Multiple GPUs and PSU

[Spherical]

Read a thread on this in the Licensed Users LightWave Plugin Forum and, seeing as how I cannot post a response in there yet, thought I would offer one here. If the Mods want to move it in there for continuity, that's fine.

The question is on how large a PSU would be needed for a GTX690 and a GTX670 and would an existing 750W PSU handle it. There are a couple of responses citing how much each card handles and adding up the wattage to arrive at the conclusion that a 750W PSU would have enough, with some headroom.

There is one analysis offered that says it will be OK.

Your 690 is listed at 300 watt max , and the 670 at 170 watt max = 470 watt + 140 watt for overhead = 600 watt so you should have power to spare as long as you are not maxing out your CPU while rendering with the GPU's

Problem is, adding up the wattage of the GPUs isn't the way to go about this.

What needs to be paid attention to is not the aggregate power draw specification of the given GPUs but the manufacturer's minimum value for system power of the larger draw card and then add the power draw of the second card to that, then add in for headroom to prevent a system shutdown or damage. So, taking that into account, nVidia states that:

• Minimum System Power Requirement for the GTX690 is 650W
• This leaves 100W for headroom
• Maximum Graphics Card Power for the GTX670 is 170W
• That's 70W short, with zero headroom to handle transient load surges
• Banking on the chance that the cards won't use that much is risky, at best

Get a 1,000W or 1,200W PSU and never worry about it. Short of that, you can use a second PSU of lower wattage and power the cards' external connectors from it. The motherboard and cards don't care where the power comes from, only that there is enough of it. If you don't have enough supplying those cards, the motherboard traces will not be able to handle the load and burn out.

Never attempt to skimp on power. False economy.