12V Rail Amperage and GFX power requirements

[dgre005]

I've been reading about the power reqirements of graphics cards such as the Radeon HD3850 and the figures quoted are in terms of the power on the 12V rail e.g. 30A on the 12V Rail. I have a 400W PSU whose maximum loads on +12V1DC and +12V2DC are both 18A. Does that mean that this PSU 's capable of supplying 36A and is therefore ok for a 30A graphic card?

Thanks for any info...

David

 

[theTHiiiNG]

What brand and model is the PSU?

There is a big difference between a cheapo 400w PSU and a good brand PSU.

But I would say yes it should be fine, depending on what else you have connected.

 

[mrmez]

My understanding of it...

PSU's amps are often split over several rails.

Since u only connect/use one rail for a single gfx card, in your situation u will NOT have enough amps.

The best PSU's have a single rail so u dont need to bother about how you hook things up.

ATM im running a 750w Corsair with a single 60A rail.

 

[mactronix]

The correct way of finding the Amperage of the 12V rail/s is to divide the wattage by of the rail/s by 12. You cant just add them up.
As an example the antec earthwatts 430 has 2x 17A 12V rails but the wattage on the rails is 360 which if you do the math =30.
http://www.newegg.com/Product/ShowImage.aspx?CurImage=17-371-006-04.jpg&Image=17-371-006-09.jpg%2c17-371-006-03.jpg%2c17-371-006-04.jpg%2c17-371-006-05.jpg%2c17-371-006-06.jpg%2c17-371-006-07.jpg%2c17-371-006-08.jpg&S7ImageFlag=0&WaterMark=1&Item=N82E16817371006&Depa=1&Description=Antec+earthwatts+EA430+430W+Power+Supply
Mactronix

 

[delluser1]

The correct way of finding the Amperage of the 12V rail/s is to divide the wattage by of the rail/s by 12. You cant just add them up.
As an example the antec earthwatts 430 has 2x 17A 12V rails but the wattage on the rails is 360 which if you do the math =30.
http://www.newegg.com/Product/ShowImage.aspx?CurImage=17-371-006-04.jpg&Image=17-371-006-09.jpg%2c17-371-006-03.jpg%2c17-371-006-04.jpg%2c17-371-006-05.jpg%2c17-371-006-06.jpg%2c17-371-006-07.jpg%2c17-371-006-08.jpg&S7ImageFlag=0&WaterMark=1&Item=N82E16817371006&Depa=1&Description=Antec+earthwatts+EA430+430W+Power+Supply
Mactronix

On the other hand, the Antec EA500 has 2x 17a rails with 408w provided to them for a total of 34a.
http://www.newegg.com/Product/ShowImage.aspx?CurImage=17-371-007-04.jpg&Image=17-371-007-09.jpg%2c17-371-007-03.jpg%2c17-371-007-04.jpg%2c17-371-007-05.jpg%2c17-371-007-06.jpg%2c17-371-007-07.jpg%2c17-371-007-08.jpg&S7ImageFlag=0&WaterMark=1&Item=N82E16817371007&Depa=1&Description=Antec+earthwatts+EA500+500W+Power+Supply
Moral of the story; knowing the wattage provided to the +12v rail(s) let's you know the amperage

 

[mactronix]

On the other hand, the Antec EA500 has 2x 17a rails with 408w provided to them for a total of 34a.
http://www.newegg.com/Product/ShowImage.aspx?CurImage=17-371-007-04.jpg&Image=17-371-007-09.jpg%2c17-371-007-03.jpg%2c17-371-007-04.jpg%2c17-371-007-05.jpg%2c17-371-007-06.jpg%2c17-371-007-07.jpg%2c17-371-007-08.jpg&S7ImageFlag=0&WaterMark=1&Item=N82E16817371007&Depa=1&Description=Antec+earthwatts+EA500+500W+Power+Supply
Moral of the story; knowing the wattage provided to the +12v rail(s) let's you know the amperage

Yes quite true i probably should have said that some do actually come out the same as if you did just add them up. Good PSU's will have the info you need ie the wattage for the 12V rails on the sticker. If not then its a good rule of thumb to assume that it will be lower than the combined rails. Its a pain but the PSU builders and the Graphics Card makers cant seem to agree on one format. Cards tend to say "needs a 450Watt psu" trouble is there is so much differance in the actual power output between makes. I have a 400 Watt unit that i wouldnt dream of putting a 3850 on its only got 18 Amps on a single rail.
Mactronix

 

[JAYDEEJOHN]

Not completely. A psu may say 20 amps per rail, and yet claim 600 watts to the 12 volt rail. What it comes down to is, whats the max available PER rail on any psu.

 

[mactronix]

Wouldnt life be so much easier if the card manufacturers quoted in AMPS
Mactronix

 

[invisik]

Not completely. A psu may say 20 amps per rail, and yet claim 600 watts to the 12 volt rail. What it comes down to is, whats the max available PER rail on any psu.

i agree! my 9800gx2 says it needs 40amps and my antec earthwatt 500 says it has 2x 17a =34 and the card is running fine with a decent overclock from 600/1000 to 722/1126 also with an overclock q6600 @ 3.0ghz, 2hdd 1 blue-ray, 3x 120mm fan. they just say u need that much power because some ppl go buy a crappy brand with 750watts when a good quality with 600watt can be comparable to it with 150watt less.

 

[JAYDEEJOHN]

Usually cheaper psus will say they have 480 watts going to the 12 volt rails, while each rail is really only capable of giving just 17 amps per rail. 17 and 17 = 34. 34x12 = 408, so in reality it can really only DELIVER 408 watts on the 12 v rail

 

[dgre005]

What brand and model is the PSU?

There is a big difference between a cheapo 400w PSU and a good brand PSU.

But I would say yes it should be fine, depending on what else you have connected.

My PSU is a Sparkle ATX-400PN 400W (a cheap PSU) and according to the customer reviews on Newegg.com, they often supply more watts than other 400w PSUs... I guess it would be enough to power a 3850 AGP?

Thanks.

 

[dgre005]

BTW I forgot to add that the total power to the 12V rails shouldn't exceed 348W; therefore according to what the posters above said, it should be able to take 29A on the 12V rails...