What PSU(s) are you using on your 8 card Vega rigs?
2 pcs CORSAIR HX1200 for every 8 VEGA cards.
I have 2 pcs of EVGA Power Supply SuperNOVA 1600 P2, 1600W, 80 PLUS Platinum for 10x vega64, and it´s a more than enough. PSU working max on 60% and this is in the range where has the best efficiency.
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That PSU only has 9 8 pin VGA power connectors, right? Where are you powering your tenth Vega from?