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I started to replace a light fixture in a closet and discovered that although 2 wires (each containing a black, white, and ground wire) come into the box, only the white wires are connected to the fixture (one on each side). Black and ground wires are connected separately, but not attached to the fixture. See picture. The light is controlled by a single switch. How can the light work (it does if the switch is turned on) without a black wire?

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    Hello, and welcome to Home Improvement. If that picture shows the original setup, then my complements: most people take everything apart before start to think about documenting. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Aug 31 '19 at 19:43
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It looks like a switch loop that was wired backwards. The always hot of a switch loop should be white and the switched hot should be black with a marking on the white always hot to identify it as a hot. I have seen this many times, it is easy to fix by swapping the white and black at each end and then marking the white wire with a black sharpie or black tape (any color other than green, gray or white would meet code)

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    You don't have to reverse the white and black on switch, right? Just at the ceiling box for the light, right? – Jim Stewart Aug 31 '19 at 23:02
  • Always hot has to be the same at both ends , changing the white and black at each end would be the only way it would work and be in compliance with code, I don’t remember the year it changed but it was a long time back. – Ed Beal Sep 1 '19 at 1:39
  • If the wall switch is just a simple mechanical switch, the contacts are symmetrical. It does not matter which way the current flows through the switch. Right? – Jim Stewart Sep 1 '19 at 10:13
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One of the white wires is a neutral, and the other white wire is a "switched hot" coming from the wall switch. [EDIT: This was formerly an acceptable practice if the white was marked black, but is not now.]

(Of the two black wires one is the line hot (always hot) and the other black wire carries the always hot to one side of the wall switch.)

When a white is used as a switched hot (or an always hot) it is supposed to be marked with a ring of black tape to indicate that it is sometimes (or always) hot.

IMPORTANT FOR SAFETY

Notice that the two contact screws are different colors--one is brass (gold) colored and one is silver colored. When you connect the wires the neutral is to be connected to the silver colored screw and the switched hot to the brass colored screw. This specification is so that the switched hot is connected to the center contact in the screw base and the neutral is connected to the outer threaded section.

Before you remove the white wires from the existing fixture use a black marker on the white wire connected to the gold screw on the old fixture. Connect this marked wire to the gold screw on the new fixture. It would be best if you had a non contact voltage tester to confirm which wire is switched hot, but if you don't have one one could assume the existing fixture is wired correctly.

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    Not anymore. You've been obliged to use the white for always-hot for quite some years. – Harper - Reinstate Monica Aug 31 '19 at 22:24
  • I agree it has been a long time , on vacation do you have a code ref? Don’t have my books until next week+ – Ed Beal Sep 1 '19 at 1:40

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