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Old light switch connected by two white wires. There are two black wires capped off in the wall.

The new light switch has one white wire labeled for a neutral line and two black wires labeled line/load.

Do I connect the two white wires from the wall to the two black wires on the new light switch, and then connect the white switch wire to the two black wires that are capped off in the wall?

Close-up of switch wiring

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  • Its an old place. They are copper wires. There is also fuses and breakers here.
    – Marc
    Oct 3 '21 at 2:09
  • 1
    Can you measure the voltage from the black wires to the ground that's visible ducking behind the cableclamp in the back right of the box please? Oct 3 '21 at 2:26
  • I will later today when I pick up a new meter. Its been wired like this for at least 3 years.
    – Marc
    Oct 3 '21 at 8:33
  • Note that the way the ground is connected (red circle) is incorrect. It should be connected to the box with a screw in the hole in the green circle. It should be either an 8-32 or 10-32 screw (can't remember which, but they sell 'em at the big box stores - they're green). The other ground wire in the other cable should also be attached via a screw.
    – FreeMan
    Oct 4 '21 at 12:59
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The picture is very helpful. It clearly shows you have cables (rather than conduit) and not a switch loop. (A switch loop would use a single cable, not one wire from each of two cables). Which makes it look like it is switched neutral. Which is not a good thing.

You need to determine 100% whether this is "switched neutral" or "wire colors used incorrectly".

You should have 4 wires:

  • Hot from panel
  • Switched hot to light
  • Neutral from panel
  • Neutral to light

The normal (code-compliant) setup is hot & switched hot = black, neutral = white. You may have that - and the switch incorrect. Or your wires may be swapped - and the switch correct.

As noted in a comment, checking voltage from each wire to ground is one way to determine which wires are hot (~ 120V to ground) and which wires are neutral (~ 0V to ground). But in addition, we need to know which cable is going to the panel and which one is going to the light, in order to determine line (hot) vs. load (switched hot).

  • Turn off power.
  • Disconnect all 4 wires and make sure they can't touch - ideally put a separate wire nut as a cap on each one.
  • Turn on power.
  • Use a non-contact tester to determine which wire(s) are hot.
  • Turn off power.

Report back here. But assuming that you can 100% determine the hot wire:

  • If that wire is black then:

    • Black from that cable = line for the new switch,
    • Whites get connected together with a wire nut
    • The other black = load for the new switch.
  • If that wire is white then you have bigger problems to be resolved before installing the switch because hot/white and black/neutral is a big problem.

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  • 2
    Thank you so much for that detailed explanation. I will get the meter tomorrow and test it and report back. This has been wired like this since my girlfriend moved in here 3 years ago.
    – Marc
    Oct 3 '21 at 8:31

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