Calculating Amps Dual Rails

[Etherian]

Trying to buy a PSU for a new system that will be plenty if I upgrade later.

Noticed a 7950GX2 has a minimum of 450 watts 28V on the 12V Rail(non SLI setup). Hoping that DX10 cards won't be more than this(probably my future upgrade) was looking at power supplies.

I found a PSU that shows 500W, +12V1@15A,+12V2@16A in the requirements. Can I add these two to know that it will run it? or it it more complex than that?

 

[randomcow]

Can't add. I'm not sure if there is a way to figure out the total unless they give it to you in the specs.

 

[zerobeta]

Each PSU has a combined rating, even if they offer 15A and 16A on 2 rails, it will have a maximum of for example 28A, and not 31A because it it cant provide that much, you usually find this info on the manufactor's site

I have read that there is a problem with dual rails PSUs, where if for example you have a dual rail PSU with 22A on each of the two rails, one rail is always dedicated to the CPU, and you have the rest (only 22A) to power the rest of the components

If this is true, then you better off with either a single 12v rail with high amps, or a triple or quad 12v rail, with alot of amps on each rail, then you will have (on a single rail PSU) all the amps for all the components, or (on triple or quad rail PSU) one rail for the CPU, and the rest for all the other components, where as with dual rail PSUs you have this problem where you only have half of the total amps for all the hardware except the CPU

I can send you a link to a forum where someones explains this and where I got the information, I cannot confirm if it is true

To be sure, either go with a single high amps PSU, or better, a triple or quad rail PSU, and get a high wattage (600W+) PSU so you will able to upgrade in the future without replacing the PSU (get even more than 600W if you are going with SLI or Crossfire)

Always check the whole PSU specs to know what you are getting

 

[13thmonkey]

If this is true, then you better off with either a single 12v rail with high amps, or a triple or quad 12v rail, with alot of amps on each rail, then you will have (on a single rail PSU) all the amps for all the components, or (on triple or quad rail PSU) one rail for the CPU, and the rest for all the other components, where as with dual rail PSUs you have this problem where you only have half of the total amps for all the hardware except the CPU

Not sure abbout the latter pieces of this statement, but the ATX spec states 20A max on a single 12V rail, this was put in on safety grounds as 20A is a LOT of current, but 22A shouldn't be too bad.

If the 12V power is quoted, i.e. max 300W 12V, then you can calculate the max 12V current, Power = Volts x Current >>> Current = Power/Volts in this case 300/12 = 25A. You 'just' need to find that 12V power figure.

 

[Doughbuy]

The 28A is for the whole system, not just for the card itself... The card pulls only around 140W or so at load... so 140W / 12V is only a bit more than 11A (P = I V)

So the numbers you quote should work fine, depending on exactly what each rail powers. I'm assuming one rail is dedicated to CPU, while the second rail is for graphics, so you should have plenty of headroom to spare.

One thing you have to be careful of though is that some PSU's don't allow the maximum amount of amps per rail at the same time, so see whats the maximum wattage on the 12V rails...

 

[Guest]

And just to add, 500W might be on the low side for DX10 cards, Id go for 600W/700W to be on the safe side...

And the brand is REALLY important, tell us the model and we'll tell you what to do =)

 

[jaguarskx]

I found a PSU that shows 500W, +12V1@15A,+12V2@16A in the requirements. Can I add these two to know that it will run it? or it it more complex than that?

You need to look at the sticker. It will tell you the information you need. Click the following link as an example:

Hiper 480w PSU Sticker

Notice the sticker states that 12v1 supplies 18 amps and 12v2 supplies 16 amps. That works out to a total of 408w; (18 amps + 16 amps) x 12v.

Look underneath where it states 350w for Max Combine Wattage. That is what you are looking for. The 12v1 can provide up to 216w on it's own. The 12v2 can provide up to 192w on it's own. But combined the two can only provide 350w of power or 29.17 amps total on the 12v rails.

 

[Doughbuy]

Yeah, Jaguar got to what I was hinting at... too lazy to post links to an actual sticky (Bad Doughbuy... Slap)

You also have to factor that into the equation... but the rails itself seem fine i.e. they can deliver the power from the limited info we know.

And yeah, 500W might not be enough for DX10, I recommend a 600W minimum just in case... if your going SLI... 700W at least, 800W recommended