Like i said, you have 18A and you will need 33Amper current or high in the 12V line for that gpu.
Read this http://www.tomshardware.com/forum/279391-28-power-requirements-specs-popular-graphic-cards-guide
, there is the explication:
"To figure out how many amps you need on the 12 volt rail use this formula (watts / 12 = amps) so if the card requires 650 watts simply divide 650 by 12. This formula only works for 12 volt for different voltages there are different dividers. "
Sorry if i make a mistake.