Originally Posted by donf
Well first of all a picture would really help us instead of a drawing. Second, why does it appear that there are two "Hot" leads coming from the GFCI breaker in the "B" or Sub panel.
Did he jumper from the Breaker "B" ox back to the Main Panel? If he did, did he remove all of the connections from the existing lights in the Main Panel ans splice them to the Wires from the "B" box.
The way a GFCI works is it monitors the difference of current between the Black (Hot) and the Neutral (Return) White. If there is a difference from 0.004 to 0.006 it will trip the breaker.