R9280x consumes 350W so 290 needs more to work and hash properly.
sorry, just not true, I have one... 
people should start to look at the spec. rather than making assumption
i'm using Asus R9280x-DC2T-3GD5 gpuclock:1080 gpumemclock:1600 using catalyst 14.6RC2. so MY card consumes 350W, dunno about yours. 24A at 12Vrail
That card btw had ROI in 3 months and since then all is profit. Was the days were doge had met moon! Epic days!!
(and 350W is probably the power drawn by the rig and not the card itself...)