Running GTX 780s in SLI with a 750W PSU

Ghostwarrior69 · Nov 4, 2015

Hey! I have been thinking about upgrading to an SLI configuration, and I was wondering if my PSU would hold up. Here are my full specs:

Case: CM Storm Trooper Big Tower
PSU: Silver Power SP-S750M 750W (Bronze plus)
Mother Board: MSI Z87 G45
RAM: Corsair XMS3 1866MHz 16gb dominator (2 sticks)
CPU: intel i7 4770k (not overclocked)
Water cooler: Corsair H60
GPU: MSI GTX 780 3gb
SSD: Samsung 840 Series 256gb
HDD: Seagate Baracuda 2tb
And a ASUS blue ray reader.

(Not sure if it has anything to say, but it also powers my mouse, keyboard and headset)

My question is this:
Am I able to upgrade to an SLI setup with two MSI 780s without upgrading my PSU?

Thanks for any response.

ko888 · Nov 4, 2015

With no overclocking of the CPU and GPU that PSU is just enough.

The Silver Power SP-S750M uses Seasonic as its OEM so the PSU has some quality to it.

JackNaylorPE · Nov 4, 2015

According to nVidia, ya need a 850 watter w/ two 8 pin and two 6 pin connectors (some models may require for 8's)

http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-780/specifications

Thermal and Power Specs:
95 C = Maximum GPU Temperature (in C)
250 W = Graphics Card Power (W)
600 W = Minimum Recommended System Power (W)
One 8-pin and one 6-pin = req'd Connectors

600 watts for system and 1st card plus 250 watts for the 2nd card. I'd normally go up 50-100 watts with water but the pump is so weak it doesn't present any significant load to the system.

While it may be a Seasonic OEM w/o knowing which platform, I am hesitant to comment on quality.

Ghostwarrior69 · Nov 4, 2015

Thank you so much for such a quick response! One followup question, 62 amps are enough for two 780s right?

JackNaylorPE · Nov 4, 2015

1. Two 780s can pull 50 amps (600 watts) on their own. MSI 780 for example, can pull 276 watts at stock settings, 312 when overclocked.

2. Label ratings are oft exaggerated.

3. Be aware of potential warranty impacts if not complying w/ published system requirements

ko888 · Nov 4, 2015

JackNaylorPE :

It's based on the semi-modular Seasonic M12II Bronze Series (SS-750AM Active PFC F3) (i.e. before the M12II EVO Edition Series SS-750AM2 Active PFC F3).

ko888 · Nov 4, 2015

Ghostwarrior69 :

It's enough if you're not planning to run Furmark for an extended period of time.

During gaming the two cards in SLI mode will draw a peak of around 430 Watts.

JackNaylorPE · Nov 4, 2015

I have a power meter on my system and I averaged 670 - 680 watts during my testing at the wall with twin Asus 780s which equates to 603 - 612 watts of DC output. CPU load on that game during the test was only 40 watts.

Going by the recommended formula where max load = 75% of PSU size

750 x .75 = 562.5 watts max load

The MSI 780 pulls 223 peak x 2 = 446 per TPU

448 + 88 watts CPU + 40 watt MoBo + 10 RAM + 15 storage + 10 optical + 5 pump + 5 fans = 620ish

Granted they won't all peak at the same time, but whatever might be shaved there is certainly eaten up two fold by capacitor aging. So the manufacturer's system requirements certainly seem reasonable and I wouldn't want to be in a position of saying 750 watts is just fine and then have a user sending in a warranty claim with a copy of their receipt that included a PSU which didn't meet the card manufacturer's published system requirements.

ko888 · Nov 4, 2015

For two GeForce GTX 780 in 2-way SLI mode the secondary graphics card does not draw the same amount of power as the primary card. The secondary graphics card, more realistically, only draws 90% of the power that the primary card draws.

JackNaylorPE · Nov 4, 2015

ko888 :

That's depends on your build.... poorly implemented case cooling yes, but not when properly done. Furmark, GPU-Z or Afterburner will allow you to test this quite easily. Each of these reports card TDP. I typically see 109-113% on one card and 108-112% on the other.

Now you might see such a thing with poor case cooling design where the top card runs 10C hotter than the other. If close to throttling temperature, top card will throttle. But, with adequate case ventilation, more intakes than exhausts so you don't suck card and PSU exhaust back into the case (as often occurs when peeps incorrectly mount CLC fans as exhausts) and a 120mm 1450 rpm-ish fan mounted on the back of the HD cages, this doesn't happen.

Both my wc'd 780s run at 39C GPU temp / 44C if I limit fans speeds to inaudible range and on my sons box (wc'd CPU only), his top 970 ran about 8C hotter than the bottom one, TDP varied by 2-4%. The fan on back of HD made that go away tho.

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