I'm working on our master bath, wife wanted a second duplex outlet near the vanity. OK, a PITA but no problem.
My plan was to daisy chain off the existing GFI outlet, running romex from it to the desired location. Problem is, the existing outlet has two romex cables coming into it...one is standard 12/2 (plus ground) - call it "A" - which is attached to one side of the outlet. The other is 12/4 romex (with two ground wires), call it "B". One pair of black & white "B" wires go to the side of the duplex opposite "A"; they then are pigtailed to the second pair of black & white "B" wires.
I'm guessing that romex "A" is what is feeding the existing outlet and that the "B" wires go somewhere - I know not where - to feed whatever.
If I want the potential duplex to be GFI, I need to feed it from the outgoing side of the existing outlet, correct? And how can I tell for sure which romex - "A" or "B" is feeding the existing outlet?