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Lot's of questions! Come one, come all! GTX 275, psu compatibility

Last response: in Graphics & Displays
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June 2, 2009 4:21:43 AM

So basically the main topic I need assistance with/help under standing, is about PSU's. Since I am specifically looking at the BFG Tech BFGEGTX275896OCE GeForce GTX 275, I decided I might as well post it under Graphics Cards. ( I think these forums are more busy anyways) I was considering ATI's 4890 also, but it appears that this particular 275 is preforming way better. Evern better than the GTX 280...( I think these forums are more busy anyways) I plan on getting the GTX 275 here within a month or two, but I noticed also that I will need to be upgrading my psu also. The requirements for the GTX 275 I am looking at requires a combined 12V current rating of 42A or more. I've been looking around, and I've got a good feeling of what I'm needing for that. The main question is, and what I need help understanding, occurs when putting two of these in SLI. ( Not looking at doing now, but in the future when prices come down) What I'm not sure about, is with two GTX 275's do you need a combined 12v rating of 82A? Or/Also, since there's two, do I need a psu with two 12V rails? If someone can help me get a better understading of the Amp requirements of running SLI cards, It would be greatly appreciated.
You can view the BFG Tech GeForce GTX 275 card I'm looking at from the below link.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Was looking at something like this for the power supply.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Has a 60Amp 12 volt rail, but once again I don' t know if I need an 82 one since there's two cards... Your answers are greatly appreciated!!
June 2, 2009 5:16:56 AM

Thats an awesome power supply I use it , but the most I would do is two GTX260's in SLI with that PSU and thats with everything in my case minimal , maybe get they're 1000watt. what do you have in your computer besides? If its mostly new stuff or you have a lot of HDD's I suggest you go with the 1000 watt. otherwise the 750 should be fine.
June 2, 2009 5:21:20 AM

I will post the correct specs of the rest of my pc tomorrow morning. I'll go ahead and say, it's very minimum though. The main thing I really do not understand though, is 1. Do you have to have two 12V rails to run two graphics cards? And if not, would I need to have a combined rating of 84Amps if it were one rail to run two of the GTX 275's as listed above?
Related resources
June 2, 2009 5:26:50 AM

No, two gtx 275s will not consume 82 amps. 82a * 12v = 984 watts of power just on the GPUs alone. I don't think I need to explain how absurd of a number that is :lol: 

Two gtx 275s will not even consume 42 amps combined - guesstimating off of this review I would say a pair of gtx 275s will need about 30 amps.

For PSU 12v rails, this is a little weird, yet very simple. The truth is that nearly all PSUs, except for the highest of the high end, have one (1) 12v rail. That single 12v rail is split into multiple cords before the wires even leave the PSU, so, as an end user, all you will ever see are the multiple cords. This is done for two reasons: There are multiple components that need the 12v power, and because having all that current running through a single 18 gauge wire would make it very, very, very hot. Like, hot to the point where it melts the insulation around the wire and then the wire itself melts. You'll often see PSUs that appear to have multiple 12v rails, but in reality they are just measuring the individual wires that are a part of the 12v rail.

Take a look at this PSU for an example. You can see that it has two 12v wires rated at 18a each, and then the entire 12v rail is rated for 360 watts. This means that while each individual wire can pull 18 amps of current, the combined load between the two wires cannot pass 360 watts, because both wires are being powered by the same rail. Doing a bit of math, we can figure out that 360 watts / 12 volts = 30 amps, so the max load on the 12v rail for this PSU is 30 amps.

Some PSUs, such as your Corsair unit, will simply list the max load for the 12v rail and not tell you individual wire ratings. On that note, the corsair unit that you initially linked would be an excellent choice.
June 2, 2009 6:07:43 AM

Thank you efeat. Your post helped a lot. It seems what I am confused about is the system requirements for the power supply listed on newegg.com. ( for the BFG Gefoce GTX 275) "575W PCI Express-compliant system power supply with a combined 12V current rating of 42A or more" You can see the real page by using the link i put up in my first post. I guess I can probably answer my own question by saying the 42A or more, is saying for your average system, you need 42A for everything on your system. Would you mind clearing this up for me? And just to clarify, you are saying that two BFG GeForce GTX 275's running in SLI, would only need 30Amps combined? 15Amp per split of the 12v rail?
June 2, 2009 6:15:26 AM

Gin Fushicho said:
Thats an awesome power supply I use it , but the most I would do is two GTX260's in SLI with that PSU and thats with everything in my case minimal , maybe get they're 1000watt. what do you have in your computer besides? If its mostly new stuff or you have a lot of HDD's I suggest you go with the 1000 watt. otherwise the 750 should be fine.


I am currently running a AMD Athlon 64 X2 4200+, of course I'm going to want to upgrade to a new one soon.
Just running one 80Gb hardrive right now (lawl, had it for 5-6 years). Whenever I do get a new one, it'll be just one 500Gb hardrive or so.

So yeah, watts definitely won't be a problem. Of course I am wanting to get a power supply that will last me about 4 years. Same plan goes for the graphics card. I'm hoping the GTX 275 will do me good for about 1-2 years, and then when I start having problems, buy a second one and run them in SLI (Prices should be way down by then). Which I'm once again hoping will carry me over for another 1-2 years.

Based on this post, how many watts do you think I should be looking at? ( To carry me for the next few years with the better processor, one big hard drive, two GTX 275's )
June 2, 2009 1:38:23 PM

Based on that post and what efeat said , the Corsair 750 watt should be perfect for you , and probably will last you the 4 years you want it to since your minimal with your computer , when you go SLI though, I suggest you dont go above more then 2 maybe 3 HDD's and 2 Disk drives.
June 2, 2009 4:21:50 PM

I have been doing some research this morning, and I have found two sites regarding the Power Consumption of the BFG GeFore GTX 275. ( Who knows how creditable these 2 sites really are though)

http://www.bit-tech.net/hardware/graphics/2009/04/17/pa...

http://www.bjorn3d.com/read_pf.php?cID=1554
You have to scroll nearly to the bottom of the page from the above link to find the chart. ( Tons of information on that link!)

The charts post that the idle Power Consumption is 167w, 216w's at idle.
Then it goes on to the peak power consumption which is 364w, 369w.

Which would mean (taking the higher posted chart for safety), you would need 30.75Amps per split on your 12Volt rail to run this card at max settings with no problems. 369w/12V=30.75Amps. Is this figure correct?
June 2, 2009 4:25:24 PM

Bit-tech used to be a site I liked.... until I grew up. >.< I've never heard of Bjorn3-D so no comment. but yes that is exactly the reason why I said he should be minimal if he's gonna SLI those 2.
June 2, 2009 4:28:07 PM

Just noticed that the first link I sent wasn't specifically for BFG's GTX 275... So really just worry about the second link, which is what I did my figuring off of anyway.
June 2, 2009 4:51:05 PM

Alright here's something else I need some assistance understanding. I noticed that newegg shows the power connectors to be 2x 6-pin. Are the PCI-E cables these 6-pin connectors it is referring to? If so, I have another question to follow it up.
June 2, 2009 4:53:07 PM

Yes that is what they are referring too.
June 2, 2009 5:05:11 PM

Next question then. Are you able to power this graphics card say.. if you had one PCI-E connector that was putting out 30Amps? Or would you be required to use two PCI-E connectors? ( I realize that it is logical to use two anyway, and that's what I would do. Just asking for the sake of learning)
June 2, 2009 5:07:39 PM

To answer a few of your questions:

I have no idea where GPU makers pull their numbers for power requirements. So far, every single amperage requirement I've seen plastered on a box has been more than double what you really need. When I guessed at 30 amps for both cards, I meant 15 amps to each card, 30 for the total. Anandtech checked power consumption in their gtx 275 review and it seems to use just a hair less than the gtx 280.

The only other component you really need to be concerned about with total system power is the CPU, which also uses the 12v rail. A stock i7 920 is a 130w CPU, which means it'll need 130w / 12v = 10.83 amps of power in addition to the ~30a your GPUs will need. That already puts you at 41a/492 watts before you even add any other components. The upside to this is that all of your other components combined will only total a few amps. HDDs typically take 1 amp at their max load (which never happens for more than a couple seconds, due to the nature of HDDs), Optical drives are the same way, and fans/LED lights are even less. The [theoretical] max load of your system will probably be in the 550w range. But then you have to ask yourself, when is every single component going to be stressed to 100%? Pretty much never, unless you like to do massive file transfers while burning a DVD and playing a couple 3D games while calculating the meaning of life.

Anyway, getting sidetracked here. The Corsair 750w would work well for your build when you finish it. If you want a little extra oomph in case you decide to play with overclocking or if you want to try and re-use your PSU for the next build, try the Corsair 850w. It's only ~$25 more expensive, and would give you a bit more wiggle room for planning your build.

Edit: Yes, you would probably need to use two anyway. The GPU will probably sense that one connector is not present then proceed to flip out at you.
June 2, 2009 5:30:22 PM

Yup , just get the 750 watt. :3 and they get they're numbers from an average based on systems they use under load. and you always want jsut a little more power then your entire system would use at load.
a c 271 U Graphics card
June 2, 2009 5:42:35 PM

Also don't forget the simple limitations of the hardware, PCIe slot on motherboard = 75w, PCIe feed (six pin) from PSU = 75w therefore a GTX275 can consume no more than 225w.
June 2, 2009 5:59:15 PM

Thank you both. I believe I have just a few more things I need clearing up. First of all, I'm a little confused how some of the Power Supply's specifications are listed. For example:

CORSAIR CMPSU-750TX 750W
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

And another one I was looking at, the XIGMATEK MC NRP-MC751 750W.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

What's a little confusing to me is, the CORSAIR shows one 12V@60A. It has 4 PCI-E and 1 x 12V(4/8Pin). quick question, is the 12V(4/8Pin) for your processor? And back to the point I'm getting with this post... is the 4PCI-E connectors sharing the 60A? As in 15Amp for each connector? Then I also have to take in account, as efeat mentioned, the PSU runs off the 12 volt rail also. ( which I'm assuming is the (4/8Pin) connector) With this in mind... does each connector just run at, however ever many Amp's the device it is plugged into required? Say PSU is using 10... and then the PCI-E connectors just use the combined 30Amps you referring to?
June 2, 2009 6:01:40 PM

They all share the amps from the PSU so your parts will get the correct ampreage they need if and only IF its plugged into the part , so dont worry about not getting enough. The 12V rail is inside of the PSU.
June 2, 2009 6:04:16 PM

I got a little side tracked with the question there. What I was getting at when I posted the link... I'm not getting the difference between the 12Volt rails on the two Power Supply's I listed. The Corsair product just shows one 60Amp 12 volt rail. Whereas the Xigmatek has It's 12V rail listed like this, "12V1@18A,+12V2@18A,
+12V3@18A,+12V4@18A". Bottom line, what is the difference between the two 12V rails on these two power supplies. Oh, and that's a very good point you made MouseMonkey. Thank you.
June 2, 2009 6:06:33 PM

I see what you mean Gin Fushicho.
June 2, 2009 6:07:35 PM

Mouse monkey always makes good points. =) and one 12V rail is a lot better then having many less functional 12V rails.
June 2, 2009 6:12:27 PM

So what it be correct to say, that with one 12volt rail, your CPU's connector, and all the PCI-E connectors share the one Max Amp rating of that rail? When there are multiple 12 volts rails, like the XIGMATEK PSU I posted, would that mean that one PSI-E connector is run by each 12Volt rail? ( seems logical since each rail is putting of 18Amps. But then which one does the PSU run off of?) I'm sure one of you can better explain this to me though =)
June 2, 2009 6:18:26 PM

When there are multiple rails it "switchs" the rails depending on the power need so yu might experince a mini brown-out in power. you are however correct about the single rail yes.
a c 271 U Graphics card
June 2, 2009 6:23:48 PM

Gin Fushicho said:
Mouse monkey always makes good points. =) and one 12V rail is a lot better then having many less functional 12V rails.

Thank you very much sir, you're too kind *blushes*

@silvertounge, I found this from a review on the Corsair http://www.hardwaresecrets.com/article/505/8
June 2, 2009 6:26:38 PM

Thanks Mousemonkey, I'll look at that now.
June 2, 2009 6:27:32 PM

Mousemonkey said:
Thank you very much sir, you're too kind *blushes*

@silvertounge, I found this from a review on the Corsair http://www.hardwaresecrets.com/article/505/8


Okay that is awesome , I loved my power supply before but now I love it even more. I'm going to open up my case and hug it , and maybe give it a naughty lick. ;3
June 2, 2009 6:32:31 PM

I'd just like to clarify something about the 1 12V rail, vs. multiple. So the conclusion would be that one 12V rail would be recommended. Multiple 12volts rails will work, but because of the switching it might have to do, it's just safer to go with the one 12volt rail, correct?
June 2, 2009 6:38:06 PM

Good. One more issue I'd like to discuss, is the difference between a SLI/Crossfire ready PSU vs. a SLI/Crossfire Certified PSU. Simply put, what is the difference between the two?
June 2, 2009 6:41:50 PM

There is no difference between the two , its just a brand prefrence on the manufacturers part. in other words if it says "Xfire ready" That means they used ATI products in the testing of the PSU , if it says "SLI ready" they used Nvidia products.
a c 271 U Graphics card
June 2, 2009 6:45:36 PM

I would assume (and it is only an assumption) that the former should be OK because the manufacture feels it's up to scratch whilst the latter has been given the nod by ATI and Nvidia.
June 2, 2009 6:54:27 PM

lol right. I realized the difference between SLI/Crossfire, my real question was between the "ready" and "certified", which Mousemonkey's assumption seems to cover the question ^.^
June 2, 2009 6:55:10 PM

silvertounge said:
lol right. I realized the difference between SLI/Crossfire, my real question was between the "ready" and "certified", which Mousemonkey's assumption seems to cover the question ^.^


Yeah I miss read it the 1st time I'm sorry.
June 2, 2009 7:05:14 PM

Alright, thanks to you guys I have a much better understanding of PSU's, and pretty much know what I'm looking for when I decide to purchase a new one. Probably the last thing I'd just like double checked, is if my current PSU will work for just one of BFG's Geforce GTX 275's.

The PSU I have is a OCZ GameXStream OCZ600GXSSLI 600W. Which you can see from the below link.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

As I discussed earlier ( and once again who knows how creditable this site is), the below link shows their results for the exact Power Consumption of the BFG GeFore GTX 275.
http://www.bjorn3d.com/read_pf.php?cID=1554
The chart is near the bottom of the page, but I will go ahead and say that it shows that the Idle consumption is 216Watts (216w/12V=18Amps), and then, during the 30 minute loop of 3DMark Vantage the peak power consumption was recorded as 369 Watts (369w/12V=30.75Amp).
Keep in mind I'm currently running AMD Athlon 64
X2 4200+ ( when I upgrade, it will just be a very good one, but no where near top notch)
4Gb of DDR2 Ram.
One Hardrive, one cd/dvd drive.
Will my current power supply suffice?
June 2, 2009 7:05:38 PM

Gin Fushicho said:
Yeah I miss read it the 1st time I'm sorry.

It happens! haha, I appreciate the response anyway.
June 2, 2009 7:06:55 PM

Yes your power supply will suffice for one card. =)
a c 271 U Graphics card
June 2, 2009 7:19:25 PM

^+1
June 2, 2009 7:22:34 PM

Glad to hear that =). Well... I'm trying to think if there's any other possible thing I could ask lol. I've learned a lot in the past few hours ^.^
June 2, 2009 7:44:59 PM

Okay one more quick question, the AMD Athlon 64
X2 4200+ I am currently running ( even though I plan on upgrading soon), won't be bottlenecking the GTX 275 will it?
June 2, 2009 7:46:04 PM

It will. you will want to get a 2.8Ghz dual core or better to get full performance out of the card.
June 2, 2009 7:49:39 PM

Sounds good. I appreciate all the responses. It really has cleared up a lot of issues for me. I'll leave this forum open a little longer, just in case I might think up something else.
June 2, 2009 7:51:23 PM

Okay. x3 Well I hope you enjoy your upgrades. =D
a c 271 U Graphics card
June 2, 2009 7:51:28 PM

Agreed, although cranking up the res and eye candy to the max will alleviate that somewhat, and by that I mean 1680 x 1050 is the lowest you want to go.
June 2, 2009 7:56:35 PM

1680x1050 is actually what I'm running at currently, and will be till I get a new monitor ;) 
June 2, 2009 9:54:47 PM

silvertounge said:
I'd just like to clarify something about the 1 12V rail, vs. multiple. So the conclusion would be that one 12V rail would be recommended. Multiple 12volts rails will work, but because of the switching it might have to do, it's just safer to go with the one 12volt rail, correct?


Incorrect. All the PSUs you are looking at only have one 12v rail. What you are getting confused on is how manufacturers label their units.
Some, like Corsair, just give the total rating of the 12v rail. This type of labeling is good for the at-a-glance value. You can easily see the capacity of the 12v rail without having to do any math. The downside to this is that you lose out on the more detailed specs (such as how much each individual wire that's hooked up to the 12v rail can carry.)
Others, like the Xigmatek, show the rating of each wire hooked to the 12v rail, and then the total wattage of the 12v rail. This type of labeling is good because you see what the individual wires hooked into the 12v rail are rated for. The downside to this is that you have to a bit of math to find out what the overall 12v rail is rated at.

Rather than type any more, I'll just send you to this long but well-written post that can explain it better than I could.
June 2, 2009 11:01:45 PM

I read the entire post, and that does make since. So tell me this then. Say if a company, such as Corsair, is giving a Power Supply a total rating of 60Amp. So are these amps split up between the 4 PCIe connectors? meaning 15 Amps each? Or are they split up even less than that, because you have to account for the connector for the CPU also?
June 2, 2009 11:46:25 PM

silvertounge said:
I read the entire post, and that does make since. So tell me this then. Say if a company, such as Corsair, is giving a Power Supply a total rating of 60Amp. So are these amps split up between the 4 PCIe connectors? meaning 15 Amps each? Or are they split up even less than that, because you have to account for the connector for the CPU also?


Yes, the amps are split between the multiple PCI-E connectors, but they do not come out to be 15 amps each. Each PCI-E connector will pull as much power as it needs - that's all there is to it. The only reason some PSUs will list 'multiple' 12v rails is because they are telling you what each individual connector is rated at, so you don't overload one. GPU makers are already aware of this, and design their cards to require enough PCI-E connectors that the average PC builder (such as you and I) will never have to worry about that.
June 3, 2009 12:25:32 AM

Thank you. One more question though. The CPU Amp requirements are being taken out of the 12V rail also? ( As discussed earlier in this chat) For example, say you have a PSU with a 12V rail rated for a total of 70Amps. (Just throwing numbers out there) Say your SLI graphics card require a total of 50 amps, and then your CPU takes up 10. Would your PSU's amp requirements come out of the 12V rail also? Leaving you with 10 Amps to spare.
June 3, 2009 1:14:49 AM

Yes, that's correct. That's what the EP-12V (or whatever it's called) is for.
!