AMD graphics suck power like nothing else.
Quote from the manufacturer's page for that card model:
REQUIREMENTS
External Power - 6-pins : 1
External Power - 8-pins : 1
Minimum Power Supply Requirement : 750 watt
See:
http://xfxforce.com/en-us/products/amd-radeon-r9-300-series/amd-radeon-r9-380-4gb-double-dissipation-r9-380p-4255
Anything less than that, may run well for a while, or you might get random reboots or other weird troubles when the system
gets warm. Power issues are no fun and can be weird to troubleshoot too.
Quote from the manufacturer's page for that card model:
REQUIREMENTS
External Power - 6-pins : 1
External Power - 8-pins : 1
Minimum Power Supply Requirement : 750 watt
See:
http://xfxforce.com/en-us/products/amd-radeon-r9-300-series/amd-radeon-r9-380-4gb-double-dissipation-r9-380p-4255
Anything less than that, may run well for a while, or you might get random reboots or other weird troubles when the system
gets warm. Power issues are no fun and can be weird to troubleshoot too.