Electrical Mystery

So, out porch light quit working a couple days ago. I figured the bulb had burned out, so I changed it. But it still didn’t work, even with a known good bulb.

I took off the switch plate, and pulled out the switch. It had power measured from ground (the box) to the input. But it also had power at the output, though a couple volts less. There was no voltage at the light. I took out the switch, and got the same result. 119 at the hot lead, and 117 at the wire that supposedly goes to the light. Any ideas what’s going on?

[Afternoon update]

So, I removed the light. With the wires disconnected at the switch, there is 119 between the hot lead and ground, and almost the same on the neutral. Neither of them should have any voltage on them. WTF?

[Saturday-morning update]

OK, here’s a new part of the mystery. The switch actually controls two lights (our main entrance is on the upper floor, and there’s another light on the ground floor below). When I get into the box where the switch is, there is a hot wire coming from the breaker, a triple wire with one of them going to the switch, and one going to each of the lights, and a pair of neutrals going to the two lights. But there is no neutral coming from the breaker.

And with the two neutrals connected and no switch, the hot, the ones going to the lights, and the neutrals all have voltage to ground.

Again, WTF?

Next step is to disconnect the two neutrals and see which of them or if both of them, are hot.


[Afternoon update]

OK, I’ve exposed all the wires in the box. There is a black one presumably coming from the breaker, and it has 119V. There are two white ones, each going to a fixture, and they are both hot as well. There is no white one coming from the breaker. There are two yellow ones, presumably the hot ones, that went from the switch to the two fixtures. One of them is hot (the one going to the downstairs fixture). The other one, which goes to the upper fixture, is the only one in the box that isn’t hot. There is no ground wire; it’s grounded through armored conduit.

I continue to scratch my head. I guess the next step is to take the downstairs fixture off the wall and see what’s happening down there.

[Update a while later]

I’m busy with another project (installing new lighting in the kitchen), but I suspect that when I look behind the other fixture, I’ll see the neutral that comes from the breaker, which then feeds the upper fixture. And that’s probably where the problem is, though I have no idea what could have caused things to change.

[Update a while more later]

Per discussion of codes in comments, the house was built in 1977.

30 thoughts on “Electrical Mystery”

  1. There was no voltage at the light.
    How measured? Same method used on the switch? Between hot lead and ground or between the two leads? If the latter you may have a broken neutral wire or a bad connection with neutral. If the former you probably have a broken or badly connected hot lead. Check the wires at the light end and make sure they are in good contact with the fixture. When you say you “took out the switch” I assume you mean you disconnected the switch altogether and just shorted the wires together? Sounds like a fixture problem to me. Try connecting the wires on the outside light to a regular indoor lamp that works or a trouble light. If it lights up its your fixture not the wires.

    1. I measured the receptacle for the bulb. What you suggest is unfortunately the next step, which I’ve been avoiding because it will involve having to recaulk when I put the fixture back on.

  2. A favorite trick of jack leg electricians and DIY’ers is to switch the neutral instead of the line. Even more common is to run from the breaker to the fixture, then run a piece of romex to the switch using the white wire for the return power leg to the fixture without marking it because a few inches of red electrical tape is soo expensive. Then there’s the chance that there’s a junction box somewhere in the attic, if you’re lucky, or buried randomly in a wall if you’re not that has a bunch of neutrals sort of stuffed in a wire nut and the one for your fixture has worked loose.

    One of the problems with most digital meters is that they have such high input impedance, that they will register phantom voltage on a floating neutral. If you put even a small load, it disappears. A light bulb with clips between the wire and ground will tell safely. Something like a soldering iron will work as well.

    With the power turned off, measure the resistance between the neutral and ground, it should be no more than a couple of ohms. If it’s an open circuit, you’ve found what’s the problem, all you have to do is find where. A bad connection on the line side can cause the same sort of problem and the voltage will nosedive when you put a load on it but I’m betting it’s a bad neutral.

    1. This makes a lot of sense. One way to put a tiny load on a line to see if you’re just floating is to measure the voltage across a resistor. If you’ve got a 100 kOhm resistor, the power dissipation through it should still be under 1/8 W even with a hot line (V^2/R). I miss having Radio Shack stores nearby.

      1. True enough. But you can buy an electrical probe at like Home Depot that will also safely put a load on the wire to tell you if its floating or not. I’m thinking MCS is onto something. Let’s hope it’s not a wire come loose from a wire nut in the wall. Wire nutting outside a junction box doesn’t sound like up to code to me.

    2. The house was probably up to code when it was built and the things I talk about were never legal, at least since the end of knob and tube. but practices vary a lot from place to place and over time.

      My dad called residential wiremen hatchet men from the practice of daisy-chaining outlets by using a roofing hatchet to notch the 2×4’s to run the wire through wall horizontally rather than bringing them out the top and across and down. Horizontal runs are still legal provided certain rules are followed but not a sign of a quality builder.

      Older houses also used a lot of junction boxes where modern practice is to run wires direct back to the breaker panel. Also, generally fewer outlets on a circuit since people want more than a couple of appliances to operate at one time without tripping the breaker. Buried junction boxes have never been legal. Splices outside a box haven’t been legal for a very long time.

      A house is a living record of everything that anybody, however ignorant and unqualified, has done since the building inspector signed the certificate of occupancy. These people are a lot more inventive than I can keep up with.

      1. We bought an 85 year old house in farm country. Had some electrical problems. I do a lot of DIY but when it comes to electricity, anything beyond replacing a switch or outlet, I leave it to a professional. Electrician and his apprentice were tracing a short in a line that apparently a previous owner had extended, and every time he removed a switchplate and looked into the box, he would mutter a frustrated “Farmers!” under his breath… 🙂

      2. My first home was a detached condo that I bought new and got to watch its construction. The outlet romex wiring ran daisy chained horizontally through the studs through 1” holes bored through them. Even on the second floor, even though it had a small attic. Less wire that way I suppose. At least there were no buried junction boxes. At least the wire was copper. This was in 1986.

  3. I might have to sit down and draw what you describe to picture it correctly, but I wonder if you’ve got the standard setup where the breaker feeds the light fixture, with a “switch loop” carrying the hot (on a white) to the wall-switch, with the switched-hot returning (on black) to connect to the socket.

    It’s the very first diagram in the following set of common ways of wiring ceiling fixtures before the 2011 code change started requiring neutral at every switch-box:


  4. “There are two white ones, each going to a fixture, and they are both hot as well. There is no white one coming from the breaker. ”

    The two white ones are coming from a switch {which is switched on}. Or you have a two or three or four way switch to outside light. You turn it on or off from different places in house.

  5. Again, if there is power to the terminals on the fixture and the fixture is not working, it’s not a wiring issue, it’s a fixture issue.

    1. There is 119V on both the hot lead and the neutral, so there is no voltage drop across the fixture. There is no reason to think that there’s anything wrong with the fixture.

        1. I’m sure it would work fine if it was being provided with actual voltage. I have no reason to think there’s anything wrong with it, give that the wiring is clearly FUBAR.

          1. Do you have one of those plug-in outlet testers handy from a previous home wiring project? If so, I’d unwire the receptacle and wire to a spare outlet you have laying around (or use alligator clips), and see what the outlet tester says. A good one will catch all kinds of things and tell you what’s wrong. However, it won’t tell you where the wiring problem is.

          2. Yup. Suggested before vis probe. You could also just wire the switch back in and with the fixture removed attach a trouble light to the wires with alligator clips and see if it works. If it does it’s the fixture, if not you have either a switch or wiring problem. If you just short the switch and joy, its the switch. If not its the wire. I forgot to ask you said two lights. Did the other light fail too or did the switch work it ok?

  6. It might be time to check the voltage between your “ground” that’s referenced to armored cable sheath to “real ground” connected to copper pipe (not Pex). If there is >100V, your ground isn’t. If your “ground” is merely floating near zero, you can check for resistance to neutral, which should be very low.

    When I had a new washer installed, I had to put in a properly wired 3 prong socket adapter. Simple, right? All I had to do was to tie the ground to the armored cable. But then I saw that the armor only went up to just below the ceiling. So I had to tie ground into the cold water pipe.

    1. Not even a cold water pipe is a guarantee if you have a well like I do. The underground connection between the pressure tank and the well is vinyl hose poking through the concrete basement wall.

      1. In my case, the cold water pipe I used was grounded (checked against properly wired outlets). That’s why I put in the Pex comment. What you really need to do is verify ground and neutral reference points throughout the house. This can probably be done with a long 3-wire extension cord, so you can measure voltages relative to neutral and ground all the way back to the main panel. I assume the panel is OK, otherwise nothing in the house would work!

    2. “into the cold water pipe.”

      A practice acceptable in Canada (CEC Subrule 26-700(7) c) but not permitted in the US NEC.

      But I’m sensing anyway from other data that you may be Canadian.

  7. You know this thread is the literal embodiment of the set-up question for a joke as old as the credentialed professions:

    Q: How many engineers does it take to change a light bulb?
    A: None, if you hire an electrician…

    1. You’re supposed to conclude with

      “You’ve been a great audience, I will be here all week, and remember to tip your waiters.”

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