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Another Question About: GTX 770, 42 amps required on +12V Rail

Last response: in Graphics & Displays
January 21, 2014 11:33:50 PM

Hey gang.. I've seen similar questions asked and answered, but none spoke directly to the question that I have regarding my setup...

I am looking to buy a GTX 770 4gb, which states that it requires 42 amps on the +12V rail

I have a Thermaltake Toughpower 850 from a few years ago, and I saw this review posted recently that threw me for a loop:

I bought this psu in August 2007 and have used it continuously. I've had no need to upgrade until now. Sadly, it can not power the latest high end graphics cards. I bought an EVGA GTX 770 that requires 42A on the 12V rail. This unit has 4, but maxes out at 30A. I want to take advantage of the power management features of the new Haswell processors and this PSU isn't certified or capable. Sadly, it will no longer power my primary rig, but will find a welcome home in my secondary machine. Apart from those two deficiencies this is the best power supply I've ever owned.

Is this indeed the case? When I bought this PSU, I went overboard considering I'm running an i7 920 @ stock speed, and a Radeon 6950 also @ stock speed. I bought it for future expansions into Crossfire or SLI. Seems I'm not going to get to continue using it if I go with the GTX 770 - is that correct? I also intend to upgrade the rest of my rig, but that's another story for now...

Here's a snapshot of the PSU Specs:

Thanks in advance.

Best solution

January 21, 2014 11:39:15 PM

You have 62Amps on your 4 +12V Rails so you should be more than fine.

Im not an SLI/X-Fire Wiz but for a single 770, Your Golden.

with 850W (744W Available) you should be able to SLI 770s

744W \ 12(V) = 62Amps

I think whomever wrote than review/post was misinformed..
January 22, 2014 12:03:26 AM

I agree. He is an idiot. Maxed out at 30 A.. Seriously.

However see how many PCI-Express connectors are there on the PSU. 1 6pin and 1 8 pin each are needed to power the two cards that you want to install. So you'll need two of each.
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January 22, 2014 9:12:25 AM

thanks for the quick responses

I hadn't considered that my +12v capacity was actually the sum of all my +12 rails. Makes perfect sense, actually.

It seems that I will be able to upgrade my system at the pace I would like, then.

I'll check out the pin config for sure, but for now I'm just going to run a single card anyway.

Much appreciated!
January 22, 2014 9:21:12 AM

DCNemesis said:

I hadn't considered that my +12v capacity was actually the sum of all my +12 rails.

Not really.
It is not the sum of 12V rails.
A rough estimate is that you can pull off 80% of the current from a given rail.
So the PSU you mentioned, the rails are 18, 18, 30 and 30 A respectively. The 30A rails are meant for the graphics card. The 18 A rails are meant for low power components like fans.
Total sum of four rails is 96A. 80% gives 76.8A. This PSU must have been built using a higher margin, that's why it's rated at 62A.

If not given, use this guideline.

TL;DR: Each rail is built keeping a current margin in mind, given is max current. The max current is generally given separately(62A). If not given, use 80% as guideline.