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pictured is the switch and wires coming from wall

Our den has a switch that does not seem to operate anything in house. There’s not a ceiling light in room and it does not affect the four outlets in the room. It runs from a 15 A circuit in the breaker box. I decided to remove the switch and use the wire to power six can lights I installed in the ceiling. They are smart lights and I have a smart switch to operate them. I hooked one of the can lights to the wires connecting black to black, white to white, ground to ground but nothing happens. I used a non-contact volt meter on the wires and both wires turned it red. Not sure what to do from here?

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    I take it the depicted cable is the only cable in the wallbox? Jan 29 at 0:45
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    Non-contact testers are great for safety. But you may be seeing phantom or induced voltage. Get a real multimeter, at least Class II, and check voltage from black and white to ground. If one has voltage and the other doesn't then this is likely a normal switch loop. If both show ~ 120V then it gets more complicated. Jan 29 at 1:34
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    I'll suspect an "extra" wire on one (or more) of the outlets, and the receptacles having been replaced but the tabs not broken - unless they are on a different breaker than this switch is. That's given no overhead light...(or junction box, unless it was improperly concealed.)
    – Ecnerwal
    Jan 29 at 1:40
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That's a switch loop. Bog standard. Perfectly legit until recently. The white should be unswitched hot, and the white should have marking of tape, paint or heat-shrink in a hot color (but often does not.)

You need to find the box that the switch loop comes from, where you will find its white connected to a black, and where there is normally also a white neutral.

Color does NOT imply function in cables. Color may guide you, but not all whites are neutrals.

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And in that socket Ecnerwal mentioned... you will find that somebody replaced the receptacle, probably for aesthetic reasons or to get a USB recep... and they did not know what they were doing, and they did not notice that the "tab was broken off" on the original recep.

They did not break it off on the new recep.

As a result, they "shorted out" the switch, making both sides of the switch hot at all times, just as you are measuring.

Once you identify the socket Ecnerwal describes by the other wires in the box (e.g. a white wire with blacks)... go ahead and snap the tab off between the brass screws. Voilà, the switch will start working again.

Turn off the circuit breaker and verify power is off before opening any box.

Also, while you're in there, wrap a few loops of black electrical tape around that white wire. It's supposed to be marked to indicate that it's a hot.

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    Maybe it would be possible to reconnect the white to neutral at the outler, to actually get hot and neutral at the switch location, like OP seems to want.
    – jpa
    Jan 29 at 15:51
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    Should "and they did notice that the 'tab was broken off' on the original recep" be "and they did not notice"? I'd expect most people who replace without breaking the tab on the new receptacle probably didn't even notice it on the old one. Jan 29 at 22:12

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