How many AMPS are enough on each 12V?

Discussion in 'Power Supplies' started by Borealis7, Nov 4, 2012.

  1. Borealis7

    Borealis7 Platinum Member

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    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...
     
  2. OmegaSupreme

    OmegaSupreme Member

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    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.
     
  3. Borealis7

    Borealis7 Platinum Member

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    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...
     
  4. OmegaSupreme

    OmegaSupreme Member

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    WOW! Those things suck that much power!!! :eek: 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.
     
  5. Borealis7

    Borealis7 Platinum Member

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    #5 Borealis7, Nov 4, 2012
    Last edited: Nov 4, 2012
  6. OmegaSupreme

    OmegaSupreme Member

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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.
     
  7. Borealis7

    Borealis7 Platinum Member

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    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.
     
  8. OmegaSupreme

    OmegaSupreme Member

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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?
     
  9. Borealis7

    Borealis7 Platinum Member

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  10. MrK6

    MrK6 Diamond Member

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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.
     
  11. Borealis7

    Borealis7 Platinum Member

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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.
     
  12. OmegaSupreme

    OmegaSupreme Member

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    Actually, according Tagan's page your PSU can only pull 360w (30A) max from the +12v rails.
    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.
    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).
     
    #12 OmegaSupreme, Nov 6, 2012
    Last edited: Nov 6, 2012