Question on Multiple 12v Rails

[1Wolf]

I've got an EVGA 8800 Ultra video card here that says it requires

"A 500 Watt Powersupply with a minimum of 35 amps on the +12 volt rail"

Comparing that to.....this PSU for example (http://www.newegg.com/Product/Product.aspx?Item=N82E16817341001).

It is a 600 Watt PSU with 4 12v rails...

12v1 = 18a
12v2 = 18a
12v3 = 18a
12v4 = 18a

So I'm not sure if I should be comarping that 35 amps on the 12v rail requirement to one of those individual listings (i.e. 12v1 or 12v2) or to the total of all the individual listings (i.e. 12v1 + 12v2 + 12v3 + 12v4).

It seems there is alot of contradictory information out there on the net (Some of the advice I've read actually seemed downright dangerous), as well as an awful lot of WAY technical info and I would really like to understand this. So please don't just answer with a "That PSU will work fine with that video card" or what not. I'm really trying to learn the proper way to actually determine that rail requirement so that I can do it in the future as well.

Thanks!

 

[HOOfan_1]

The 35A requirement listed for your 8800Ultra is not for the card itself. It is for the card as well as all the other components that require 12V power. nVidia estimated what other components would be in a fairly high end computer that one would run an 8800Ultra with and figured out how much 12V power they would need.

35A 12V is 420W. The 8800Ultra is not going to use 420W of power.

Second 12V rails are almost never addative.

I explained why in this post
http://www.hardforum.com/showpost.php?p=1034871778&postcount=13
A comprehensive PSU label would tell you exactly how much combined 12V power was available...but as with many FSP Epsilon units...that one does not. It is not 72A however.

Furthermore...as you can see the breakdown is given for what components are on what rail. The 8800Ultra has 2 6pin PCI-E plugs. Each 6 pin plug is not supposed to pull more than 75W. 1 PCI-Ehas its own 18A (216W) rail. The other is shared with the second CPU connector which you may not even need to use. Even if you did again, it is capable of pulling 216W...and the PCI-E only needs at most 75W leaving 141W available.

 

[Dangman]

The 35A requirement listed for your 8800Ultra is not for the card itself. It is for the card as well as all the other components that require 12V power. nVidia estimated what other components would be in a fairly high end computer that one would run an 8800Ultra with and figured out how much 12V power they would need.

35A 12V is 420W. The 8800Ultra is not going to use 420W of power.

Second 12V rails are almost never addative.

I explained why in this post
http://www.hardforum.com/showpost.php?p=1034871778&postcount=13
A comprehensive PSU label would tell you exactly how much combined 12V power was available...but as with many FSP Epsilon units...that one does not. It is not 72A however.

Furthermore...as you can see the breakdown is given for what components are on what rail. The 8800Ultra has 2 6pin PCI-E plugs. Each 6 pin plug is not supposed to pull more than 75W. 1 PCI-Ehas its own 18A (216W) rail. The other is shared with the second CPU connector which you may not even need to use. Even if you did again, it is capable of pulling 216W...and the PCI-E only needs at most 75W leaving 141W available.

QFT.

 

[1Wolf]

Thanks for the help For whatever weird reason...its very confusing to me

The 35A requirement listed for your 8800Ultra is not for the card itself. It is for the card as well as all the other components that require 12V power. nVidia estimated what other components would be in a fairly high end computer that one would run an 8800Ultra with and figured out how much 12V power they would need.

Ah...I follow. After reading your post I went back and re-read the 8800 Ultra label. Now that I've read your explanation...it makes much more sense. I was reading that label to mean "Requires a powersupply that can supply 35a on a 12v rail for this video card". Now I know it really means "Requires a powersupply that can supply 35a on 12v rail for this video card and any other components using the rail".

A comprehensive PSU label would tell you exactly how much combined 12V power was available...but as with many FSP Epsilon units...that one does not. It is not 72A however.

I see. In another thread Danny had recommended steering clear of the OCZ PSU I linked above and so I was going with my second choice which was...

http://www.newegg.com/Product/Product.aspx?Item=N82E16817153040

After reading your thread, I went out and took a look at the listing on the Thermaltake site and I can see they have a chart which lists that PSU as "+12v 52a". So this must be what you mean by a comprehensive PSU label. I'm glad I asked, the information on some of these labels seems very misleading unless you know exactly what to look for.

I have to admit though, I still don't quite exactly understand the math. I so understand that the rails are usually not additive.

However, lets say I picked up the Thermaltake PSU I linked above. Thermaltake lists the following for this product on their site...

Next generation four +12V rails(12V1, 12V2, 12V3, 12V4) supports high-end graphic card and PC system (combined loading of 52A)

Just like the original PSU I asked about, this Thermaltake one has 4 rails:

12v1: 18a
12v2: 18a
12v3: 18a
12v4: 18a

So I can also see that "Combined Loading" figure there at 52a. So I know that the combined loading isn't additive (72a) but is really 52a.

So when the 8800 Ultra's requirements say...

A 500 Watt Powersupply with a minimum of 35 amps on the +12 volt rail

Does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 18a (216W) that I won't be able to use that video card? Or...does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 52a (624W) that I won't be able to use that card?

Or...are all the rails (12v1, 12v2, 12v3, 12v4) collectively called the "+12 volt rail".

 

[Dangman]

So when the 8800 Ultra's requirements say...

A 500 Watt Powersupply with a minimum of 35 amps on the +12 volt rail

Does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 18a (216W) that I won't be able to use that video card? Or...does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 52a (624W) that I won't be able to use that card?

Or...are all the rails (12v1, 12v2, 12v3, 12v4) collectively called the "+12 volt rail".

All of the rails (12v1, 12v2, 12v3, 12v4) are collectively called the +12V rai. Also note that the 35A rating is for the ENTIRE SYSTEM + the 8800 Ultra, not just the video card alone.

 

[HOOfan_1]

Does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 18a (216W) that I won't be able to use that video card? Or...does that mean that if everything I have plugged into that particular 12v rail of the 4 available on my system exceeds 52a (624W) that I won't be able to use that card?

Or...are all the rails (12v1, 12v2, 12v3, 12v4) collectively called the "+12 volt rail".

Any decently designed PSU with the 12V source divided into multiple units will make sure that it is almost completely impossible for the components those rails are designed for to overdraw that rail.

Let's say theoretically the Thermaltake you are looking at has the 4 12V division....the upper limit on each division is 18A (216W). That is a circuit which tells the power supply to shut itself off if it detects more than 18A flowing through that circuit. So that is not a physical limit of the components, but a limit set by the manufacturer. However, the divisions are coming from a single source which is physically limited to 52A.

12V1 is for the Molex, SATA, and floppy connectors
12V2 is for the CPU connectors and the 24pin ATX connector
12V3 is for one GPU connector
12V4 is for another GPU connector

Any one of those divisions can load up to 18A without shutting off your PSU, but combined they can be no more than 52A.

Since your videocard needs 2 GPU connectors, and those GPU connectors are limited by specification to 75W or 6.25A, there is no way they SHOULD reach that 18A limit, even if they were on a single rail. However, if there is a short circuit in the card itself, then it could keep drawing more current from the connector. The wires on that connector can only carry so much current before the resistance causes them to overheat and melt the insulation....that is where that 18A limit comes in. If something malfunctions and draws more power than it ought to, it will shut off your PSU and keep it from being damaged or causing further damage to your computer.

 

[1Wolf]

Thanks Hoofan and Danny for all the great explanations. They really do help

At least I think I understand the most important parts now...

* The 52amp number is the one I need to worry about
* The individual rails (12v1, 12v2, 12v3, etc) are not additive.
* The video card's requirements are for an entire system plus the video card, not just for the video card alone and should be compared to that 52amp number.
* I should be suspicious of any PSU that doesn't list that 'collective' 52amp number...or...if they DO list that number and its greater than or equal to the sum...i.e...52amp is a reasonable number, 72amp on that same example PSU is not.
* Stay away from that OCZ PSU
* If my video card plugs into 2 of those rails, the theoretical limit that it could get would be 432 Watts (18 amps * 2 = 36 amps * 12v = 432 W ). However, the GPU connectors are limited to 75W or 6.25a.
* If on either of those, a draw of greater than 18a were to be attempted, the PSU would shut down to protect itself. If collectively there was a draw greater than 52a, the PSU would shut down to protect itself.

I'm probably still "off" a bit, but I think I understand this a whole lot better than when I started.

Thanks alot for the help!

 

[Zero82z]

You've pretty much got it down. One other thing is that the individual OCP ratings on the label are usually a bit conservative and the actual OCP trip points are higher than the listed values (by a few amps, not much).