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500w psu with 2 18a 12v rails enough for GTX 260?

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February 4, 2009 2:57:56 AM

I have an OCZ StealthXStream power supply at 500w.
It has 2 12v rails at 18A each. According to EVGA's site their gtx 260 ssc requires 35A of power on a 500w PSU. Now I realize my psu does not exactly meet that standard, so I started looking at other cheap psu's that could handle it. the cheapest looking one is this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817148010
at 34 a. if this is enough it would be a minimal cost in upgrading. If it isnt, I would likely move up to this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341011
A much bigger cost, but would allow for sli of a second 260 later.

The first PSU I listed has one 12v rail with 34A, Just short of the 35 required. Would this suffice, not just the amperage, but say it were a 35A, it only has one rail, would it still have enough power with every other component also running on it?

Now for my real question. I started to realize that the fact that my psu has 2 18A rails may be to my advage when I saw this post:
http://www.hardforum.com/showthread.php?t=1389830
When asked what psu he used for 5 gpu's (2 gtx 295's) and an 8800 gtx, none of which are low power cards, in one computer I went to read the Amperage specs on the one he listed.
they are as follows:
+3.3V@40A,+5V@40A,+12V1@18A,+12V2@18A,+12V3@18A,
+12V4@18A,+12V5@18A,+12V6@18A,-12V@0.8A,+5VSB@3. 5A
It has 6 12v rails and is 1000w, but each rail is only 18A.

This leads me to beleive that my 2 18a rails just might suffice for a single gtx 260, if the 2 rails were added together.
My PSU has one pcie 6 pin connector, maybe I would need to isolate the 2 (4 pin peripheral) adapter that would account for the second pcie plug needed to the second rail.

anyone have any advice on this? It would be great to not have to buy a new psu, as the card is expensive enough as it is.
Thanks
February 4, 2009 3:18:42 AM

It will work, even if you had a GTX 280 and had a sum of ~25A on 12V PSU (but good brand). Nvidia and ATI overstate the power requirements almost x2 because there are still crappy noname PSUs around.

The real problem you may have is NOISE. Most PSUs after they work more than 50%-70% (your case) of their wattage start to heat and the psu fan usually reaches annoying noisy speeds.

If you decide to buy a new psu you should not watch for watt/amper amounts but for low temps and low fan speeds underload. A good 500w psu is enough for anyone minus 3-sli-cf.
February 4, 2009 3:45:20 AM

I am hoping to keep this psu for my first gtx 260, as when I add more, I will also be upgrading to an i7/x58/ddr3 200 setup, so a new psu will not be the biggest deal in the world
Related resources
February 4, 2009 3:15:04 PM

I have a newer modular TT 550 with 16A on one rail and 17A on the other for a total of 33A. I am running a Phenom II 940 OC to 3.2Ghz. 8 gigs of 800mhz ram. 7 fans, and a EVGA 260 core 216 55nm. plus the HD, dvd etc. I can run Prime 95 and folding@ home at same time--No problems.
a c 130 U Graphics card
February 4, 2009 4:45:35 PM

Hi febisfebi.

Your last statement in your opening post was this,
"anyone have any advice on this? It would be great to not have to buy a new psu, as the card is expensive enough as it is.
Thanks"

You also said somewhere that you are doing some pretty major upgrading.

So my question to you is this,
Is your new card expensive enough that you dont fancy taking a chance at killing it by having it and your system underpowered ?

Yes ATI etc do exagerate but not anywhere near the x2 sugested, your PSU has a total of 27Amps which personally i dont think will cut it.
So you have a PSU that is under recomended if exagerated spec by roughly 20% and you would also need to use an adaptor from the peripherals to make up the extra power lead needed. Its not sounding good to me.
My recomendation to you would be to save yourself the possable grief of damaging your system and get the power supply you intend to use for the new system now and run the new card on it untill you get the rest.

If its a newer 55nm card then you may get away with it but if its the 65nm card then i dont fancy your chances.

Mactronix
February 4, 2009 9:08:37 PM

itll be the 55 nm model.
Where do you come up with 27A? it has 2 18A 12 rails, I would think that would equal 36A total. Making it one amp above the exaggerated requirements. If what jas340 says is true, this should be plenty, as my psu is more powerful, and I have less peripherals.
I will be doing major upgrading, in the not so near future, certainly after the gt212 comes out. If I get lucky, I will be able to upgrade to the gt212 on this card using evga's step up program. In which case I would move that card to the new system, but that is like I said, sometime in the future. I just recently did some major upgrading to the phenom 9950, so i'm fine for a while. An i7 is in my plans though. I already bought a psu for this computer, and would like to be able to use it for this one, and use the new supply for the new computer I build.
February 4, 2009 9:10:18 PM

GNR said:
It will work, even if you had a GTX 280 and had a sum of ~25A on 12V PSU (but good brand). Nvidia and ATI overstate the power requirements almost x2 because there are still crappy noname PSUs around.

The real problem you may have is NOISE. Most PSUs after they work more than 50%-70% (your case) of their wattage start to heat and the psu fan usually reaches annoying noisy speeds.

If you decide to buy a new psu you should not watch for watt/amper amounts but for low temps and low fan speeds underload. A good 500w psu is enough for anyone minus 3-sli-cf.

how can you tell if a psu has low temps and fan speeds under load, without physically trying out each one?
a c 130 U Graphics card
February 4, 2009 9:30:01 PM

febisfebi said:
itll be the 55 nm model.
Where do you come up with 27A? it has 2 18A 12 rails, I would think that would equal 36A total. Making it one amp above the exaggerated requirements. If what jas340 says is true, this should be plenty, as my psu is more powerful, and I have less peripherals.
I will be doing major upgrading, in the not so near future, certainly after the gt212 comes out. If I get lucky, I will be able to upgrade to the gt212 on this card using evga's step up program. In which case I would move that card to the new system, but that is like I said, sometime in the future. I just recently did some major upgrading to the phenom 9950, so i'm fine for a while. An i7 is in my plans though. I already bought a psu for this computer, and would like to be able to use it for this one, and use the new supply for the new computer I build.


It dosent work like that you dont just add up the Rails, you haev to divide the total wattage of the rails by the Voltage which equals the actual Amps.
This is what i got when i googled your PSU
http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...

Now we only want the 12v rails so we have to take the 152 from the 481.5 which leaves 329.5 which when divided by 12 equals 27.45Amps i rounded down as its best to be under.

Mactronix
February 5, 2009 1:59:52 AM

Just so you know, Mactronix is right about that you can't just add the rails. Take the voltage that's leftover from the other rails (generally the 3.3v and 5v together are shown.) and divide by 12.
February 5, 2009 9:40:26 PM

Thank you Mactronix for explaining in a way that is not just your opinion. Will i really be at risk for damaging equipment other than the psu though? It sounds like other people have succesfully used a gtx 260 with similar ~500w psu without problems.
a b ) Power supply
February 5, 2009 10:04:45 PM

mactronix said:
It dosent work like that you dont just add up the Rails, you haev to divide the total wattage of the rails by the Voltage which equals the actual Amps.
This is what i got when i googled your PSU
http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...

Now we only want the 12v rails so we have to take the 152 from the 481.5 which leaves 329.5 which when divided by 12 equals 27.45Amps i rounded down as its best to be under.

Mactronix


This is partially correct; you cannot simply subtract the 3.3V and 5V wattage to obtain the 12V wattage.

As an example, the OCZ EliteXStream 800W has a 3.3 and 5 total of 180, 12V is 744, plus 6 and 20 for -12V and 5Vsb.
http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...
a c 130 U Graphics card
February 6, 2009 8:22:41 AM

^
No but its the best you can do given the info you have to work with,and as the 152 is within the 12v box and not just written on the label like some are its more accurate than just adding the rails together. If all PSU's came with both voltage and Amperage numbers for the seperate rails we would know exactly where we were.

@
Oh its possable it will run and never give you a problem, then again its possable that the components will try and draw more power than the PSU can give and then its down to luck which get damaged first.
Im not trying to scare you needlessly, but for example the psu jas340
is talking avbout has a much stronger +12v rail than yours and i would expect his unit to run the card ok, although why the need for 7 fans i dont know ?
In the end its your decision as to if you take a chance or not, im just trying to give you all the info so you can make an informed decision.

Mactronix :) 
February 6, 2009 11:27:55 PM

mactronix said:
^
No but its the best you can do given the info you have to work with,and as the 152 is within the 12v box and not just written on the label like some are its more accurate than just adding the rails together. If all PSU's came with both voltage and Amperage numbers for the seperate rails we would know exactly where we were.
Mactronix :) 


in the manual it lists what you said, as follows
voltage________min load________normal load_______max load
+3.3v _________.5A_____________15.0A___________30.0A
+5v___________.3A______________15.0A__________30.0A
+12v1_________1.0A_____________9.0A___________18.0A
+12v2_________1.0A_____________9.0A___________18.0A
-12v___________0.0A____________0.25A___________.5A
+5vsb__________0.0A____________1.25A___________2.5A

how is jas340's psu much stronger on the 12v rail he has 16A and 17A, which he added up to 33A. the only real difference is 50 more watts. If I added mine together it would equal 36.
Thanks again to Mactronix for the help. I appreciate everything you explained here, I am just trying to understand this as best I can, and as you say, make an informed decision.
a b ) Power supply
February 7, 2009 5:18:41 AM

febisfebi said:
in the manual it lists what you said, as follows
voltage________min load________normal load_______max load
+3.3v _________.5A_____________15.0A___________30.0A
+5v___________.3A______________15.0A__________30.0A
+12v1_________1.0A_____________9.0A___________18.0A
+12v2_________1.0A_____________9.0A___________18.0A
-12v___________0.0A____________0.25A___________.5A
+5vsb__________0.0A____________1.25A___________2.5A

how is jas340's psu much stronger on the 12v rail he has 16A and 17A, which he added up to 33A. the only real difference is 50 more watts. If I added mine together it would equal 36.
Thanks again to Mactronix for the help. I appreciate everything you explained here, I am just trying to understand this as best I can, and as you say, make an informed decision.


As Mactronix said, you cannot simply add the 12V rails to get the total. The PSU will usually state the 12V total, but this one does not.
a c 130 U Graphics card
February 7, 2009 6:59:53 AM

The problem here is that you need to understand the numbers printed on the rails are usually the max numbers which by default means that both rails cant run the given Amps at the same time.
On a good PSU the rails do work out as being the same amperage as is printed on them.
The only way to be sure is if th ewattage of the +12v rail is printed on the label then you can work it out.
This supply is nice and simple http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...

However this one is harder
http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...
It actually puts out around 52 Amps and not 76 which would be more power than the whole PSU is rated at.
Mactronix
February 7, 2009 11:47:46 PM

Thats what I was thinking before I saw the requirement on evga's site. But I didnt know if the rest of the system's power draw would affect the available 18A Assuming it was only coming from one rail. Do both rails run to all power cords, or do I need to isolate one rail for the pcie adapter for the second plug?
anyone have comments on THG's observations that it only pulls 16A? sounds like this might just work.
a c 130 U Graphics card
February 8, 2009 7:06:56 AM

Evga's specs are for the full system load not just for the card, there is no way the card need anywhere near 35 Amps.
However as i tried to explain earlier there is not 18 Amps available guaranteed.
I dont know how your actual PSU is wired up but the drawings suggest that its a single rail split into two, this would mean that the amount available on one rail would in effect depend on what was being drawn on the other, thats why you cant just add up the rails.
On the subject of the actual draw of the card 16A sounds slightly high to me, i have seen a few reviews and they average around 14A for the 65nm card and between 10 and 12 for the 55nm card.
Very roughly the card can take up to 75 watts (6.25 Amps) from the PCIE slot which would leave it needing another 120 watts (10Amps). But even that isnt as straight forward as it sounds because it depends how EVGA have the power circuit set up. I wouldnt think they would draw the max from the PCIE slot and then it would make sense if they split the remaining requirements evenly between the two power cables, but again you cant say for sure.
So basically how much the rest of the 12v system of your PC is drawing will determine how many Watts/Amps are left for the 2nd rail, if you try and overdraw this ammount it will either make your Graphics card unstable, this will either damage it probably capacitors, or it just wont work.
The other option is something else on your system CPU etc does the same thing.
Im sorry to keep bringing it up but it is a possability that you need to consider.
As i said a few posts back as its a 55nm card it could well work but i wouldnt go as far as saying it definatly will.
The just not working option is the most likley these days as most cards will give a beep or just throttle itself back if it dosent get enough power.
I get the feeling that you are leaning towards giving it a go which in the end is up to you. i just wanted to advise you of the risks and that you dont actually have 36Amps.
Good luck if you decide to give it a go.

Mactronix

February 12, 2009 3:41:41 PM

thank you again! it arrives today. I'll be sure to let you know how it goes. I have had graphics cards throttle themselves back in the past due to the power cable not being plugged in heh but thats hardly the case here. It sounds like theres a good enough chance that, yes I believe I will give it a go. but If I start to see signs of things going wrong ill stop using it, and post here. and probably get a new supply if thats what is needed.
thanks all, and thanks especially to Mactronix
a c 130 U Graphics card
February 12, 2009 6:52:28 PM

Yes please do post back with your findings, not enough people do when these things come up and if you prove it one way or the other then we can better advise others.
Oh and your welcome
Mactronix :) 
February 12, 2009 11:55:31 PM

damn ups. they always leave it on the porch every time I order from newegg, even with much bigger/more expensive packages, but this time they just had to leave a delivery notice after I was gone for 5 minutes after waiting til 6pm all day long without leaving.
oh well, at least theres tommorrow, and I can enjoy a weekend of gaming if all goes well.
February 13, 2009 2:31:58 AM

febisfebi said:
I have an OCZ StealthXStream power supply at 500w.
It has 2 12v rails at 18A each. According to EVGA's site their gtx 260 ssc requires 35A of power on a 500w PSU. Now I realize my psu does not exactly meet that standard, so I started looking at other cheap psu's that could handle it. the cheapest looking one is this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817148010
at 34 a. if this is enough it would be a minimal cost in upgrading. If it isnt, I would likely move up to this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341011
A much bigger cost, but would allow for sli of a second 260 later.

The first PSU I listed has one 12v rail with 34A, Just short of the 35 required. Would this suffice, not just the amperage, but say it were a 35A, it only has one rail, would it still have enough power with every other component also running on it?

Now for my real question. I started to realize that the fact that my psu has 2 18A rails may be to my advage when I saw this post:
http://www.hardforum.com/showthread.php?t=1389830
When asked what psu he used for 5 gpu's (2 gtx 295's) and an 8800 gtx, none of which are low power cards, in one computer I went to read the Amperage specs on the one he listed.
they are as follows:
+3.3V@40A,+5V@40A,+12V1@18A,+12V2@18A,+12V3@18A,
+12V4@18A,+12V5@18A,+12V6@18A,-12V@0.8A,+5VSB@3. 5A
It has 6 12v rails and is 1000w, but each rail is only 18A.

This leads me to beleive that my 2 18a rails just might suffice for a single gtx 260, if the 2 rails were added together.
My PSU has one pcie 6 pin connector, maybe I would need to isolate the 2 (4 pin peripheral) adapter that would account for the second pcie plug needed to the second rail.

anyone have any advice on this? It would be great to not have to buy a new psu, as the card is expensive enough as it is.
Thanks

StealthXStream OCZ500SXS ~34A
http://forums.guru3d.com/showthread.php?t=205763

Normally the formula is:
maximum total combined wattage on the combined 12v rails divided by 12 = amp rating for psu
but OCZ did some creative labeling as you can see here:
http://www.newegg.com/Product/ShowImage.aspx?CurImage=1...
February 16, 2009 12:51:12 AM

well im back, from 2 days of gaming hehe:)  its working great, wanted to let you all know, and also let you all know how much I appreciate your help with all this. almost looks like I started a trend of (will my crappy psu work with this card) threads. Its good people are realizing the need for good psu's. Looks like it really comes down to the quality of the psu more than anything, as this guy
http://www.tomshardware.com/forum/260466-33-powersupply...
is running a 585w with 19 and 20A rails gets a low power warning on a 9600gt. I am assuming that my card takes more power, as the listed requirement for that card is 26A.
March 19, 2009 10:07:50 AM

You've gotta have a pretty crappy PS to get a low power warning. I have had it a couple of times, but only because of a loose connection. Best way to see if your power supply is up to par, would be to use a multimeter and measure the voltage, sitting and under load. Test the first rail (motherboard/cpu power) voltage threw a molex connector using the yellow and black wires. And the 2nd rail through the back of the pci-e connector. If your anywhere in the range of 11.70-12.20v under load you will be good to go. If its under 11.50v though. You probably want a new power supply. Furmark seems to put the most stress on my system, Ati tool also does a good job.
March 6, 2011 3:48:29 AM

Yeah, I know. Ooooold ass thread. I was just reading through , trying to get some understanding on this whle 12V Rail and Amps stuff. Y'all are saying it isn't as simple as adding the Amps on each independent Rail? This adding and dividing thing i'm seeing here really does look odd (and confusing)
March 6, 2011 4:27:14 AM

iam2thecrowe said:
http://www.jonnyguru.com/forums/showthread.php?t=3990 read that and close thread pleas mods.


Oh ship! Thanks! Wow, exactly what i've been scouring the net for. And yeah mods, close this thing down, since people are so touchy about posting in a thread that has been inactive for months

~Des
!