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Thread: Two wires on one screw

  1. #1
    DIY Junior Member arctic's Avatar
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    Default Two wires on one screw

    OK, so I was looking in a box today because I thought I heard a noise. There are two light switches and one receptacle on this box. The one light switch has a red wire fastened on a screw at the top and a black wire fastened to the screw at the bottom. The other light switch is the same however there are two black wires on the bottom screw.
    There is no evidence that I can see of any arcing, burning or melting in the box. I am not even sure if the noise I heard, a clicking noise, was from the box (but it was in that vicinity)- anyway my question is: are there any issue with having two wires on one screw on the light switch?

    I know nothing about this stuff so I just look at it, never touch it or take it apart- I learned that lesson the hard way a while back.

    Any thoughts?

  2. #2
    Test, Don't Guess! cacher_chick's Avatar
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    2 wires stacked under the head of a screw is not a proper installation.

    A wire nut and 3rd short piece of wire should be used to make a "pigtail" connection.

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  3. #3
    DIY Junior Member arctic's Avatar
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    OK Do you think there is a danger to this? I spoke to a local electrician, he said it was the lazy way of doing it, but if it has caused me no problems, which it hasn't, then to leave it. He thought the clicking sound I was hearing might be the fridge "de-energizing", as it is about 5 feet away from the area I heard the noise.
    I have been in the house for over 2 years and the switches, lights, receptacle have all worked fine. No tripped breakers or anything else either.

    I don't know what to think, or if I should take any action (which would be calling a professional because I could not do this myself).

    What are the professionals opinions?

  4. #4
    Test, Don't Guess! cacher_chick's Avatar
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    It has probably been like that since the house was built and it hasn't caused any issues. Are you going to pull out all of your switches and receptacles to inspect them? Are you doing this with the power on?

    I would not consider it to be a issue worth calling in an electrican, but when the switch goes bad or if one day I was changing colors, then I would fix it when I installed the new switch.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    New houses are required to have arc fault circuit breakers...if you had them, they might trip. Is there any evidence of arcing (often black or pitted) on the switch? If so, fix it. The average switch doesn't have a big load on it, so the wires don't heat up and cool off with the load changing much. If it was controlling a big load, you'd likely have a problem as that expanding and contracting from the heat tends to loosen the screw...with one, properly tensioned, it'll stay tight; with two, probably not.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    DIY Junior Member Throwbax's Avatar
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    Cool 2 Black wires connected to 1 screw, plus another single black wire in light switch.

    Hello
    I recently bought an older house (built in early 1960's)
    I have a 3-way pole Switch for a Dimmer I'd like to install (2 switches for hallway light fixture). I don't own a circuitry tester, to test the wires.

    I have this same issue with 2 black wires connected in the same screw, (lower left screw side of the switch)
    and another black wire connected to the upper left screw and 1 red wire.
    I have a red wire connected to the upper right side of the switch screw. There is no green wire, however I can connect the green wire (on my new dimmer switch) to the back of the box because of a screw.
    My Question is;
    I'm curious as to know whether the 2 connected black wires in the same screw on the left side is the Black wire connection I need to make?
    And the single black wire in the screw above the 2 in the box is the White wire I need to connect to my new dimmer.

    Thank you in advance.

    Throwbax

  7. #7
    DIY Senior Member kreemoweet's Avatar
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    Sorry, all of us out here on the internet have circuitry testers of our own, but we can't figure out how
    to apply them to YOUR wires. Maybe you should splurge on the twenty bucks or so it costs for a tester,
    and then study up on the instruction sheet that came with your new switch. I CAN tell you one thing: making
    any assumptions at all based only on the color of a wire's insulation is a sure route to serious UNHAPPINESS.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you're replacing a 3-way with a new 3-way, they are often labeled so making the new connection is fairly straight forward. BUT, keep in mind that, depending on how it is wired, it may not have any neutral wires in the box and none would be connected to a mechanical switch. You may need to run a new cable to get one. For a non-electronic device, all you need is common, traveler, and switched (can't remember what they actually call them!) - no neutral is switched, you always switch a hot.

    Plus, it is NEVER a good idea to have two wires under the same screw - it's impossible to get the connection properly torqued down, and it will loosen eventually, creating problems.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    Moderator & Master Plumber hj's Avatar
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    since we do not know HOW the switch manufacturer made his switch, we do NOT know whether the "traveler" wires connect to those two top terminals, but you do NOT need a "white" connection for the switch but you probably have black, white, and red wires because that is how they make the cable.
    Licensed residential and commercial plumber

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