I have 12/3 wire running out to my garage. I have a double 20 amp breaker in main box and a subpanel with two single 20 amp breakers in the garage with black to one breaker and red to the other. The black works but the red keeps tripping. What can I do?
Is anything connected to the 'red' side? I am not 100% clear on what is connected to the subpanel but the debugging sequence should be the following:
turn off all the breakers on the 'red' side of the subpanel. Check if the main breaker still trips. If it does not, the issue is with whatever hooked up to that side (shorted wire, etc)
If it still does, swap with the 'black' breaker to see if the breaker itself is the problem.
If the problem continues, check the wiring.