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I have an old switch box that has a single cable coming into it with a black wire and a white wire. I've confirmed with a voltage tester that the black wire is hot. Before installing an electronic remote switch, I want to make sure that the white wire is a neutral.

My doubt comes because my house has a number of switches that are set up as a switch loop, where a single cable comes into the box, and one of the conductors is always hot (usually fed by power coming into the fixture) and the other conductor is used as a switched hot. In this old switch box, I'm wondering whether the cable is an abandoned switch loop where the always hot conductor is still connected but the switched hot doesn't connect to a load.

Is there a way to test this? I believe that measuring hot to neutral on a multimeter should give 120V, correct? What would measuring hot to switched hot give? I'm just looking for a way to safely and definitively confirm that the non-hot wire is a neutral on the same circuit as the hot wire.

  • Is this a NM cable or a BX (armored) cable? Can you post a photo of the inside of the box? Can you find where that cable leads? – ThreePhaseEel Jan 3 '17 at 23:45
  • BX. I'll post a pic when I get home. Unfortunately it won't really be possible to definitively determine where the cable leads because all the walls are finished. – ajs317 Jan 3 '17 at 23:49
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    Measure the voltage from the white wire to the box then, it should be near 0 volts if it's a neutral. – ThreePhaseEel Jan 3 '17 at 23:49
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    It'll show up as an indeterminate reading, perhaps several volts, perhaps close to 120V. – ThreePhaseEel Jan 4 '17 at 0:00
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    Is the hot wire connected to a known breaker? Turn the breaker to off, connect a receptacle to the pair and plug in circuit tester and turn the breaker on. – Jim Stewart Jan 4 '17 at 2:42
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Short answer, check for continuity between White and Ground.

If you have 120V between White and Black, and White is tied to Ground (check for close to 0 ohms resistance from white to ground), then White is neutral, Black is Hot.

There are some miswired options that may be in play (ie switched neutral), but hopefully this was wired by someone who knows what they are doing.

To answer your other questions: I believe that measuring hot to neutral on a multimeter should give 120V, correct? Correct

What would measuring hot to switched hot give? 120V, assuming that there is a functional light bulb or functioning device plugged into the switched socket. If not, then you would read 0v, or some floating voltage much less than 120V.

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