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Our bathroom fan recently stopped working. Even when the switch is on, it didn't turn at all. No motor hum either, just silence. So we pulled the fan out of the box and tested the voltage in the receptacle: it read only 16V from hot to neutral. I got the same reading from measuring the voltage across the bare white and black wires.

The fan is on the same circuit as the smoke detectors in the house (see photo). I figured this out my having my brother and sister help me out by watching the multimeter reading as I flipped off breaker switches one by one.

The connections, at least to my untrained eye, look fine. So what is causing the low voltage reading? The other bathroom fan receptacle reads 120V like I expected.

I also plugged the 'broken' fan into another bathroom fan's receptacle and it started up fine. Spun somewhat slow, but I think it's a little gummed up after 20+ years of use. So I know the fan is okay, but we still might get another one or at least give the poor thing a good de-dusting.

Thanks in advance for your help.

*Edit #1 I did some more circuit testing. Something I didn't mention before was that in the fan receptacle neutral to ground read 3V. What is interesting is that in the upstairs alarm receptacle, the neutral that reads only 16V also read 3V to ground. The other 119V one read 0V to ground as it should. Hot to ground in the alarm receptacle read 120V as it should.
Edit #2 My dad came home and I told him what I'd figured out today. We will be going into the attic next week to see if we can find the fault. Thanks everyone for your help! Edit #3 Just to be clear, the two photos of measuring voltage from hot to neutral are in a fire alarm receptacle that comes earlier in the circuit. The two neutrals were tied together, along with the small wire that goes to the little three prong plug for the alarm. I only separated them to see if I got different voltage readings. So I don't think a floating neutral is the issue.

FINAL EDIT Thank you manassehkatz for your patience with my repeated misunderstanding of what you were saying. The receptacle is back to 120 and the fan will be promptly reinstalled. For those curious, the issue was in the smoke alarm neutral connection. The white neutral wire reading 16V (as I incorrectly called the bad neutral) must have somehow worked itself loose either over time or when it was jostled around at some point. Once I tied the wires together properly, I read 119V hot to neutral in both the smoke alarm wires and in the exhaust fan receptacle. Woohoo!

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    Do the smoke detectors work? There should be an indicator light to tell you whether they are running on AC vs. battery.
    – manassehkatz-Moving 2 Codidact
    Mar 12 at 20:24
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    Yes downstairs they do. See my comment below for more detail. It seems to me that one of the neutrals or grounded conductors if you want proper terminology is wonky. Now just to find where the wonkiness is.
    – cam2323
    Mar 12 at 22:36

2 Answers 2

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TL;DR White Neutrals Should Be Bundled

(Note: All of this is based on standard NEC = US/Canada wiring. Other countries do other things.)

Most of the time, white is neutral. Neutral is always white (in cables; in conduit it can be gray) but white is not always neutral.

However, while a white wire in a cable can be:

  • Hot in a switch loop
  • Switched Hot in a switch loop (really shouldn't be, but it happens)
  • Traveler in a 3-way switch setup
  • Hot in a 240V circuit

neutral is always white.

Unlike hot/switched hot/traveler/etc. wires where there are very often very specific and different meanings of each black or red wire in box with a bunch of stuff (switches, receptacles, etc.), most of the time the white neutrals are connected together with a wire nut or lever nut (Wago) connector. In fact, a common problem is that someone disconnects the white wires and then wonders why things don't work. Or pulls out a "spare" white wire from the convenient bundle and wonders why it doesn't work (or why it works but now the others don't).

A wire that is not connected to any of ground, hot or neutral will be floating. Which ends up meaning that it is subject to low levels of induced/phantom voltage - and 16V is typical of that.

In the pictures, 16V is with one white wire - presumably a neutral wire going on to another fixture. 119V is with another white wire - presumably coming from the breaker panel or from some other bundled neutral wire group in another junction box.

Connect all the loose white wires in that box together, adding a pigtail to a fixture if needed, and you should be all set.

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  • I get what you're saying, but in the photos of the wires being tested, the neutrals are only separate because I removed the wire nut that was keeping them together. Before testing they were tied together, along with the white wire that runs to the little fire alarm plug where the three small pins plug into. So everywhere I can see the neutrals are connected which leads me to believe a floating neutral isn't the issue. Thank you for your response though I appreciate it.
    – cam2323
    Mar 13 at 1:18
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    My point is that the reason one of the white wires is showing 16V rather than 119V is because it is disconnected from the other one.
    – manassehkatz-Moving 2 Codidact
    Mar 13 at 2:12
  • Ahh I think I see what you're saying now. You mean somewhere else in the circuit other than where I took the photo. Lol! Sorry about the misunderstanding.
    – cam2323
    Mar 13 at 22:18
  • Actually I mean in that box. You show 16 from one white to the black. Then 119 from the other white to the black. 16 is phantom. 119 is real. The whites should be together and then they'd both show 119 to the black.
    – manassehkatz-Moving 2 Codidact
    Mar 13 at 23:09
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    Sorry for all my misunderstanding, you are correct! I tied them together and tested, got 119V hot to neutral. Then tested the fan receptacle and got 119V. So somehow the two neutrals got separated inside that box, and that caused the issue. Thanks so much for your patience! You saved me and my dad from unnecessary exploration in the attic.
    – cam2323
    Mar 14 at 14:24
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Usually a reading like 16v is phantom voltage that the wire is picking up from another hot/live wire cable it comes close to.

You will probably need to check that circuit back to the panel and find the loose/corroded connection/eaten wire. Check the easy stuff first, switches/lights/receptacles/junction boxes.

Your smoke alarms are probably down also or before the circuit problem, which should limited the circuit distance to check.

You should get a reading from the black wire and the ground wire, it should normally give a reading of ~120v(or local volts). It might help in seeing if the problem is in the black or white(neutral) wire. Neutral and ground should be 0 volts.

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  • Yes excellent point. The smoke alarms downstairs are fine. But I just checked the upstairs alarm receptacle and it was dead. So I measured the voltage from black to white, but kept the whites separate. One showed 120V and the other showed 16V. In addition, when measuring voltage from the bad white wire to ground it read 3V, whereas on the good white it read 0V. We are getting closer!
    – cam2323
    Mar 12 at 22:35
  • Note that if the circuit was turned off or disconnected, 16V (at near-zero available watts) could indeed be present as phantom voltage. If so, turning it on/reconnecting it should give you house voltage across those contacts. Now you just have to trace down where the circuit is open...
    – keshlam
    Mar 12 at 23:03
  • I tried shutting off the breaker then reading hot to neutral voltage. Hot to good neutral read 0V as expected, but hot to bad neutral initally read ~100V and slowly counted down to 0V as I held the leads in place. Weird. My dad and I will be exploring further once ice fishing season is up next week. He thinks the wiring will be visible in the attic, and we can explore there to find the fault. Thanks!
    – cam2323
    Mar 12 at 23:52
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    It isn't "good neutral" and "bad neutral". There is only one neutral per circuit. When the wires are connected to each other they will both show correct neutral values (~ 120V relative to hot, ~ 0V relative to ground). When they are disconnected, one is "neutral" and the other is "a white wire not connected to anything, affected by random induced/phantom voltage but meaningless numbers".
    – manassehkatz-Moving 2 Codidact
    Mar 14 at 14:24
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    You are correct manassehkatz, thank you for your patient teaching! See my comment on your answer below.
    – cam2323
    Mar 14 at 14:57

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