Power Supply causing GPU+System Crash? How much power do I need??

[unseengundam]

I think I have problem with my power supply not giving enough juice for possibly even 2 GPUs (I have 3 GPUs). I currently have a Corsiar RM 850W with +12V at 70 AMP (~850W) on that rail.

I have 3 cards:

Sapphire Radeon R9 280x
MSI Gaming R9 280x
XFX R9 290x R9-290A-ENFC

#1 Test -> The power usage according to my Belkin monitor goes to near 800+w and which I believe eventually causes my system to crash (< 15 minutes).

#2 Test -> I have tried to lower it down to ONLY 2 of the 280x. This time the Belkin monitor goes to around 650w and then my system crashes in less than 15 minutes. I would think the RM 850w is enough for this at least?! 

#3 Test _.Anyhow, I am currently down to test using only ONE 280x. The total watts usage is at 335watts and the system is running over 15 mins. I will keep on testing this.


So my question is does my Corsair RM 850w PSU just suck? Does it seem like cause of #1 and #2 test failing?

Finally, what power supply (or additional PSU) can I get for my rig? How much wattage and 12v amp on the rails do I need?

Thanks in advance!

[Warning__3]

Have you tried to undervolt the cards? i managed to lower my 2 280x with 100w by undervolting from 1200mV to 1050

[Gator-hex]

12V x 70A = 840W

Sapphire Radeon R9 280x = 8-Pin=150W + 6-Pin=75W + MB=75W = 300W TDP (I have seen some Sapphire Vapor X are 2x 8-Pin though! 375W)
MSI Gaming R9 280x = 8-Pin=150W + 6-Pin=75W + MB=75W = 300W TDP
XFX R9 290x R9-290A-ENFC = 8-Pin=150W + 6-Pin=75W + MB=75W = 300W TDP

CPU and other stuff = 100W (maybe 150W if AMD)

Total TDP = 1000W

[rograz]

280x at stock is nowhere near 300W

[Gator-hex]

280x at stock is nowhere near 300W

Reference clocks, yes, but it has been designed to be able to call on 300W at peak load and chips vary in their power requirements (silicon lottery).

You only need a nano second spike in power demand to crash your system and it all depends how far you push your overclock, too far causes crashes.

Then you've got to ask why Sapphire decided to put 2x 8-pin (375W) on their 280x Vapor-X OC (My guess 8+6-pin wasn't enough for overclocking!)

http://www.eteknix.com/sapphire-amd-radeon-r9-280x-vapor-x-oc-3gb-graphics-card-review/15/

[sub-80]

at some youtube videos showed 3 r9 280x at full load consumed +950 watts, but from my research as am trying to build a rig worst case scenario is 400watt per card full load.

[Equate]

You will need a minimum of 1000 watts for smooth function of the system