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Thread: Why would there be 2 wires connected to one light bulb?

  1. #1
    Unregistered Guest

    Default Why would there be 2 wires connected to one light bulb?


    I know only the basics with electricity, and just bought my own home.

    The previous owner half finished the attic. He sheetrocked everything, but in two spots there are just wires out of the sheetrock with temporoary lights hooked up to them.

    The first is on a two way switch (what i would call it) where you can turn the lights on/off from two different switches, one at the top and one at the bottom of the stairs. There are 2 lights attached to these switches--a track light, wich only has 1 set of wires (the black, white and ground) connected to it. the other has 2 sets of wires, with the blacks together to the black of a temporary light socket and the two whites together to the white of the temporary light socket. then the two grounds are together. Seems wierd to me--why would there be two wires????

    The second is even more confusing to me. There is a light switch that controls a temp light. there is only wire coming out of the switch, so the power must be in the ceiling. but out of the ceiling is coming 3 wires (i.e., 3 wires that contain the 3 electrical wires, not sure of what to call them). two of the whites are attached to the white of the light socket. then, the other white of the 3rd wire is attached to the black of the light socket. all three black wires are nutted together and the three grounds the same. my guess is that somehow the light switch is involved here, and there is another outlet in this part of the room that must be involved. just seems like an awful lot of wires............

    thanks for any help.

  2. #2
    Join Date
    Dec 2003
    Kent, WA


    For the first situation, the switch is really a "3-way". I know, seems confusing, but that's what they are called. The switched power from the switch goes to your temporary light on the black and white wire. In order to get to the track light, it needs these same two wires. So that is why the second cable is at the temporary light. Most all of your power outlets are wired like this too -- power in, and one or two power outs to other outlets. The last outlet or light on a circuit segment will have just one black and white wire. If you didn't have the second cable at the temporary light, the track light would have to connect at the switch box (which usually wastes wire).

    The second situation is also common. This is called a "switch leg" and is what is done when always-on power is brought to the ceiling light box first. Yours is wired backwards from the convention, but it still works. When correctly done, a 2-wire cable will be connected such that the white wire goes to the always-on black wire and the other end goes to the wall switch. The black wire goes from the other switch screw to the light fixture brass screw or black wire. The light fixture white and always-on white are tied together. You have a third cable in there which is providing always-on power to something else -- probably a wall receptacle or a light on a different switch.

    The switch leg method isn't used as much now as it was in the 60's. I much prefer having power brought to the switch first, and only take switched power to light fixtures. But both ways are legal. It is also a good idea to tape the white wire of a switch leg black (at both ends). That way, people will know its a "hot" wire and not a grounded white wire.

    Lots of wires in a box is common in order to quickly branch out different directions to power receptacles on all walls. There is a limit to how many wires of a given size can be placed in a box. This is based on a certain number of cubic inches that changes per wire size. The cubic inch capacity of plastic boxes is stamped in them nowdays, and metal ones you look up in a table in the NEC.
    Kent, WA

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