This is a new thread that was started , because I realized continuing discussing it in the dual GTX 970 thread would have made it perhaps OT . Anyways , here goes: I propose that it is possible to install two GTX 980 video cards for Compute purposes inside a Nehalem Mac Pro 5,1 or an upgraded Mac Pro 4,1 > 5,1 . And , furthermore, to power the cards entirely internally and have enough power remaining to operate a high performance PCIe secondary storage device . We should also use just a single processor six core machine with a reasonably useful amount of memory . All these components are necessary to make a GPGPU array useful in rendering operations . The Nehalem Mac Pro has a large PSU rated for 980 W . It is a very efficient power supply and provides system builders with a lot of opportunities to install high performance after-market components . We just have to be careful where we acquire the power sources for all these components since Apple made the Nehalem Mac Pro beautiful inside . Components are not directly powered by the PSU , but receive power indirectly through motherboard traces . It is vital that the power drawn through these motherboard traces do not exceed their rating , or the motherboard will become damaged and there is the possibility of a case catching on fire . And let's be conservative with overall system power draw , so no beefy 12 Cores and 128GB memory configurations . A nice humble Hexacore will do . Remember, rendering is not CPU centric anymore , but having lots of energy sipping threads available probably won't hurt . Here's the proposed system configuration : Mac Pro 5,1 (or 4,1 > 5,1) One x L5639 Xeon . Six Cores @ 2.13 GHz . 12 threads . 60W TDP . 32 GB 1333 MHz DDR3 ECC 1.35v low voltage memory modules (4 X 8GB) . 2 X nVidia GeForce GTX 980 Reference video cards (PCIe slot 1 and 2) . 1 TB PCIe Sonnet Tempo Pro Plus card with 2 X 512 GB Samsung 850 Pro SSDs in RAID 0 (PCIe slot 4) with 6G eSATA . No optical drives . Blue tooth , factory . Wifi , factory . I will dispense with instructions on how to install the processor , memory and Sonnet Card . They are available elsewhere and these particular upgrades are not difficult to install . We just want to discuss a possible method of installing two highest end Maxwell GPUs available . Nvidia GeForce GTX 980 reference cards consume 165 W of power, officially . But there are some concerns they actually consume 183 W at load in our Macs . So, let's over-engineer our system so there are no nasty surprises . Here's how we can install two GTX 980s right now . Additional components needed for power delivery are two ordinary Mac PCIe 6 pin power booster cables, two powered 2.5 to 3.5 SATA HDD form factor adapters and two SATA power to 6 pin PCIe booster adapter cables . Not too many parts , but they are uncommon and need to be special ordered . Anyways, here's our installation so far : GTX 980 card number 1 powered from PCIe slot 1 (75 W) + PCIe booster Aux A (75 W) + SATA Backplane #3 (33 W) . Total 183 W consumed . PCIe slot 1 . GTX 980 card number 2 powered from PCIe slot 2 (75 W) + PCIe booster Aux B (75 W) + SATA Backplane #4 (33 W) . Total 183 W consumed . PCIe slot 2 . That was quick ! And it will work , too . But dream time is over, folks , because ... We have our array up and running internally powered , but we also just consumed all available PCIe power resources (300 W) . There's no juice left for the two remaining PCIe card slots . Hmmmm . And we really need one of them . How are we going to fire up that gorgeous Sonnet PCIe high performance drive card we will definitely need to write all the data our GPGPU array is so busy processing ? We'll need to reroute just one of those 6 pin power booster cables and stick it somewhere else But where internally ? We're not gonna cheat and route from the exterior . Clients hate external aux power supplies . Every time I mention it , I get cold stares . So, time to think like a Borg again ! Enter the ODD power point on the mobo . That point fires up two energy hogging optical drives through a harness in the Optical Bay . We reroute the optical harness from the Bay and into the middle of the enclosure , where the video cards are . There are two SATA power connectors on that harness . We get a PCIe 6 pin booster connector to dual SATA power connector splitter cable and connect it to one of GTX 980 card number 2's booster connectors . This will require some minor cutting and super-gluing . But , at least, we have all our devices connected and powered . Concern is , how much juice does Apple allow to the two Optical drives ? A DVD-RW drive likely needs 27 W max . So, the two drives will be provided with 54 W bare minimum through that harness . And there is going to be some extra provided as a reserve . What this amount is , is anyone's guess . So, we now have the following GPU installation : GTX 980 card number 1 powered from PCIe slot 1 (75 W) + PCIe booster Aux A (75 W) + SATA Backplane #3 (33 W) . Total 183 W available . GTX 980 card number 2 powered from PCIe slot 2 (75 W) + ODD SATA (54 W) + SATA Backplane #4 (33 W) . Total 162 W available . We now have more than enough juice (75 W freed up) to power the PCIe Sonnet high speed drive . But, will a GTX 980 work properly with just 162 W in a Mac ? And , we also need to be concerned about melting mobo traces with all the draw the components will be demanding . We're grabbing resources from all over the place . So, maybe we should take baby steps, one step at a time, with this build, folks . And, if it does work as described as above, there may be some automatic down throttling involved , due to insufficient power provided. Who knows until someone tries the design , but ... If the above set up does not work, then we will need to take a page from Prince134's beautiful dual 7970 Mac Pro experiment and down voltage the cards to reduce the power draw . I bet this system build would work , especially with down volt-aging the cards as a last resort . The 980s perform no higher than 20 percent compared to the 970s in the passmark direct compute score . Compute is what this is all about . And I think my Dual 970s draw 153 W peak each in one of my other Macs right now . So, add 20 percent to the 153 W and we get 183 W peak . The 970s at load probably averages around 135 W each . This is really a close call... And all of this is still theoretical , as no one has done this yet (install two GTX 980s in a Mac Pro entirely internally powered .) Given enough time , I'll do this build and write up precise instructions . But someone else will probably beat me and these notes are intended to help and to open a general discussion . Overall , the system should not consume a large amount of power relative to the high performance the build should deliver , especially once drivers are optimized for the Maxwells . I estimate total system power consumption at around 500 - 600 W at load . And just as a disclaimer , by the way, try all this at your own risk . Sparks may fly . You're on cutting edge of internal Mac Compute, after all . And the cutting edge is rarely safe territory .