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I started replacing a light fixture with a fan + light, and ran into a weird issue. My contactless electrical tester indicated current despite having turned off the breaker. The light was off in the room, and outlets were all disabled, but I am kind of spooked and want to get a some advice before I touch this stuff.

Any suggestions for what might be happening? I'm afraid there might be a second circuit. It's an old 1906 house, and I just don't trust it to be wired up in a sane way.

https://photos.app.goo.gl/eLhqrqzK6k8KRdzZ6

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Switch off all the breakers, then test again.

Turn on the breakers one by one and that way you will locate the one causing the issue.

Many times I have found that what is on the label is not true.

Always test with a multimeter before using fingers...

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Contactless electrical testers are notorious for picking up phantom voltage (crosstalk from nearby wires). However that doesn't matter. The purpose of that type of tester is safety, not diagnostics. It's not a usable diagnostic tool (so don't use it as one). As such, it's totally OK for the tester to "err" on the side of caution. Just keep turning stuff off until it stops indicating.

If you want a real and serious reading, get a DVM and a bit of skill (mainly, the ability to recognize phantom voltage when you see it!)

Is a second circuit a possibility? Oh, you betcha! There were lots of looped hots, stolen neutrals, etc. These can bite you when you disassemble a circuit.

There's even a wiring method permitted today (and done properly, perfectly safe) called a Multi-Wire Branch Circuit, where 2 hots deliberately share a neutral. Until recently the hots could be on totally separate breakers (now they must be handle-tied).

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  • Don't all circuits share the same neutral in the box though? I'm guessing you're talking about something like having an outlet where each of the two outlets is on a separate breaker but they have the same neutral wire? – Dean MacGregor May 26 at 13:59
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    @DeanMacGregor Ugh, I do not have the energy to explain again why we separate each circuit's N from each other and from Gs,... automotive and electronics people often go "why?" because they are used to having a HUGE (compared to their needs) neutral of either the groundplane or the vehicle chassis. But in home wiring neutrals are no bigger than hots, and don't have breakers. But yes, the legit method, MWBC, does allow splitting a recep just like that. The breaker handles must be tied to protect maintainers. – Harper - Reinstate Monica May 26 at 14:15

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