Attic fan wiring question!?

I just installed a attic fan, gable-mounted, and the circuit keeps going out as soon as I turn the breaker on. It is on a 15 amp breaker, running 14/2 wire and nothing else is on the circuit. I connected the black to the black/ white-white/ ground to the green screw in the thermostat wired to the fan. Any ideas on why it keeps popping the breaker?