+12 Volt Rail Amp Rating

That means the PSU must be able to provide at least 26 amps of constant amperage. However, most PSUs list peak amperage which is very different than constant amperage.

What is your price range? $100 is a good start.

List your system components as well.

Check out the PSUs with a single +12V rail. For example, my PC Power & Cooling Silencer 750 Quad has a single, powerful +12V rail capable of providing 60A. It is SLI approved, and has two 6-pin PCIe and two 6/8 pin PCIe connectors for the latest video cards.

Regards,

Altazi

As Altazi said get a PSU that has a large single rail. The silencer 750 is kind of expensive and larger than you need. Get the Silencer® 610 EPS12V Power Supply instead. $119.00 factory direct with a 49A single rail. Check Neweggs price. The Corsairs are also single rail, even though they don't say so. I don't really like the cheesy modular cables on the Corsair though. Here's a link to Power Supply Myths Exposed! with some good information.

Edit: It's also a tier 1 PSU Official XS Tiered PSU Manufacturer Brand Listing Phase III

It actually should say something like "power supply with 'combined' +12 Volt current rating of 26 Amps".
And yes using ONE PCI-E 6-pin power connector will get the 8800GTS the extra power it needs and can't get from the PCI-E slot.
Corsair VX450W 33amps on the single +12v rail $70 after rebate

No, you do not need it on one single rail. The 26Amps stated are for the entire system. So dual 18Amp rails would be enough.

Im using a £20 (~$35 $40) PSU, no named brand from ebuyer. It provides 600 watts with 30 amps on the +12v rail. Im using an 8800 gts 320mb, the amp requirements are the same as the 640mb version. I have 1 hard drive, E6750 at 2.67ghz, and a dvd drive. Its working fine so far, under full load on games like oblivion  .

I dont know much about PSUs or if my PSU is actaully giving my gfx card enough power, is there a way to check?

If everything is working fine, that's a good start. Are you running any kind of system monitoring software, like ASUS PC-Probe? It will notify you if a voltage drops below a preset threshold.

You can always use one of the PSU calculators to see if yours is "in the ballpark". I like this one.

Regards,

Altazi

It says I need 388 watts, Ill try out pc-probe though.

Thanks for the replies everyone.   I think I understand now:



So if the power supply that I'm buying says this for output: +3.3@30A,+5V@50A,+12V1@18A,+12V2@18A,-12V@1A,+5V SB@2.5A

It will work for the video card because the two +12 rails combined are more than 26 Amps?

yes

This is incorrect. WRONG!
"djcoolmasterx wrote :

No, you do not need it on one single rail. The 26Amps stated are for the entire system. So dual 18Amp rails would be enough."

You do not add the rails together. My old 305w psu has 2 18a rails = 264w = 22amps

What kpo6969 means is you need to check the total watts available to the +12v rail(s)

In the example of the 500w PSU above you can just make out that +12V1=18A and +12V2=20A for 38A.
But if you read the fine print you see that only 360W are available to +12V1+12V2. So 360W / 12W = 30Amps.
So instead of the expected 38Amps that power supply will only support 30Amps of +12 volts.

Watts divided by Volts = Amps

My suggestion is to stick with the single +12V rail. This avoids the hassle of trying to balance the loading on the multiple rails. Much easier this way.

Altazi

Currently the best deal around for a PSU on a budget, $65 shipped:
http://www.buy.com/prod/corsair-vx-450w-power-supply/q/...

That is true, but if it is truly in compliance with the ATX12V specification then any unused current on the CPU rail will be unusable. Therefore, you are limited by the maximum amperage available on the second rail. With a single rail any unused current is available.

No never said that you add the amperage, instead you divide the wattage supplied to the 12v rails by 12v to get the overall amperage. In the example I gave you could typically expect about 30Amps for a 550W PSU(which I would expect is the minimum someone buying a 8800gts 640 would get).

Sorry if you misunderstood I just wanted to make clear that the 26 amps does not have to be on a single rail.

I need to buy Gainward 7800 GS, and Chieftec GPS-450AA-101A 450W psu, which has +12V1 14A and +12V2 15A = 29A total!

Should this psu be enough for this card? Thanks in advance

I can't find anything about Chieftec PSUs. My guess is that it came with the case. Either way I don't think they are reputable, I could be wrong. I would replace it with a name brand single rail. Corsair HX series is single rail even though they say they are multi rail.

You can find it on Chieftec site: www.chieftec.com

And Corsair is better I know that, but also a much more expensive... I don't have money for Corsair; it cost 4 times more the Chieftec! For example Chieftec 450W cost 50 Euros at my local shop, and Corsair 520W costs 100Euros, that far more expensive!

And why would you replace it with single rail??? What's the difference?

Thanks

7800GS suggested PSU is 350watts and 18amps.
You probably already know its needs extra power plugged in via a 4-pin hard drive power connector.

Yes, I know that! But, Chieftec has 29A on both +12V! Gainward 7800 GS Golden Edition needs 6-pin PCI-E power connector!

What I meant is that I can't find it on any PSU lists or for purchase separately. I'm guessing that it isn't a very good PSU, but that's only a guess. For the single rail question see my post above or read this Power Supply Myths Exposed! #8.

thanks for that link, I now understand why You recommend a powerfull single rail PSU, rather than dual rail PSU; but on other side that powerfull single rail PSU costs much more then some normal dual railed    

Well you get what you pay for. One thing you don't want to skimp on is the PS. If it blows it can take your whole system with it. Also a cheap PS can give you weird problems, due to out of spec voltages, that you will never be able to track down. The Corsair HX series is single rail even though they claim it is multi rail.
Here are some other links

jonnyguru - Good reviews on a lot of PSUs.
VX450W power supply review
Power Supply Fundamentals | silentpcreview.com
Official XS Tiered PSU Manufacturer Brand Listing Phase III

Most psu's are single rail.....they just claim multi rail to be fully atx complaint....They use virtual rails so they all drive from one spot(one big ass +12 volt rail) and in most cases just have over current protection to stop more then 18-20 amps per virtual rail....

Keeps things safe and in spec.....It may not happen but pulling 50+ amps off one cable can and will melt the protective cover and do other damage....PC Power Cooling is good(stable quiet and 2nd to none if you got the $$$)...but one can overload a cable and damage something even with the fact that the psu can handle it...in many cases the wires can not...

At OP.....at the end of the day all that matters is how much you have on the combined +12 rail..... corsairs 450 should give you plenty of power for most systems....if you are worried step up to the hx520 or hx620....Also OCZ's 600 and 700 watt GameXtreme's seem to be stable as well as many of Antecs current line....

Computers don't draw as much as people think...unless u are extreme with multi gpu and multi socketed cpu's

My Sytem:

Case: Antec 900
PSU: OCZ 700watt
Board: P35 DS3R
CPU: Q6600 @ 3.00
CPU cooling : Zalman 9500 @ ~2200 rpms
Memory: 2x 1024MB ,2X 512MB
Video: 8800GTX 600(core) 1400(shader) 900 x2(memory)
Storage:
2x WDC 250gig (Raid0)
2x Seagate 320gig(Storage)
2x Seagate 500gig(Storage)
1x 500gig Seagate (Backup-external .not part of the final number)
Optical drive: Samsung SH-W162
TV card: PVR 250
Sound: Creative SB Audigy 2zs
Screen: Samsung 950b (LCD .not part of the final number)
Speakers: Altec Lansing Select 641 (not part of the final number)

Full load folding + games = 350 watts from plug
Idle + speedstep = about 200-230 watts from the plug

Take off about 20%(assumes the psu is 80% efficient) for conversion/heat loss and you will see it does not take that much to run a computer...Not saying to get a 350 watt psu as most will not have the needed +12....but a 1000 watt is a waste for most people....

You are correct they would need to have separate regulation for each rail in order to be true multi rail, however the term rail is used to indicate over current protection as you stated. Apparently more than a few PSUs are made with a single rail (no over current protection per "rail") and yet state that they are multi rail and ATX12V compliant. I thought it was only the less reputable companies but the Corsair HX series is an offender.

Absolutely, If you short something out, but it isn't a direct short, you can burn down your house. Caution is the word of the day.

Technically this is not true. Lets say you have a dual rail PSU with 18A + 18A and 33A combined, which is truly current limited to 18A on each rail, and fully complies with the ATX12V standard. Lets also suppose, for arguments sake, that the motherboard 12V rail is only drawing 7A maximum. you are still limited to 18A for all of the other components in the system. So your true limitation for fans VGA HDs etc. is 18A not 26A (33-7). So you loose 8A that is locked away on the first rail. That's why I like the single rail design even though it does require more care in dealing with. That Corsair VX450 is a single rail with 33A.

I understand now! Thanks for all of your help!    

No problem.

Hi,

I’m not really sure I understand this. Are you saying that the Corsair HX series is a bad choice, because of this rail thing? I was going to get one for my Q6600 / Geforce 8800 build.

Should I rather go with a PSU that is officially single rail?

Thanks,
Jonesy (who didn’t know there was even something called a rail until a few days ago - still not too sure what it is) ;_)

(Sorry for bumping btw.)

Okay, I've been googling for a couple of hours, so now I understand that you're saying it is a bad thing that the HX's don't have over current protection. (Because of safety issues.) But is there any alternative if you want a system with high end graphics etc.? I mean can you get a PSU with over current protection that can run a high end system?

Sorry for the noob questions, still very new to this power stuff. (Didn't pay much attention in physics class.)  

There's more: Some of the cheap psu manufacturer fudge their specs to claim the highest wattage, knowing that the uninformed shop by $ per watt.
1) Advertising peak power vs continuous power.
2) Specs are taken at 25c which is not realistic. Any good psu needs to operate closer to 50c.
3) putting more wattage to the lower voltage rails instead of the 12v rail where it is needed.
4) Using cheaper components which fail sooner. Look at the warranty.

In short, stick with the top tier brands. http://www.xtremesystems.org/forums/showthread.php?t=10...

---good luck---

No, what I am saying is that it is good to have a single rail whether they lied about it or not. Single rails require more care in the installation to ensure that there are no "almost" shorts (really no shorts of any kind) that won't trip the maximum amperage of the PSU but will still burn the wires mobo etc. and possibly catch fire. Use a single rail just BE CAREFUL. The only thing that bugs me is that some people want the extra protection from possible fire that they choose a PSU that meets the ATX12V standard of no more that 240VA (~20A) per rail. This is for safety reasons. If they buy a muli rail PSU, that is ATX12V compliant, then that is what they should get, not a mislabeled single rail.

<snip>

Okay, thanks Zorg. I see your point now.

Now however I’m starting to worry about short circuits.   I take it you’re talking about the home PC builder causing a short by installing a PC part wrong somehow? Could you tell me what specifically I should be careful about when building a computer? And how great is the risk of a short occuring? (Oh, and what is an ”almost” short?)

Cheers

(Btw., if Geofelt’s post was adressed to me then thanks for that too.)  

An "almost" short means it is shorted but not a direct short with zero resistance. I don't want to get over your head, but here goes. Current(I)(amps)=Voltage(E)/Resistance(R)(ohms) Written as I=E/R algebra gives you R=E/I. Let's assume a Silencer 610 PSU with 49A(I) and 12V(E). Plug in the numbers and you get R= 12/49 or R= .2448. So the PSU is designed to drive a maximum of 49A, and then shutdown due to overload, into a resistance (R) of no less than .2448 ohms. This is fine as long as all of the components take the proper amount of current based on their resistances and the current is distributed properly across the PSU wiring and the system. If you have a true short ~0 ohms the PSU will deliver full current for a split second and shut down. You may damage something but you won't burn your house down. Now lets say that you have a short but it is not a complete short and has a resistance of .3 ohms. I=12/.3 I=40A. So you can deliver 40amps of current through one set of wires, and traces on the mobo etc. They will heat up very fast and the PSU won't see a problem because it is designed to shut down at 49A. Wires and components will melt and catch fire. The power supplies that are ATX12V compliant will shut down after reaching ~20 amps so there is less current, therefore less heat and less chance of fire.

That said, I still prefer large single rail PSU's, due to "lost" current on underutilized rails. You need to be extra careful due to the possibility of a similar situation to the above e.g., the right number of mobo standoffs etc.. No matter what PSU you use you need to be very careful anyway because you could wind up with a brick for a computer. The ATX12V standard does not guarantee you won't have a fire it just minimizes the possibility.

I hope that explanation helps.

What exactly does "being careful" with a single rail mean?

Does this just mean making sure you have a firm connection to the motherboard? Make sure you don't play soccer with your new 70A Single +12v before you install it?

Being careful means being careful with the installation of your hardware to ensue that it is installed correctly so that nothing is shorting out. Like not having an extra mobo standoff that is shorting the chassis to the traces on the mobo, or having a PSU wire get pinched somewhere and shorting to the chassis. You know just general build care.

<snip>

Okay thanks a bunch, Zorg. I think I understand most of what you’re saying. Have I understood you correctly that if a short occurs as a result of a building error, it’s most likely to be an almost short (and therefore more dangerous), because the motherboard or whatever will offer some resistance?

Anyway, could you elaborate on what kind of errors would cause such shorts - you’ve mentioned ”redundant” standoffs and PSU wires getting pinched (that would only cause a short if the insulation was broken, right?) Are there any other things?

Or maybe there are some principles that could make it easier for me to know what I should be on the lookout for? (Like do certain components have to be involved; do shorts only occur when exposed metal and/or exposed wires touch each other, or something.)

Sorry to dump all these questions on you - it’s my first build, and I’m already in overdrive imagining all the things that can go wrong. (Maybe my brain is short circuiting.) ;-)

All this fuss about power supply rails/voltages are so boring and blahhhhhhhh

Oh gotcha, that sounds pretty easy to ensure  

Not to me, unfortunately. I'd love some more feedback on what to be on the lookout for. (First time builder/clueless noob here.)  

I'm new to the PSU thing. I'm really unsure if I can find/want a good combined multi-rail system or a huge single-rail.

Seems to be the multi has lacking performance or you have to be careful which card/socket you plug which rail into to distribute power evenly or such if I've gathered the info correctly. The large, single 12 rail seems to just offer all the power on one line, thus making it "easier" to manage and shop for (Ie +12V 70A). If it is a matter to make sure you don't pinch a wire or squeeze it between the case and the motherboard or something silly like that, it seems you should make sure these things don't happen with ANY PSU or cords for any piece of hardware, and not really any special attributes to a PSU alone.

And if I'm misinformed, I encourage to be corrected and I'm sure I will be  

For both you and Jonesy, I think the best place to look for information about what is a good PSU is Jonnyguru.com. I see some people have quoted the article from PC P&C myths to support their ideas of single verses multiple rails. Keep in mind that PC P&C makes single rail PSUs and what they print out will have a bias toward their PSUs. They also put out their take of modular vs their own non-modular wiring. And last, they make the claim that larger fans are not as good as their small fan.

If you check Jonnyguru, he tests a Thermaltake Toughpower 750 PSU with modular wiring, large fan, and multiple rails and scored it equally with a PC P&C 750 Silencer, giving both his seal of approval. In fact if you read through the articles, you find that he knocks down the Thermaltake because it doesn't look as nice, while he knocks down the PC P&C Silencer because it has a loud fan. So the Thermaltake, with its large fan, modular wiring, and multiple rails is scored as well as the PC P&C Silencer with a small fan, solid wires, and single rail. That should tell you something about the respective qualities of the two PSUs and the real effects of fans, wires, and rails.

For my own part, I like the idea of a modular wiring that means I only use the wires I need and don't have my case cluttered, I don't really care if the rails are single of or multiple, and I really like the idea of a quiet fan, so I own a Thermaltake. I also don't care if its looks aren't as good as a PC P&C. Like any PSU, its stuck inside my case and I never look at it like it was a piece of art. I just care whether or not it does the job. So check Jonnyguru and see what he rates as good, bad, or average PSUs.

The Thermaltake 750 is a nice PSU. It is also a quad rail not a dual rail, which mitigates the loss of current on underutilized rails. Any problems associated with the modular connectors shouldn't show up on a new PSU, but possibly on one that has been in service for a while. Jonny likes modular cabling, I guess it's a matter of preference. I would rather go with a Corsair HX520 or VX550 then a similar dual rail. I like the VX series better because of better non modular non ribbon cabling, just my preference.

I think that sums it up very well when a person is considering high quality PSUs. We each have personal preferences, and the variety of PSUs insures that we each will find what we want. The main trick is sorting out the high quality PSUs from the mediocre, and the garbage.

Truth.

The Thermaltake 750 sounds interesting. Now I'd actually prefer a multi rail PSU, because of the improved safety. Do you think it could run a system with a Q6600 CPU, a Geforce 8800 GTS GPU, 2 HDDs, 1 optical drive, 2 GB DDR2-800 RAM, and 1-2 PCI sound cards?

(It only costs slightly less than the HX620, but maybe it's because it's of the lost power on the rails?)

Thanks for mentioning this PSU, Sailer.

Yes, it could do it with ease and leave you room for overclocking, going SLI, or other hardware.

Yes it will power that easily. I assume you read Jonny's review. One thing that I didn't understand is that he gave it a total pass after this comment.