I have a Dell motherboard (OHY9JP), A GTX 1060, and a GTX 960. My motherboard features a PCI-E x16 in x16 and a PCI-E x16 in x4. I currently have the GTX 1060 in the x16, with the GTX 960 in storage. I wanted to run both GPUs concurrently to speed up GPUgrid, but when I plugged the GTX 960 in the x4 alongside the GTX 1060 (no SLI), the computer would not boot. I *think* I have enough power feeding the GTX 960, the 1060 and 960 run on the same rail, one requiring only a 8 pin and the other a 6 pin, but I am wondering if there could be come other issue, like motherboard compatibility, that is stopping it. Why is it not working? Does the motherboard have to support something, or is it a power requirement?

I think it has something to do with the motherboard. I remember trying to do that also with a dell. Even installed a larger PSU. Would never boot.

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That is what I figured since this motherboard's apparent goal is to do nothing I want it to. Thanks for your time.

I'm surprised a 4x electrical has a 16x physical slot. Technically it should still work.

This says it can supply 25W with the 16x slot at 35W.
http://clascsg.uconn.edu/download/specs/O790.pdf

Max power is 200-265W (depending on form factor) for the system so probably not enough for two cards. Or do you just have the motherboard and not the whole system?

Some things you could try to narrow it down.
Run just the 4x slot w/o a card in the 16x slot.
Disable onboard video.
Flash the 960 card to pull more power from the PSU cables instead of the PCI-E slot.

Thanks for the ideas. In case it help you or anyone else visiting, the 960 is an EVGA SC with a single fan. It has only a 6-pin power input. I have a full system, and the PSU is a 520 watt, 70 of which is fed to the CPU, according to Open Hardware Monitor.

the PSU is a 520 watt, 70 of which is fed to the CPU

Did you already take into account that the PSU is maybe 80% efficient? Which means there are ~410W available only. Having said this ... 70W needed by the CPU, maybe yet another 50W by the HDD, RAM and other devices, leaving 290W to both GPUs. The 1060 pulls 120W at full load and the 960 yet another 100W. Which means the PC should power up even so.

Maybe the Dell Mainboard fails to fulfil the PCI standards to deliver sufficient power by all slots? That reminds me of the RX480 power issue when Vega was released. mmonnin already touched on this...

This says it can supply 25W with the 16x slot at 35W.

yes, that may be the reason ... the question is, what will a gtx 960 do if the PCIx4 slot provides 25W only?

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I would love to see HCF1 protein folding and interaction simulations to help my little boy... someday.

Alright. Thanks for that thought! I'll try just the 960 in x4 and see what happens.

the PSU is a 520 watt, 70 of which is fed to the CPU

Did you already take into account that the PSU is maybe 80% efficient? Which means there are ~410W available only.

Usually the total output power is the rating of a given PSU, so you did the math in reverse. If the given 520W PSU has 80% efficiency, fully loaded the 520W is the 80% of its power draw from the wall outlet (which is 520W/0.8=650W).

However it is not recommended to load a PSU above 50% continously, as most PSUs are the most efficient around 50% load, and they are not intended to have high load 24/7 (except for the professional/server/gold or higher rated product line.)

If the PSU has many 12V lanes, each lane has its own (lower) power rating for security reasons (overcurrent protection).

Molex to PCIe power connector converters are highly unrecommended.

About the original issue:

It's a DELL product, and they have very strict product specifications. If it's not stated expressively that this product supports two discrete GPUs, then it won't work with them. This limitation is probably preprogrammed in its BIOS.

I have assumed that the 520W already is the primary power as there are not many PSU with uneven numbered wattage. Sounded like a 400W PSU to me. But if the 520W is the output load indeed, then you are correct, it is the other way round.

Side note. Even with that calculation of reducing available power from primary to secondary side by the power efficiency, you need to be careful with that figures, see this Image

https://goo.gl/images/P8Re7D

It may be that another portion must be deducted for sizing the PSU as there are limitations in maximum load in the fine print... and the nominal capacity is not really meant to be withdrawable all the time. Frankly, there are many weird PSU designs on the market and you sometimes are not even safe by choosing a brand name.

it is not recommended to load a PSU above 50% continously, as most PSUs are the most efficient around 50% load

I agree to that. I normally run my PSU at about 60% load.

Molex to PCIe power connector converters are highly unrecommended.

Generally speaking, yes. But I wouldnt be much concerned having a non overclocked tiny gtx 950 in a PEG, not really drawing near the maximum specification on its 6pin connector. But I would keep my hands off those adaptors (especially when it provides an 8pin 150W rated connector) having a thirsty 780ti or a R9 290X. That would possibly cause a shut down right away. In any case, reading the specs and the power limits per line on the type plate is a good idea. Especially those HP, Dell, Acer... integrated systems are full of surprises.

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I would love to see HCF1 protein folding and interaction simulations to help my little boy... someday.

the PSU is a 520 watt, 70 of which is fed to the CPU

Did you already take into account that the PSU is maybe 80% efficient? Which means there are ~410W available only. Having said this ... 70W needed by the CPU, maybe yet another 50W by the HDD, RAM and other devices, leaving 290W to both GPUs. The 1060 pulls 120W at full load and the 960 yet another 100W. Which means the PC should power up even so.

That's backwards. A 500W PSU supplies 500 DC watts to the PC but pulls more than 500 AC watts from the wall. The efficiency loss is in the conversion. A PSU should be able to deliver it's rated DC power.