Great question — answering as the UphillTech team:
Well, I am not sure what causes the issue but have found an exterior timer/remote switch (has app for cell phone), that apparently has solved my problem. The ENERGIZER SMART Wi Fi Outdoor plug. This plugs into your exterior outlet, and has two individually programable outlets(15 Amps per outlet). I had tried several other timers with no success. Even contacted my local electrical company about the issue. They told me that they would research it but never got back to me. Every other timer I used tripped the GFI as soon as I plugged it in, but this one WORKS ! Hope this solves everyone else's problem like it has mine.
Full answer by Jim Ommert: https://diy.stackexchange.com/a/295841 (CC BY-SA 4.0).
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