I have a GFCI located in my garage which feeds into my bathroom and then goes to an external junction box and finally underground to my deck. Upon doing some routine testing, I noticed by deck receptacle showed "Open ground". Upon initial investigation, I noticed that the wiring from the bathroom outlet to the external junction box was missing a ground wire. I assumed that was the problem so I replaced that span with brand new wires, inclusive of a ground.
I reconnected everything the same. The only difference is that the new ground from the bathroom now connects to the ground wire coming from the deck, which meets in the external junction box. I assumed all was good, but no chance. The GFCI kept tripping.
Long story short, I can confirm that the short happens when I connect the bathroom ground and the deck ground in the external junction box. If I do not connect them, then everything works, but I still get the "open ground" at my deck.
Why is this happening? I thought the ground wire can't trip a GFCI? Is water, likely in the underground cable/conduit, causing the ground to short things out?
Not to complicate things, but the end of the underground cable at the external junction box is 12 AWG; however, the opposite end at the deck is 14 AWG. I do not see another external junction box. Is it possible they connected two different cables underground and that's what's causing this?
Appreciate any feedback. I am a novice at electricity but I love learning.