I have a sub panel in my garage the is fed from the Square D main panel in my house and I am attempting to install a Square D 20A GFCI breaker in the Square D sub panel. The GFCI breaker is going to replace a regular 20A breaker. The only items on this circuit will be 8 outlets. I have the breaker wired according to code. The GFCI breaker trips as soon as I flip the breaker switch. My main panel is 100A and the neutral and ground wires are landed on the same bus bar. The sub panel has the separate neutral and ground bus. The main panel is bonded, the sub panel is not bonded. What could cause the GFCI breaker to trip immediately?