Specs of secondary PSU for GPU

sam0605 · Dec 17, 2017

Hi guys, I have a 4 years old PC
i7 3770
16 GB DDR3
GTX 650 ti
450W PSU

I recently got a great deal for an used GTX 780 (http://www.gigabyte.com.au/Graphics-Card/GV-N780OC-3GD-rev-20#sp )
I know my current PSU cant handle the power requirements for the GTX 780 (550~600W recommended)
Also, I have an old 400W psu (from another old PC) laying around. The 12V rails have a total capacity of 300W [+12V1 (13.0A) & +12V2 (14.0A)]

I want to use this PSU (as secondary) to power only the GTX 780.
So my question is "will this PSU be enough for power the GTX 780?".

PS: I know how to setup secondary/dual PSU for GPU but I'm just not sure about the wattage requirements (GTX 780 is a power hungry beast).

Any help is appreciated. Thanks!

aquielisunari · Dec 17, 2017

According to https://www.anandtech.com/show/6973/nvidia-geforce-gtx-780-review/19 that PSU doesn't seem to offer enough. That's total system usage. I then look to its needs with a complete system- 600W (with one 6-pin and one 8-pin external power connectors). That means 75W PCI-e + up to 75W(6 pin) and up to 150W(8 pin). That means up to 300W the card has access to. The PSU can offer up to 300W. Toss out 20% because of efficiency issues. That means it can sorta be depended upon for up to about 240W. Add in the abysmally low amps on the dual rail PSU and I'd not take a chance with it.

sam0605 · Dec 17, 2017

aquielisunari :

Thanks for your quick reply. Appreciated.
I agree that the PSU will be able to provide maybe ~ 240W stable, but since the GPU will also be drawing ~75W from the PCI-e, does it means its viable.

PS: I'm not sure about the interaction/priority of power source (PCI-e vs external 6/8 pins)

aquielisunari · Dec 17, 2017

I agree that the PSU will be able to provide maybe ~ 240W stable, but since the GPU will also be drawing ~75W from the PCI-e, does it means its viable.

Possible? Yes. Something I would try? No. Why?

Amps? It needs

600 Watt or greater power supply with a minimum of 42 Amp on the +12 volt rail.

That PSU's combined amps? 27A(Thanks to Capt. Obvious

)

That 300W PSU isn't a good candidate. Bad, very very bad.

4745454b · Dec 17, 2017

Toss out 20% because of efficiency issues. That means it can sorta be depended upon for up to about 240W.

It doesn't work like that. Not even close.

I very much doubt a 300W PSU will have the 8 and 6 pin plugs you need. I know it won't be powering a system at the same time, but I'm going to give you my standard advice when looking to use molex to PCIe adapters. Don't. Using adapters to force a PSU to power something it wasn't designed to power is a bad idea. Even if the math "in theory" checks out, practice will show you why it's a bad idea. I know you don't want to hear this, but getting a proper PSU to handle the load is a much better idea.

aquielisunari · Dec 17, 2017

4745454b :

Toss out 20% because of efficiency issues. That means it can sorta be depended upon for up to about 240W.

It doesn't work like that. Not even close.

I very much doubt a 300W PSU will have the 8 and 6 pin plugs you need. I know it won't be powering a system at the same time, but I'm going to give you my standard advice when looking to use molex to PCIe adapters. Don't. Using adapters to force a PSU to power something it wasn't designed to power is a bad idea. Even if the math "in theory" checks out, practice will show you why it's a bad idea. I know you don't want to hear this, but getting a proper PSU to handle the load is a much better idea.

Thanks for stating the painfully obvious answer that I missed

sam0605 · Dec 17, 2017

I see. But isn’t 18A out of 42A (recommended) goes to the mobo?

In any case, thanks for your input. I might consider upgrading my PSU.

4745454b · Dec 17, 2017

Thanks for stating the painfully obvious answer that I missed

I wasn't trying to single you out. Just correcting factually inaccurate information. People say that about efficiency all the time. Sorry, it's wrong.

I see. But isn’t 18A out of 42A (recommended) goes to the mobo?

Again, the math might add up, but practice will say otherwise. Your talking about a 300W board, and if I'm right it has no PCIe plugs on it. None. It wasn't designed to actually put out that much power over its 12V rail. You'll fry it. Maybe not the first time you try, but the 12v rail will fry when you plug a 200-250W GPU into a 300W . Even if 75W is coming from a different PSU.

aquielisunari · Dec 17, 2017

sam0605 :

The specs of the PSU are now moot. Without the 6 and 8 pin connectors it's useless.

sam0605 · Dec 17, 2017

4745454b :

Thanks for your reply. I'm thinking of getting a 600W PSU. I believe that should suffice.

Just for your information, that secondary PSU is 400W with a 6-pin PCI-e adapter and multiple molex and sata lines.

4745454b · Dec 17, 2017

400W or 300W? If it's a good 400W with a 6pin it has a chance. If it's a 300W with no PCIe plugs I wouldn't even try. Getting a "600W" is meaningless and doesn't tell us anything. With PSUs it's all about the model.

sam0605 · Dec 17, 2017

4745454b :

Well, its a 400W PSU with 300W on 12V rails and the remainder on 3.3V & 5V.

Building on that, how about I switch the PSUs? Use my current 450W (80 plus) to power the GPU and the 400W for power the system?

4745454b · Dec 17, 2017

With PSUs it's all about the model.

For all I know the 400W is the better unit.

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