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I am using a multimeter to find which of two black wires in a ceiling light fixture is hot, so I can install a new ceiling fan. However, when I touch the black probe to the metal box and the red probe to either of the two wires, I get no reading on either wire. I do get a 120V AC reading across the two wires.

Likewise, on other outlets throughout the house, I get no reading on the hot slot if the black probe is in the third ground slot or is touching a screw, but I do get a 120V AC reading if the black probe is in the neutral slot and the red probe is in the hot slot. The house was built in 1950 and has three-prong outlets, but apparently they aren't really grounded.

My question is, how I can tell which of the two black wires to the ceiling light fixture is hot?

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    do you have a ground wire in the box ?
    – Ruskes
    Jul 19 at 19:54
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    Do you only have two black wires in the ceiling box, or two white wires as well? Are any of these wires connected to each other? This related question might help. Jul 19 at 19:56
  • It looks like there are two white wires tied together. I have uploaded some photos. !Valid XHTML. !Valid XHTML.
    – kwksdy
    Jul 19 at 20:37
  • Are there any switches that control (used to control? supposed to control?) this fixture? If so, (a) upload pictures of the switch box wiring and (b) do the readings in the ceiling box change when the switch is on or off? Also, try a NCVT and see which wire(s) that shows as hot. Jul 19 at 21:13
  • Not to answer your question, but you really should fix those outlets. If there is truly no ground in the box (very likely in a 1950s house), you can install GFCI outlets (make sure to mark them "no ground").
    – DoxyLover
    Jul 20 at 0:22

3 Answers 3

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The two white wires tied is a Code violation, in two ways. First, the switch is switching neutral - which means the lamp socket is always hot. That's never OK.

The white wire which is in the same cable as the hot black wire, is true neutral and must connect to the lamp's neutral.

Second, this is a switch loop, where (on the switch leg) the white wire is being used as a hot. That was banned in 2011 generally, but for pre-2011 installations the white wire must be used as always-hot. That is to make it easier for people with voltage detectors to realize this black/white pair are NOT hot and neutral.

Therefore, the always-hot (supply) black wire must splice to the white from the other (switch loop) cable. This won't connect to the lamp.

That means the remaining wire will be black, and connects as expected.

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  • Thanks. This is helpful. I am not excited about using a system that is out of code, so I'll call an electrician to find out what it will take to fix it.
    – kwksdy
    Jul 20 at 1:50
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    @kwksdy Don't think using switching neutral was ever in code.
    – crip659
    Jul 20 at 11:02
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    @kwksdy Current Code requires two things: #1 when white is used as hot, mark it with black tape or paint or shrink tube. And #2 a /3 cable (black-white-red) on a switch loop, with white as actual neutral. This is for the benefit of smart switches and motion sensors, most of which need neutral, and also modern LED dimmers, because LEDs do not play well with "leaking current through the bulbs to power the dimmer". I for one wouldn't worry about it unless I planned to use a smart switch. Jul 20 at 16:12
  • @Harper-ReinstateMonica For just changing from switched neutral loop to switched hot loop, which code version does OP need to obey? I imagine this is consider a minor change and could be grandfathered in an older code that does not require a neutral to switch.
    – crip659
    Jul 20 at 20:59
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Use a neon-probe.
Its that weird screwdriver thing, that is not a screwdriver.
You touch the point to the wire in question, and hold your thumb on the metal end. The neon vial inside will glow when touching a live wire.
They go for cheap in any decent home improvement store.
Make sure you are using a Neon Probe and not a screwdriver with a through-handle metal core, because that would... hurt

Alternatively, if you cannot be bothered to go shopping, you can use an extension wire with known good ground to measure against. Just connect one of voltmeter leads to radiators/water pipes/any large object buried into ground and test that way.

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    These are officially called Non-Contact Voltage Testers. The modern, well-designed ones such as this Klein are far, far safer than the old neon testers. They use LEDs instead of neon, they include audible alerts, don't require direct contact and, most importantly, have no exposed metal. Jul 20 at 14:06
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    @manassehkatz-Moving2Codidact Good addition, but the one Thomas is describing is a contact voltage tester :)
    – MiG
    Jul 20 at 14:08
  • Thomas' comment was also about using it as an insulated screwdriver (rather than a regular one) to reduce the chance of accidentally getting shocked (there's always the possibility of an oversight). The non-contact kind can't help there.
    – MiG
    Jul 20 at 14:13
  • @MiG Thomas's answer was referencing a commonly confused old tool instead of the modern, safer, equivalent. I am saying don't use those. Actually, I did find one Gardner Bender at Home Depot that is still a contact tester, with a small actual screwdriver tip. But even that is LED and not neon any more. But I wouldn't recommend that one either. Non-contact design with no exposed metal is inherently safer. Which matters a lot when you are dealing with live wires. Jul 20 at 14:27
  • I think both of us are talking about different sections of his post here. For the first comment, the guy is clearly not referring to a non-contact voltage tester in the first section, for the second comment, in the bold part he refers to the added safety of using it as an insulated screwdriver. You can both use an actual non contact voltage tester to check, and the other kind to (literally) insulate you from oversights and other oopsies. Sure, could be written more clearly, but I like the idea of some extra protection (so using it as a screwdriver rather than a voltage tester).
    – MiG
    Jul 20 at 14:36
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In addition to Harper's excellent answer, you should probably double-check. You need an extension cord that will reach from an outlet to near the ceiling box. Then you can check with your multimeter between the neutral (wide slot) on the extension cord and the two black wires.

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    Thanks. The extension cord gave me the answer I was looking for. I now know which black wire is hot and which isn't. That is enough to install the fan, but it doesn't address the code violations Harper referred to.
    – kwksdy
    Jul 20 at 1:47
  • @kwksdy Will need to change from neutral switching to hot switching. This might cause other problems in the rest of the electrical system, depending on how many others mistakes were made. Turn off the breaker, find the white from the switch and connect it withe the always hot from the panel. Now the switch black connects to fan black and fan white to panel neutral. That box should now be in the older code.
    – crip659
    Jul 20 at 14:09

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