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I have a hallway light that doesn't light up with the switch is flipped. My first move was to change the bulb. This did not work. I used a multimeter to test the voltage across that socket and I'm getting 80V. I found this surprising because the other bulb sockets I've tested in the house get the expected 120V.

Any thoughts? Is this something where I need a special bulb or do I have a bigger problem that requires some additional work?

  • You may have a loose wire say at a receptacle. Do you have old back-stab connections at receptacles? – Jim Stewart Nov 4 '17 at 16:12
  • @JimStewart, I don't know. I will do some research on what that means and check out my situation. – Nick Criswell Nov 4 '17 at 19:42
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I can't disagree with Paul's response especially when it comes to safety. There are certainly a few things that can be considered here and if you are uncertain I would recommend an electrician because what you describe should never be able to happen and now that is has been identified correcting it is crucial.

Given that you can measure near 120, but it is significantly lower than expected, you almost CERTAINLY have a high resistance connection and this is a recipe for fire. It is possible this is a phantom voltage, but should be treated as live until proven otherwise. Intentionally presume fire is imminent, it could happen at any moment. Now that I got the warning "no kidding this is serious" part out of the way:

Possible likely causes that come to mind:

  1. Aluminum wiring: If your home has aluminum wiring, let us know. There are other posts on this topic that go into details how to resolve this issue short of replacing all wire.
  2. Stab locks/backstabs: these are just... bad. If in the path of your investigation you find these types of connections, fix it. Your future self will thank you.
  3. Bad wire nut connections: this can happen with poor or sloppy twists on wires, or people improperly joining stranded wire with solid wire, loose connections, over-tightening, or just vibrations for a poorly mated connection or a cramped box
  4. Bad Switch, Light Fixture, Outlet, or Breaker: There may be more options here based on what you are running on this circuit, but basically, ANYTHING in the path could be worn out

Steps to resolve: [Notice: when opening or touching wire, secure power, turn it back on when ready to test]

  1. Identify everything ON THE CIRCUIT shared with this light
  2. If you have an outlet, or other easy access location (be it the switch or the light), test HOT to GROUND, do you get 120V? If so, this means you have a bad Neutral (typically white) return to the box, anywhere in the path (but likely near your light/switch)
  3. If you tested less than (say 114V) from the Hot to Ground, then the issue is on the HOT (typically black); with same location notes as #3
  4. Was there anything else on the circuit from #1? If so, isolate those items, and test the light outlet again, is the voltage higher or lower than the initial 80v? (say +/- 3v) when tested HOT to NEUTRAL
  5. If the voltage on #4 goes up or fluctuates, greater than a few volts, then the issue is most likely upstream and this helps you identify where to check first. Check the items HOT to NEUTRAL moving towards the breaker until (if) you get near a 120v reading. If the voltage stayed near the 80V or you cannot perform a test like #4, then just start from the light fixture and work backwards.

In either case, there is and has been a bad connection in your circuit for possibly a long time. You should check all connections from the light back to the breaker panel. High resistance connections draw more current, thus, more heat. So other connections in the circuit could be, or become, compromised.

Correct any and all bad connections, and replace wiring if sheathing is melted. Contact an electrician if any of this is even remotely questionable.

  • Thank you, this is helpful. I am no electrician; my limited knowledge of multimeters comes from a physics degree with about 10 years of dust on it. I am going to research how to check the voltage at the light switch. In terms of fire hazard, I will make sure to keep this switch off until I can resolve the issue. Do you recommend checking the voltage on other circuits? We have an old house that was totally gutted and redone about 3 years ago-just moved in a month ago and this light never worked. – Nick Criswell Nov 4 '17 at 19:47
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    The switch is not necessarily going to reduce the fire hazard, keep the breaker off, that will. – noybman Nov 4 '17 at 19:50
  • And in answering the question on "checking the voltage at the switch" that is fairly easy. Turn off power, disassemble the wall plate and the retention screws on the switch. 1.) Are the wires SCREWED on to the switch? if not, they should be. There should only be TWO TOTAL on the switch, not incl. the ground. 2.) are there wirenuts in the box behind the sw? At least ONE with two whites in it? Check the connections. What color is the bare wire aluminum or copper? Are the sheaths burnt? With power back on, test white to each side of the switch. Do you read120 or 80? – noybman Nov 4 '17 at 19:53
  • For the record i did not downvote, but I do think this answer suffers somewhat from the "describe every possibility because OP didn't give us enough info to be more specific" problem. Of course we all do that sometimes, and it is compatible with what SE aims to be: encyclopedic and a FAQ. Better to avoid the other problem, an answer so specific it only helps one person. – Harper Nov 4 '17 at 20:40
  • If you had a bad connection on an unloaded circuit you would read near 0V or near 120V with the voltmeter. – Hot Licks Nov 4 '17 at 20:49
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You used a DVM? 80V sounds a lot like phantom voltage, induced by nearby wires. It's a bit like how a crystal AM radio can use the radio waves itself to power a tiny earpiece. This can be picked up by very sensitive digital meters (as contrasted with an analog meter where the voltage must have enough motive force to move the needle). It is not real, usable voltage, and actually means the wire is disconnected.

The easy way to test for phantom voltage is use a less sensitive meter, or put a tiny load on the circuit. The tiny load will quickly extinguish the phantom voltage.

  • Shut off breakers (until the voltage indication is gone) and attach the two wires to a receptacle, or
  • just screw in a light bulb socket adapter...

... plug in an incandescent night-light with a switch. If the voltage disappears when you switch the night-light on, that was phantom voltage. Treat it the same as you would a 0 volt reading.

Of course, then you'll need to chase the problem as normal. The good news is, barring nails driven through Romex, 99% of wiring trouble is at the terminations, and Code requires every termination (with rare exception) be inside a junction box which is accessible without deconstructing the building.

It goes without saying that you treat every gun as loa--- I mean every circuit as hot, even when you personally cut the breaker you are sure it is. Things like MWBCs can boomerang back on you.

Once I overhauled the wiring in a factory. The 480V feeder to the buses, and the service entrance wire off the 120V transformers had been stolen. The 1000A 480 service and 480-120 transformer supply knife switches were off. This was way beyond LO/TO, the service wires were gone. I still treated every wire like it was live. You don't want to wire your brain to ever think it's OK to touch copper. This is one reason I pigtail receptacles at a bench instead of hooking them onto screws at the box.

  • And I really fail to see how my advice is dangerous. Do I really need to disclaim "turn the breaker off before attaching wires to a receptacle"? Okay then. – Harper Nov 4 '17 at 20:05
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    @noybman I didn't say in the same Romex. Installers parallel Romex all the time, and sky's the limit in conduit. – Harper Nov 4 '17 at 20:08
  • Side question: "Suppose the solenoid-based tester doesn't show you the 80 V sitting on that energized wire, and you grab it — what then?" – fluke.com. Then I'm guessing you failed to find a proper ground. How would a VOM be any better in that situation? If it doesn't vibrate my Wiggy, then it can't vibrate me, correct? – Mazura Nov 5 '17 at 1:32
  • Never in my life have I tested for line voltage with a VOM - because this answer, and they will blow up in your face if it's set wrong. Also correct, yes? and that's not really a concern when checking 120 with a Wiggy, right? But 480, yeah, I don't go anywhere near that stuff. – Mazura Nov 5 '17 at 1:33
  • A Wiggy will not register phantom voltage. This may be good or it may be bad. – Hot Licks Nov 5 '17 at 13:02

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