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My wife's cousin keeps losing power to her bathroom lights and fan. It will be out for a week or so, then come back on for a day to a week, then go back out. I went over and checked the line at the switch, the lights, and the fan. The readings were good (120v-124v) without any bulbs in (she had removed them before I came over). Since I had a good reading in the bathroom, I checked the breaker. The breaker is a 2-pole 15 amp WR and had a good reading as well (123v), but one of the wires was broken and away from the breaker. I reconnected it and retested, everything was checking out. Now, for the weird part that is beyond my basic comprehension. After fixing the breaker, I checked the power in the bathroom and it was still reading fine, but when I put a light bulb back in either of the two light fixtures, it drops to 4v, but does not trip the breaker. When I unscrew the bulb, the power goes back up to 120ish volts. I changed out the breaker and retested. 123v without a light bulb, yet still drops to 4v once a light bulb is plugged in. What can be causing this?

  • When you quote a voltage measurement, I assume you're measuring to the black/hot wire or to the terminal in the middle of the lamp socket. To what point is it referenced -- in other words, where's the other meter lead attached? – Greg Hill Jan 7 at 23:19
  • Can you post photos of the breaker panel please? I'm not familiar with that type...also, where are you on this planet? – ThreePhaseEel Jan 8 at 1:39
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    What's your test equipment? If I get a "good reading" it vibrates the Wiggy in my hand, which means it will light up a light bulb. – Mazura Jan 8 at 1:49
  • 1. Measured hot to neutral as well as hot to ground at the line to the fixture. It read 123v both ways. At the breaker I measured from the breaker to the neutral bar. 2. I can post pictures of the breaker tomorrow and we are located in New Mexico, USA. 3. I use a commercial electric MS2033C multimeter. – Gerry DeNardi Jan 8 at 2:56
  • Do you read 120+V on the breaker when the light bulb is in and on? I'm guessing yes; and there is a voltage drop on a bad connection in the circuit when the circuit is complete. If you are reading the low voltage at the breaker with a bulb in, then I'd be questioning the breaker, (assuming your meter is touching the wire set screw of the breaker on one leg, and solid gnd/neutral on the other lead. – noybman Jan 8 at 4:18
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To me this sounds like a bad connection. My first look is for backstabs prior to the switch or at the switch, it can also be a broken wire sometimes a Knick on the wire when striped the wire can break over time, and last a wire not fully in a wire nut. All of these can show full voltage without a load but once a load (the light) is added it opens. I see this most often with backstabs the push in wire connections , I have found a few broken wires and a few more loose wire nuts over the years but most often a bad backstab.

Although I have found this problem hundreds of times on outlets I have found it a few dozen times fed from an outlet on a lighting circuit, Usually lighting circuits are not as heavily loaded but in a daisy chain feed the bad connection is usually close to the switch a outlet that feeds the switch within a few feet. Or the switch itself.

You mentioned a double pole breaker so it might be a multi wire branch circuit, but since both hot to ground and hot to neutral voltage are good I would be looking for a bad hot , black or red wire.

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  • If OP confirms he still has 120+ at the breaker with a bulb load downstream this helps prove this condition. makes sense RE: GND&Neutral although the "cause of the broken HOT" is begging why? Is the wire properly secured in the panel and to wood framing as it leaves the box? Is the wiring secured properly elsewhere..? – noybman Jan 9 at 2:20

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