XPS4 Installation issue --Neutral?

[patches]

Attempting to install an XPS4 to control a dock light in a back wired(no neutral) situation.  However; in the same 2 gang box is a switch for a porch light with a neutral, which I jumpered from.  Upon installation, it immediately tripped the GFI.  After fussing a bit, I think the problem is that the porch light comes from a 200 A. house panel, and the dock light is out of a 100A outside panel.  So the neutral terminates in a different panel than my dock light power.

Sound like I am SOL?  Or am I missing something else.  Any thoughts appreciated.

[Brian H]

Are you sure it is a Neutral?

If the White Wire is connected to one of the porch light switch's terminal. Along with a Black wire on the other terminal.
You have a switch loop.

Another indication of a switch loop would be a black piece of tape or black marker on the white wire that you think is a Neutral.

In a switch loop. The white wire {should have a black indicator on it} is actually the Line from the fixtures electrical box and the black is the load to the light.

If the white wire you used is in a bundle of white wires then it maybe a neutral but on the other breaker box. As you thought.

[JeffVolp]

Upon installation, it immediately tripped the GFI.  After fussing a bit, I think the problem is that the porch light comes from a 200 A. house panel, and the dock light is out of a 100A outside panel.  So the neutral terminates in a different panel than my dock light power.

Unless the neutral return goes back to the same GFI that sources power for the light, the GFI will see an imbalance and immediately trip.

Jeff

[patches]

Thanks Jeff,

That's what I thought the problem was.  Now I guess my next option is an XPFM in the panel box.