough to trip a Ground Fault Circuit Interrupter. This prevents the tester from showing voltages that are merely induced into adjacent conductors of the circuit by capacitive effect.



    A friend brought over a voltage tester like described above and it read travelers read 0V.
     
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  7. Feb 11, 2023 at 4:47 PM
    #7
    NWPirate

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    Some guys run the travelers through lighting boxes especially when running individual conductors, like you've got.

    So no voltage on the travelers, everything hooked up and working?
     
  8. Feb 11, 2023 at 4:59 PM
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    Sundog

    Sundog [OP] Zoom Zoom

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    Yes, no voltage when not using the DMM. Everything working. :thumbsup:
     
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  9. Feb 11, 2023 at 5:25 PM
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    NWPirate

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    Nice :thumbsup:
     

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