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Old 11-04-2012, 11:08 AM   #1
Borealis7
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Default How many AMPS are enough on each 12V?

Hey there,
when trying to figure out if a certain PSU could power a certain graphics card (or 2 of 'em) what can i learn from looking at the specifications?

i know the maximum power draw of the GPU would be around 300W since 75W come from the PEG + upto 225W from the PCIE connectors, but how can i know if the PSU is able to supply those reliably? do the Amper number tell me that?

i see many mainstream 350W-500W PSUs with 18A on each 12V rail, some with 20A, i wish VGA card makers would specify more details regarding the minimum power requirements and not give us marketing bullshit to promote high-end PSU sales...
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Old 11-04-2012, 11:12 AM   #2
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Quote:
Originally Posted by Borealis7 View Post
Hey there,
when trying to figure out if a certain PSU could power a certain graphics card (or 2 of 'em) what can i learn from looking at the specifications?

i know the maximum power draw of the GPU would be around 300W since 75W come from the PEG + upto 225W from the PCIE connectors, but how can i know if the PSU is able to supply those reliably? do the Amper number tell me that?

i see many mainstream 350W-500W PSUs with 18A on each 12V rail, some with 20A, i wish VGA card makers would specify more details regarding the minimum power requirements and not give us marketing bullshit to promote high-end PSU sales...
amps x 12(v) = watts.

So those small PSU's with the 18/20A rating would be insufficient for a single card that draws 300w. (Is it a GTX 480 per chance?)

Ideally, you'd just buy a power supply with a single +12v rail, like a SeaSonic, Corsair or a PC Power & Cooling.
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Old 11-04-2012, 11:24 AM   #3
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Originally Posted by OmegaSupreme View Post
amps x 12(v) = watts.

So those small PSU's with the 18/20A rating would be insufficient for a single card that draws 300w. (Is it a GTX 480 per chance?)

Ideally, you'd just buy a power supply with a single +12v rail, like a SeaSonic, Corsair or a PC Power & Cooling.
or a highly overclocked 7970 GHZ edition...not that i have one, i'm just asking in general.

"Doh!" moment of the day...i forgot my 12th grade physics...W=IA...
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Old 11-04-2012, 11:30 AM   #4
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Originally Posted by Borealis7 View Post
or a highly overclocked 7970 GHZ edition...not that i have one, i'm just asking in general.

"Doh!" moment of the day...i forgot my 12th grade physics...W=IA...
WOW! Those things suck that much power!!! That's actually more than 480's.
According to my Kill-a-watt meter, my whole rig with 2 480's at full load only draw ~570w!

Not sure if physics applies (I've never taken it)

But I assume it applies to all electricity

Amps x Volts = Watts therefore, Watts/Volts = Amps and I assume Watts/Amps = Volts.
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Old 11-04-2012, 11:34 AM   #5
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if you look here:
http://www.hardocp.com/article/2012/...card_review/10

[H] says total system draw without card is 67W, fully loaded and OCed its 366W maximum.
but that is also with an OCed CPU.

EDIT: i just noticed they bench with 12.9 BETA drivers and not 12.11 BETAs? but that belongs in another forum.
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Last edited by Borealis7; 11-04-2012 at 11:37 AM.
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Old 11-04-2012, 11:57 AM   #6
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Originally Posted by Borealis7 View Post
if you look here:
http://www.hardocp.com/article/2012/...card_review/10

[H] says total system draw without card is 67W, fully loaded and OCed its 366W maximum.
but that is also with an OCed CPU.

EDIT: i just noticed they bench with 12.9 BETA drivers and not 12.11 BETAs? but that belongs in another forum.
That does seem like a lot. But better to err on the side of caution and get a good PSU. Try to get single rail if at all possible.

I just took a quick gander at newegg and found this Corsair 500w It packs 38A (456w) on a single +12v rail. Plenty of juice for that power hungry 7970.
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Old 11-04-2012, 12:09 PM   #7
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That does seem like a lot. But better to err on the side of caution and get a good PSU. Try to get single rail if at all possible.

I just took a quick gander at newegg and found this Corsair 500w It packs 38A (456w) on a single +12v rail. Plenty of juice for that power hungry 7970.
well yes, obviously on a single rail 38A is plenty. but what if i see PSUs with multiple rails, each having 20A on it? if i use 2 separate rails to power a single 7970 then the combined Amperage is not 20+20, it's a bit less.
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Old 11-04-2012, 12:13 PM   #8
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It just depends on the power supply in question because some are regulated differently than others internally and may never even get to amperage requirement you need to run it. Which one are you using or planning on using?
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Old 11-04-2012, 04:28 PM   #9
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i'm wondering how far can i stretch my Tagan 500W PSU.
is it going to be enough with an overclocked i5 CPU and say...a 7950 card?
according to this: http://uk.hardware.info/productinfo/...specifications (i haven't opened my case in a while so i can't see it now)
it has a total of 40A on a split 12v rail so i think i could hold on to it for a future upgrade.
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Old 11-04-2012, 08:49 PM   #10
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If you can figure out the railing separation (or if a review had it laid out) and you can get your mobo/CPU on one rail and the video card on another, you should be fine. The problem with older power supplies, even a few years older, is that they lack amperage on the 12V rails. Over the last 20 years computers have shifted from using 3.3V and 5V rails to 12V rails. More modern power supplies with the latest specs put more and more emphasis and power on the 12V rail(s).

This said, if that 12V rail is split 20A/20A, you could get away with powering a 7950 easily. For reference, with my 7970 at a stock voltage overclock (typically what I game at) of 1125/1600MHz, my entire system pulls about 280-290W from the wall, or about 240W-250W actual after accounting for efficiency. However, if I crank the voltage to 1.3V on the core and get the clocks to 1.3GHz+, it'll pull 450W from the wall (or about 400W actual load). That's still below your 500W capabilities, but you might go over your 20A if you have anything else but the graphics card on a 12V rail. Therefore, if you don't overclock or lightly overclock, you'll be perfectly fine. However if you do push the clocks you might run into trouble.
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Old 11-05-2012, 03:19 AM   #11
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I see, nothing inside the case except 2 HDDs so I think i'm good.
Lately, I get to recommend hardware to uninformed people quite a lot, and i'd hate to have them buy an uber VGA card just to have it borked by a PSU, so i'm trying to figure out a general rule on PSU requirements.
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Old 11-06-2012, 12:23 PM   #12
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Quote:
Originally Posted by Borealis7 View Post
i'm wondering how far can i stretch my Tagan 500W PSU.
is it going to be enough with an overclocked i5 CPU and say...a 7950 card?
according to this: http://uk.hardware.info/productinfo/...specifications (i haven't opened my case in a while so i can't see it now)
it has a total of 40A on a split 12v rail so i think i could hold on to it for a future upgrade.
Actually, according Tagan's page your PSU can only pull 360w (30A) max from the +12v rails.
Quote:
Originally Posted by MrK6 View Post
If you can figure out the railing separation (or if a review had it laid out) and you can get your mobo/CPU on one rail and the video card on another, you should be fine. The problem with older power supplies, even a few years older, is that they lack amperage on the 12V rails. Over the last 20 years computers have shifted from using 3.3V and 5V rails to 12V rails. More modern power supplies with the latest specs put more and more emphasis and power on the 12V rail(s).

This said, if that 12V rail is split 20A/20A, you could get away with powering a 7950 easily. For reference, with my 7970 at a stock voltage overclock (typically what I game at) of 1125/1600MHz, my entire system pulls about 280-290W from the wall, or about 240W-250W actual after accounting for efficiency. However, if I crank the voltage to 1.3V on the core and get the clocks to 1.3GHz+, it'll pull 450W from the wall (or about 400W actual load). That's still below your 500W capabilities, but you might go over your 20A if you have anything else but the graphics card on a 12V rail. Therefore, if you don't overclock or lightly overclock, you'll be perfectly fine. However if you do push the clocks you might run into trouble.
So going by these readings, I'd say that Borealis's PSU is inadequate to safely run his GPU.
Aside from that, the ripple on that PSU is absolutely horrible.
Quote:
Originally Posted by Borealis7 View Post
I see, nothing inside the case except 2 HDDs so I think i'm good.
Lately, I get to recommend hardware to uninformed people quite a lot, and i'd hate to have them buy an uber VGA card just to have it borked by a PSU, so i'm trying to figure out a general rule on PSU requirements.
Dude, if I were you, I wouldn't risk it. If it were a higher quality power supply, like a SeaSonic or a PC Power & Cooling or a Corsair unit, then I'd say that you'd be fine because their single +12v rails supply a lot more amperage than the split rails. Not to mention that the ripple suppression is far superior to your Tagan. Honestly, I wouldn't push that Tagan anywhere close to its limits, it could very well fry and take everything else with it.

For peace of mind, get a GOOD power supply.

Power supplies are often the most skimped on and are probably the most important component of a computer. I learned the hard way years ago after switching from a cheapo Antec unit to a PC Power & Cooling. After that switch, I was able to get another 200MHz out of my CPU without any change in CPU VID because of the awesome ripple suppression and voltage regulation the PCP&C unit had.

Also, a good rule of thumb to follow is you want at least 10% overhead of total system draw. However, ~15-20% would be better for longevity as capacitor aging lowers efficiency and power output capacity.

Remember, a power supply does not supply it's rated power all the time, that is the number that it should be able to, theoretically, supply. So it's its capacity to supply that power. However, a lot of lower end unit cannot supply even their advertised power without blowing capacitors out of them (literally).

Last edited by OmegaSupreme; 11-06-2012 at 12:28 PM.
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