Wiring – How to connect additional wires to a receptacle

If you want two breakers, you can't share a neutral and ground. The shared neutral and ground could be exposed to too much current, because they don't have breakers for protection, only the hot does. An example:

Assuming you've got an existing circuit with a 15 amp breaker and 14 gauge wire, you add a second 15 amp breaker, running a new 14 ga hot, and sharing the neutral and ground from the existing circuit.

You have one load plugged into the "old" outlet, drawing 10 amps. 10 amps comes out through your old breaker and your old hot, the old 15 amp breaker sees 10 amps and doesn't trip, and the 10 amps goes back through your old neutral.

Now you plug in another load to your "new" outlet, drawing 12 amps. 12 amps comes out through your new breaker and your new hot. The new 15 amp breaker sees 12 amps and doesn't trip. 12 amps goes back through your old neutral. Now you've got a problem.

Your old 14 gauge neutral is now carrying 10 amps from the old circuit + 12 amps from the new circuit = 22 amps: thats more than the rated capacty. You haven't tripped a breaker, but you've overloaded your wiring. This is a very dangerous situation.

There must be a fault in the down stream load wires someplace. This fault is causing an imbalance detection by the GFI between the current in the LINE and NEUTRAL load wires.

Things you can try:

Disconnect the wires that feed the down stream outlets at those outlets and the wires to the lamp circuit. This should include unplugging anything that may be attached to those outlets. If this allows the GFI to reset then you know that the fault is in one of those down stream wires or circuits. You could then proceed to reconnect one load at a time in attempt to locate the wire or outlet that causes the GFI to freeze in the fault position.

If, in the initial test above, the GFI still persists in staying reset then you know that there is a fault someplace between the load wire connection of the GFI and the end of that wire (which should not be connected to any loads if you had properly disconnected every load).

The fault on the circuits could be a bad outlet, bad lamp connection, bad lamp fixture or bad device plugged into one of the downstream outlets. It is also possible that the fault could be caused by a nail or screw someplace that has penetrated one of the wiring cables or the wire insulation someplace has been cut or punctured by the sharp edge of a metal electrical box.

Yes...it is possible that the fault that is causing this problem may have existed in the prior situation before you brought the new GFI into the picture.

There is also the remote possibility that the new GFI outlet itself is bad although this seems unlikely due to the tests that you have already performed.