if it s a good 400w  it should run a 390 with a low cpu

i run my 390 with an athlon 620 what use 90w   on a corsair cx430 (died after 5 month)  when i realize the 12v rail only pull out 336w  when it was actually using close to 400w


i run 4x 270  on corsair 650 (600w on 12V)  for a year before modify the ring (now i have watt meter, i test and  it use 605w at the wall)
5 x 380  on a 1000w   using 1100 at the wall ( now decrease power limit to keep under 1000)

now my 390 runs on a zalman 700w  with an fx8300 oc and an 470 under clocked  (using 690w) max 700w on 12V, i limited the power limit to keep under 700w at the wall