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Sorry for not having pictures or drawings. If the description is not clear let me know and I will add some.

Upstairs bathroom has a light fixture on the wall above the sink and an exhaust fan in the ceiling. They are controlled by separate switches in the same 2-gang box.

The old incandescent bulbs finally burned out a few months ago so I replaced them with LED bulbs. After a few weeks the LED bulbs went dim and started flickering. I replaced them. The new bulbs had the same issue after a couple of weeks. In both cases the bulbs would be brighter if I turned the fan on in addition to the lights.

So I pulled the switch face plate and looked at the switches. There is one hot wire coming into the box. It was connected to one screw on the switch for the fan then continued to one screw on the switch for the light. Each switch had its own wire going from the second screw back into the wall. There are a number of ground wires in the box connected with a wire nut. One of those ground wires was connected to the switch for the fan and then continued to the switch for the light. With the light switch off I measured the voltage across the two screws on the light switch. The voltage was low but I don't remember exactly how low. Maybe 80 volts. The voltage went up when I turned on the switch for the fan.

I cut the hot wire and connected separate pigtails going to each of the switches. I did the same with the ground. I took down the light fixture and connected my voltmeter to the wires coming out of the wall. With just the light switch on the voltage was ~105 volts. With both on it was ~115 volts. I replaced both switches but it did not change anything. I disconnected the pigtail from fan switch and the voltage at the light fixture stayed at 105 volts. I noticed that the pigtail I connected to the switch for the fan was 14 gauge but everything else in the box was 12 gauge. I replaced it with a piece of 12 gauge. Now the voltage at the light fixture reads 120 volts with the light switch on and the fan switch either on or off. Success!?!

If I have both switches on and turn off the fan then the voltage at the light fixture momentarily jumps from 120 to ~150 volts. If I have the light off and turn the fan off then the voltage momentarily jumps from 0 to ~20 volts. Is the temporary jump in voltage an issue?

Why would using 14 gauge wire (or nothing) to send power to the fan switch cause low voltage at the light fixture but using 12 gauge wire results in 120 volts? I thought the point of using the pigtails was so the two switches would not impact each other.

The lights are not dimming but it still seems like there is something strange going on.

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It turns out that there was no problem with anything in the switch box. The light and fan in that bathroom and another bathroom are on the same circuit as a number of outlets on the second floor. All of the outlets were wired in series (rather than using pigtails) and most of them had low voltage. When I found the first one that did not have bad voltage I removed it and found that the ground wire going to the downstream outlets was not connected well (it was a backstab connection). I replaced that outlet and now everything works perfectly.

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Volt meters are not accurate when measuring instantaneous voltages. There will be small fluctuations when operating switches that wouldn't be noticeable with incandescent bulbs but are with LED bulbs.

The change from 14 to 12 AWG shouldn't make a difference in the voltage. My guess is you just made a better connection. The original wiring of the hot to the switch and continuing on to the other switch is very common. Electrically speaking, pigtailing those will not make a difference. The "common" point will now be the wire nut in the pigtail instead of the switch and you'll still see the slight fluctuations especially when operating the fan.

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