12V Rail Amperage Confusion

[BigMoosey74]

Ooooo kay, this pops up a lot but the threads get really confusing. I purchased a PSU with four 12V rails rated at 18A on each. What I gather, is that this has limited the GPU that I can use now. What I don't understand / confuses me are statements regarding the collective amperage over all 12V rails.

Q1: The 8800 GTS requires 26A. What confuses me is that people say...that is the total system requirement so if I had four 12V rails at 18A each, I would have a collective at 52 (based on 624W for the rails) and be able to run it. I can run it right since it isn't 26A on a single 12V rail right?



Q2: The 3870x2 has a 6 pin connector and an 8 pin connector yet requires two 6 pin connectors in the description and also needs 30A but isn't specific if that is per rail or between the two connectors. So three sub questions arise from this one.


A: What is with the 8 pin connector for if it requires two 6 pins and what is different about the 8 pin connector? Is it supposed to carry more amps?


B: Is that 30A over each rail or between the two rails. Or what I really want to know, would my two 12V rails at 18A each meet the specs to push the card?


C: Assuming I can run a 3870x2, I cannot run dual 3870x2 because based on 624 / 12, I only get a max of 52A and I would need 60 for both cards...is that correct?


Q3: So basically what I should have done is got the 750W PSU with four 12V rails at 20A, 20A, 36A, 36A to be able to run two 3870x2s (since it is 720W for the 12V rails which = 60A)?




So if all of these or parts of them get answered I think I will have a firm grasp on PSU requirements. Thanks dudes!

 

[mpilchfamily]

Amperage requirements for GPUs as stated by the manufacture are the power recommendations of the average system with that card and not just the GPU.

The 3870 X2 need both the 6 pin and 8 pin connected but you can use just 2 6 pin though you will not be able to run all the eye candy.

The 8 pin is able to offer more power that the card needs to run all its features.

Now don't pay any attention to the amperage levels of the individual rails of the PSU. Now if we knew what PSU you where talking about we could help you better. But the important thing you need to know is the total wattage that the unit offers to the 12V rails. Most PSU will mention the combined 12V rail wattage either on the label or in the manual. But as long as the total amperage to the 12V rails in total is greater then 30A you will be fine. But know that you don't add up the amperage ratings of the individual rails to get the total. The PSU may only offer 600W to the 12V rail. In which case the total amperage would be 50A not the 72A you get by adding the rails. The 18A ratings are just that. They are the max amperage that rail is rated to handle. PSUs are all wired differently but for the most part one rail is for the motherboard/CPU, one is for the PCI-e connectors. Often with 4rails 2 of them are used for the PCI-e connectors. Then all the peripheral connectors on on there own rail.

 

[jonnyGURU]

Just to reiterate what mpilchfamily is saying.....

Originally posted by: BigMoosey74
Q1: The 8800 GTS requires 26A. What confuses me is that people say...that is the total system requirement so if I had four 12V rails at 18A each, I would have a collective at 52 (based on 624W for the rails) and be able to run it. I can run it right since it isn't 26A on a single 12V rail right?

The 26A on the +12V IS for the whole system, but you don't add the +12V rails up. There's a combined maximum output wattage listed for the +12V that you would go by.

Originally posted by: BigMoosey74
Q2: The 3870x2 has a 6 pin connector and an 8 pin connector yet requires two 6 pin connectors in the description and also needs 30A but isn't specific if that is per rail or between the two connectors. So three sub questions arise from this one.

Again, the card doesn't need 30A. They're talking about the power requirement of a typical system with a 3870 X2.

The can be run with either two 6-pin or a 6-pin and 8-pin. I believe some performance is throttled if you use two 6-pin, but you can easily find a 6-pin to 8-pin adapter like this one.

Originally posted by: BigMoosey74
A: What is with the 8 pin connector for if it requires two 6 pins and what is different about the 8 pin connector? Is it supposed to carry more amps?

The 8-pin is supposed to have a +12V sense wire that tells the PSU if the +12V drops so the PSU can compensate accordingly. This should allow the PSU to deliver more +12V to that connector, but I've yet to see a PSU manufacturer implement a +12V sense wire on the 8-pin PCIe. I've only been seeing them use ground wires to "fool" the card into thinking there are 8 plugged in. This is how the adapters work.

Originally posted by: BigMoosey74
B: Is that 30A over each rail or between the two rails. Or what I really want to know, would my two 12V rails at 18A each meet the specs to push the card?

Neither. Like I said, It's 30A for the whole system. But to break down PCIe for you: Up to 75W can be delivered to the car via the slot. Up to 75W can be delivered to the card via each 6-pin PCIe connector. Theoretically, up to 150W can be delivered to the card via 8-pin connector.

Originally posted by: BigMoosey74
C: Assuming I can run a 3870x2, I cannot run dual 3870x2 because based on 624 / 12, I only get a max of 52A and I would need 60 for both cards...is that correct?

No. Since the card doesn't need 30A. Once again, that's for the whole system. If you're suspect on whether or not your PSU can handle a pair of 3870 X2 cards, you should probably post what kind of PSU it is. I'm willing to be that it won't work because with two 3870 X2 cards you're essentially running four GPU's.

Originally posted by: BigMoosey74
Q3: So basically what I should have done is got the 750W PSU with four 12V rails at 20A, 20A, 36A, 36A to be able to run two 3870x2s (since it is 720W for the 12V rails which = 60A)?

Not necessarily. All of your questions are based on the same two misconceptions: The "required amperage on the +12V is a power requirement for the card" and "you add the +12V rails up to determine +12V capability." Both are wrong notions.

Originally posted by: BigMoosey74
So if all of these or parts of them get answered I think I will have a firm grasp on PSU requirements. Thanks dudes!

I think "firm grasp" is a gross overstatement. But we'll be happy to help and a good start would be to list the type of PSU we're talking about here and what exactly your upgrade path is (one card, two cards, which card, blue fish....)

 

[BigMoosey74]

Originally posted by: mpilchfamily

Now don't pay any attention to the amperage levels of the individual rails of the PSU. Now if we knew what PSU you where talking about we could help you better. But the important thing you need to know is the total wattage that the unit offers to the 12V rails. Most PSU will mention the combined 12V rail wattage either on the label or in the manual. But as long as the total amperage to the 12V rails in total is greater then 30A you will be fine. But know that you don't add up the amperage ratings of the individual rails to get the total. The PSU may only offer 600W to the 12V rail. In which case the total amperage would be 50A not the 72A you get by adding the rails. The 18A ratings are just that. They are the max amperage that rail is rated to handle. PSUs are all wired differently but for the most part one rail is for the motherboard/CPU, one is for the PCI-e connectors. Often with 4rails 2 of them are used for the PCI-e connectors. Then all the peripheral connectors on on there own rail.

I know all of this regarding the watts, it was the amps and how they are distributed among the rails that was confusing me. I didn't mention the PSU total watts because I didn't want to focus on them. I did however mention that the 12V rails have 624W. I am appreciative for the amp info though, thanks!

 

[BigMoosey74]

To JonnyGuru, thanks for the input. I was actually hoping that you would jump in on this one. I see the point now about the amperage criteria that the video card specifies. That was my main confusion... if it targeted one rail or the entire system.

I don't plan on running two x2 cards, I was just asking in theory to better understand the amp requirements since the calc 624W/ 12V gives me 52 amps for all of the 12V rails...I know it isn't additive and I know it is relative to the rest of your system and what PSU you have. But again, like you said, it came from my misunderstanding of the amperage requirements.

So what I did was purchased the Xigmatek 650W PSU with four 12V rails at 18A each that has 624W over the four rails. I saw the 8800 GTS amperage requirement and was thinking...should I have ordered the 750W PSU with the two 12V rails at 30A? My plan is to have xfire with something like two 3870s or a hybrid 3870 / 3850 or even wait for the 4xxx series to come out and do the equivalent. Maybe 3 GPUs at max later down the road but nothing huge like two x2 cards or tri SLI with the GTX. It isn't definite yet but I thought I blocked off some single card options with the PSU purchase because I figured the amps didn't need that much attention as long as you had the multiple 12V rails.

Haha, yeah compared to what you know...I wouldn't even say my knowledge is a solid base but again, thanks for both of your replies...they definitely helped.

 

[jonnyGURU]

Originally posted by: BigMoosey74
...should I have ordered the 750W PSU with the two 12V rails at 30A? My plan is to have xfire with something like two 3870s or a hybrid 3870 / 3850 or even wait for the 4xxx series to come out and do the equivalent. Maybe 3 GPUs at max later down the road but nothing huge like two x2 cards or tri SLI with the GTX. It isn't definite yet but I thought I blocked off some single card options with the PSU purchase because I figured the amps didn't need that much attention as long as you had the multiple 12V rails.

Well, I'd bet dollars to donuts that each pair of PCIe connectors are on their own rail, so I don't think it'll be a problem. Without knowing what 750W PSU you're talking about with 30A on each of two +12V rails and how they managed to distribute that power to the connectors, there's actually no guarantee that would be any better at all.

 

[Old Hippie]

From ExtremeSystems Forum......

Already have a contact. I think Wolf and Tony might have a contact already too. I don't do the reviews any more due to conflict of interest, and Wolf has a pretty full plate.

Say it ain't so! :frown:

Come outta retirement, and I'll buy ya :beer: for life!

 

[jesss]

My psu is 450 watts. It only has 1 12v @ 18a.
Does it have enough power to run the 8800 GT?

 

[flexy]

jesss, i dont think this is enough.

You can get a very good impression about requirements about your whole system here:

http://extreme.outervision.com/psucalculatorlite.jsp

 

[xeysz]

Hi.

I am sorry that this is an old topic but I think that replying to this with my question is better than starting a new topic. That's because this topic is exactly what my question is about.

My question is if the Antec Basiq 500 Watt Power Supply (with two 12v rails @ 18A, supposedly) has enough power to support my system, actually my new HD 3870. I used to have a tool that told me everything about my PC including the amperages for each rail but when I reinstalled Windows I lost it. So I will tell you what I am using manually.

I have one SATA HD.
I have one DVD drive.
I have one Media card reader.
I have one PCI card (network adapter).
I have one Dual Core processor.
I have one 19'' (digital and analog support) monitor.
I have one Bluetooth mouse.
I have one USB keyboard.
I have the Dell stock mobo.
I have, currently, the Dell stock PSU (300 watt).

The Antec Basiq is the cheapest one from Antec that is at my local Circuit City. The online Web page link is here for Circuit city. http://www.circuitcity.com/ssm...Detail.do#availability

And here is the link for Newegg.com http://www.newegg.com/Product/...x?Item=N82E16817371004

I need to know if this can power up the above and my HD 3870 video card without any issues. This is important because if it can't I will have to spend another 50 dollars for something I am sure will work.

 

[jonnyGURU]

Originally posted by: xeysz
Hi.

I am sorry that this is an old topic but I think that replying to this with my question is better than starting a new topic. That's because this topic is exactly what my question is about.

Yeah, but it's a month later concerning a different PSU. You should've just started a new thread.

Then again, if we could get everyone to post their "is this PSU enough" questions in one thread, that would be even better!

Originally posted by: xeysz
My question is if the Antec Basiq 500 Watt Power Supply (with two 12v rails @ 18A, supposedly) has enough power to support my system, actually my new HD 3870. I used to have a tool that told me everything about my PC including the amperages for each rail but when I reinstalled Windows I lost it.

No. There aren't any programs that can tell you how much power your power supply puts out unless there's an interface between the motherboard (usually USB) and the PSU itself.

Originally posted by: xeysz
So I will tell you what I am using manually.

I have one SATA HD.
I have one DVD drive.
I have one Media card reader.
I have one PCI card (network adapter).
I have one Dual Core processor.
I have one 19'' (digital and analog support) monitor.
I have one Bluetooth mouse.
I have one USB keyboard.
I have the Dell stock mobo.
I have, currently, the Dell stock PSU (300 watt).

The Antec Basiq is the cheapest one from Antec that is at my local Circuit City. The online Web page link is here for Circuit city. http://www.circuitcity.com/ssm...Detail.do#availability

And here is the link for Newegg.com http://www.newegg.com/Product/...x?Item=N82E16817371004

I need to know if this can power up the above and my HD 3870 video card without any issues. This is important because if it can't I will have to spend another 50 dollars for something I am sure will work.

It can handle one 3870 no problem.

 

[xeysz]

Okay, it's really great to hear that.

With this PSU it's 15 dollars off at the local Circuit City so I won't have to order another one online (higher quality but at the $100 marks) and have the three day wait.
And I don't have many things in my rig, but I will probably add a blu-ray drive in the future which I would bet this could handle too.

Thank you for giving me your time.