Just yesterday, ironically enough, we changed out a 600w Kichler transformer because something was tripping the breaker in the panel. We installed this lighting system 10 years ago, but he only started having this issue within the last 6 months. Every time the irrigation came on, the breaker in the panel would trip. I have beat my head against the wall about this because I couldn't find the problem. Initially, I insisted that moisture was getting into an outlet, maybe not the one the transformer was plugged into, but certainly another one on the same circuit. This is the only logical assumption that made sense. We even brought an electrician into the equation to assist us, but he couldn't find anything either. After all else failed, I exchanged the transformer under warranty. After the change out, we ran the irrigation through all zones multiple times and no trip. Later in the day, it rained.... and rained..... and rained. Still, no trip.
I still have no clue as to why the new transformer fixed the problem, but the fact is that it did. Problem solved.... finally.
Take my advise. Turn in the transformer for a new one. What's the worst thing that can happen? Maybe it won't turn out to be the issue but I'm betting that you will thank me. My 2 cents