Little background before I ask question. I pulled a lines from main panel to outside subpanel, some 130 ft away. Put double pole regular breaker in the main panel (60A). In the sub panel I placed on 15A double pole GFCI breaker and one regular 15A sinle pole breaker. Single pole breaker line feeds to GFCI outlet. That outlet never trips, I can test it, I can reset it, I can put load on it, and no problem. I even tested it with outlet tester. No problems. Now if I turn on other double pole breaker, it trips immediately. If I leave that twisted wire from the breaker connected to the white bus, but disconnect white wire from my breaker, my breaker naturally does not trip. I measure voltage and it shows 282V between the poles on the breaker, and 141 between each of the poles and white bus. Then I thought, maybe the breaker is bad. So I took out double pole, and installed single pole GFCI breker, and same story with that one too. It immediately trips when I plug in white wire.
What else can I try and what could be the issue. I don't think that wire is damaged as pulling it was very smooth, and it is #6 AWG wire, and most of the wire is above ground.
Any ideas? How can I trouble shoot it further?