I think this has been shuffled around here before, but I don't recall any very definitive answers. As a learning tool, I would love to hear from the working electricians as to how they would proceed.

Here is the situation-

You have been dispatched to newly built home. The complaint is that the circuit breaker for the bathroom trips. In the home you find that the bathroom receptacles and lighting are powered by a dedicated 20A circuit which is being fed by a GFCI breaker. The receptacles and lights are working. Any motor load such as a hair drier or vaccuum cleaner will trip the breaker. About 20% of the time, flipping on the lights (4 60w cans) alone on will trip the breaker. The homeowner states that it has been doing this since they moved in.

As a test, a standard non-GFCI breaker was temporarily installed and found to operate properly without tripping. You do not have another GFCI breaker on hand.

As a professional technician, what steps would you begin with in diagnosing this problem?