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Timeline for answer to Wiring advice - two circuits wired to a single GFCI outlet by Robert Chapin

Current License: CC BY-SA 4.0

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Aug 17, 2021 at 5:37 comment added Robert Chapin Added more to the answer regarding tied neutrals and NCV testers.
Aug 17, 2021 at 5:35 history edited Robert Chapin CC BY-SA 4.0
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Aug 17, 2021 at 5:22 comment added KZ. Yeah, I'll need to correct this ASAP. I'm scared to look behind my other outlets and switches now.
Aug 17, 2021 at 5:18 comment added Robert Chapin If you're confident in that diagnosis, then you have 3 branch circuits sharing a wire somewhere in that panelboard. That should never happen in a 2-phase system.
Aug 17, 2021 at 5:12 comment added KZ. Well, for the other question I have breaker #4 and #23 with their neutral tied. #23 and #25 are on the same MWBC. Thinking through this, #25 is one of my heavist circuits - with two (big) computers and a toaster on the same 20A circuit (up to 2kW - it has tripped once). I guess I need to separate those neutrals sooner than later. #23 has only about 200w in it (mostly lightning).
Aug 17, 2021 at 5:06 comment added Robert Chapin More to the point, opening up the panel would allow you to determine if and where the hot wires are grouped together with a common neutral wire. Now that doesn't explain why you have two different circuits attached to a GFCI, but for your other questions it might give you an 'aha' moment.
Aug 17, 2021 at 5:02 comment added KZ. I think I need to open up my breaker panel to mark which breakers are on the same MWBC. I can't tell right now because none are marked and their handles are not tied. This will at least tell me if I need to be looking for a red and black hot pair.
Aug 17, 2021 at 4:58 comment added KZ. I opened up the first box on that circuit and ran in to this: diy.stackexchange.com/q/232092/99275 (the other circuit in that post is a different breaker from what I'm asking about here). I got busy with other projects last weekend and didn't look at thethe other boxes yet.
Aug 17, 2021 at 4:56 vote accept KZ.
Aug 17, 2021 at 4:28 comment added Robert Chapin Any luck looking in the other boxes? The only ways to explain this cross connection are that there's a 2nd box with both circuits involved, or someone just put too many cables in there for no good reason.
Aug 17, 2021 at 4:07 history edited Robert Chapin CC BY-SA 4.0
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Aug 11, 2021 at 6:19 comment added KZ. I still don't understand why they would have run two circuits to the same box. I need to open up all my other boxes on this run to figure out what's going on.
Aug 10, 2021 at 18:47 comment added Robert Chapin Also, "breaker #6 has been a mystery"... you might eventually discover that breaker was intended to serve a dedicated bathroom circuit. If there are no other loads, it would be a perfect way to run a hair dryer.
Aug 10, 2021 at 4:51 comment added KZ. Yeah, that makes sense. I guess I'll go through all my outlets.
Aug 9, 2021 at 10:01 history answered Robert Chapin CC BY-SA 4.0