daisy chaining duplex GFI

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?
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dadiOH
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dadiOH wrote:

Can't you find a marking like LINE(your A side) and LOADB). Sounds like that one is already daisy chained to some other outlet. If you want to find which outlet is connected, trip it by pushing the test button, and find out which outlet lost power.
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wrote:

Trip the breaker, remove the device and separate all of the wires, turn on the breaker and measure them to see which one is hot.
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Tony hit the nail on the head. Somewhere on the existing gfci body it has to say "line" and "load", Flashlight or other bright light should make it easily visible.
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On 02/13/2015 11:45 AM, snipped-for-privacy@sbcglobal.net wrote:

Yep, it should be clearly marked.
IIRC the top side it line-in
Of course (with the breaker off) the wires could be removed from the outlet...then with the breaker on...confirm with a volt meter
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Thanks, folks. I'll take a closer look at the existing receptacle. Also, just after I hit "send" for the OP it dawned on me that I could just take off the wires, turn on the breaker and check. duh... :)
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wrote:

power on the "line in" side.
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Even easier. Thanks.
Who says newsgroups aren't useful? :)
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dadiOH wrote:

Dumb questions will get dumb answers, LOL!
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On 2/14/2015 10:31 AM, Tony Hwang wrote:

Do not perform this test while in bath tub full of warm water.
- . Christopher A. Young learn more about Jesus . www.lds.org . .
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