Adding new light and switch

Discussion in 'Electrical Forum discussion & Blog' started by Steven JP, Jul 5, 2020.

  1. Steven JP

    Steven JP Member

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    Apr 8, 2019
    Location:
    Virginia
    I need to add another switch to a current 3 way switch system. I attached a diagram and hopefully I can explain what I am trying to achieve. The green is the white common wire in the drawing. The faded red is a mistake I couldn't erase. Switch #1 is at the bottom on the stairs. The wire runs up through the wall and is not accessible until it gets to the attic and then subsequently ties into the light. Switch #2 is at the top of the stairs and the wires that go to the light are all behind drywall until it gets to the attic. I have access to the wires right before they tie into the light through the attic floor. How can I tie into this current situation?
     

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    Last edited: Jul 5, 2020
  2. Reach4

    Reach4 Well-Known Member

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  4. Steven JP

    Steven JP Member

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    Yes I understand that but this in this diagram you provided the power goes through all three switches before it get to the light. In my situation the power goes to switch #1 then to the light and then to the final switch in which case i can't put a switch in the middle. I cant run any new wires from the last switch back to the new switch. How would I wire a 4 way switch without doing that? Thanka for your help.
     
    Last edited: Jul 5, 2020
  5. Reach4

    Reach4 Well-Known Member

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    Location:
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    They want you to avoid having white wires carrying switched hots. Mark the ends of the white hot (or sometimes hot) wires, and that still might not be officially enough. Not sure.

    See dwg. Black and gold could represent whatever colors they use for the screws on the 4-way.
     

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  6. Bannerman

    Bannerman Well-Known Member

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    Location:
    Ontario, Canada
    With 3 and 4-way circuits, there is no true 'OFF' position as each switch will only direct power to an alternate path. If the path to the light is not complete, then the light will not receive power, but changing any switch position should result in a complete electrical path to cause the light to illuminate.

    If I understand correctly, the 3-wire cables run through an octagon box in the attic, and only a 2-wire cable runs to the light fixture octo box.

    Since the 3-wire cables feed through the octo box in the attic, I will assume you will be able to install a 4-wire cable from the intended additional 4-way switch location to the octo box in the attic. Connections from the 3-way switches can then be reconfigured within the attic octo box to connect the new 4-wire cable without needing to even remove the 3-way switches from their boxes.

    The 4-way switch does not need to be physically located in the middle, but a length of 4-wire cable will be allowing the electrical connections to be located in the middle between the two existing 3-way switches, regardless of where the 4-way switch is physically located.

    Here is a YT video showing the appropriate connections:



    Although you maybe connecting the white in the 3-wire cables to switches, that white wire is no longer a 'common' but is a hot conductor (traveler) and so should be indicated as such by wrapping each end with an alternate color tape.
     
    Last edited by a moderator: Jun 4, 2021
  7. wwhitney

    wwhitney Well-Known Member

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    Location:
    Berkeley, CA
    The 4 new conductors (plus ground) can be run from any of the 3 existing boxes (switch, switch, and light) to the new switch location. Whichever is most convenient and has enough room (see below). Could be either a /4 or a /2/2 NM cable, or you could run two separate /2 cables. In which case it is required that both cables enter a metallic box through the same knockout hole, and it is best practice, although not required, to route the two cable identically (e.g. stack them up the same staples, using staples listed for 2 cables).

    One thing to watch out for is box fill. Right now the octagon box has a required volume of 7 allowances (3 conductors to each switch, one allowance for the grounds), which means 14 in^3 for #14 wiring, or 15.75 in^3 for #12 wiring. By adding 4 more conductors you're at 11 allowances, which means 22 in^3 for #14 wiring, or 24.75 in^3 for #12 wiring. A 1-1/2" deep, 4" octagon metal box has a volume of only 15.5 in^3, while a 2-1/8 deep octagon has a volume of 21.5 in^3. If the canopy of the light has a volume stamped on it, then you can add that volume to the box volume.

    Similar calculations apply to the two boxes with switches, but the device there counts as 2 allowances. So the switch with the power feed needs 8 allowances currently, and the other switch needs 6 allowances.

    Cheers, Wayne
     
  8. Steven JP

    Steven JP Member

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    Running wire from #1 to the new switch would involve a lot of drywall removal. Is there any other way I could make this work?
     
  9. wwhitney

    wwhitney Well-Known Member

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    Location:
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    As I mentioned, you can run a new 4 conductor cable starting from switch #1, from the light, or from switch#2. Your choice based on what's easiest and has enough room in the box.

    And as Reach4 suggested, you could instead intercept either the existing cable from switch #1 to the light, or the existing cable from the light to switch #2 (which his diagram shows). Doing that is complicated in practice, unless the cable runs right through the location you want switch #3, and it has about 12" of slack you can make use of. Otherwise intercepting a cable requires at least one extra junction box.

    Also, if running new cables is hard, you could use a wireless solution.

    Cheers, Wayne
     
  10. Bannerman

    Bannerman Well-Known Member

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    Ontario, Canada
    While the 4-way switch could connect to either 3-way switch box as mentioned, I had recommended the octo box in the attic as it sounds as though the existing cables from the 3-way switches already run through it. Fishing a new cable to the attic from the 4-way switch location may also be fairly simple and may not require any drywall removal.

    If the existing attic octo box is filled near capacity, an additional octo box mightbe easily added directly beside the existing and the wiring from the 3-way switches transferred to the new box which the new wiring from the 4-way would also run to.
     
    wwhitney likes this.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Retired Systems engineer for defense industry.
    Location:
    New England
    If you need more room in the box in the attic, and it opens up into it, instead of adding another box, you could use one of these...
    steel-city-boxes-brackets-551511234-25r-64_1000.jpg
     
  12. WorthFlorida

    WorthFlorida The wife is still training me.

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    Retired
    Location:
    Orlando, Florida
    I think this is the wiring you need. In the attic disconnect the wires from one of the switches at the light. Run a new cable (NMC 14-3) from the attic to the 4 way switch location. You'll need to add a junction box in the attic. From the attic side it may not be possible to extend the junction box easily.

    Google search "4 way switch wiring". This pretty much is Bannerman's suggestion.

    Screen Shot 2020-07-06 at 9.42.34 AM.jpg
     
  13. wwhitney

    wwhitney Well-Known Member

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    Location:
    Berkeley, CA
    If a single new cable is installed from the new location (switch #3) to the light, it needs to be 4 conductors.

    But as your drawing shows, if a new cable is run from the new location (switch #3) back to switch #2, then that cable only needs to be 3 conductors.

    Cheers, Wayne
     
  14. Steven JP

    Steven JP Member

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    Location:
    Virginia
    I believe reach4s diagram will be the easiest for me. I have access to all the wires. I might just have to make my junction box at the light bigger because I'll be making all my splices there. Thanks so much!
     
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