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For power requirements can you add two 12v rails together

Discussion in 'General Hardware' started by crazy pyro, Jan 16, 2009.

  1. crazy pyro

    crazy pyro New Member

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    My friend's recently got a 7900GT and needs a PSU capable of providing 20 amps on the +12V rail, I've found him an FSP one capable of providing 18A on each 12V rail but it's got two +12V rails so would that be plenty to power his system?
    Specs here:
    Pentium D that was 2.8GHZ@ stock running at 3.5
    7900GT
    ASrock dual vsta mobo AFAIK, may be a different model but think it's got a 965 chipset
    3 sticks of RAM@667
    1 DVD drive
    2 HDDs
    DVD drive through USB
    6 USB devices.
    Aftermarket cooler
    If it can't reccomend one that would preferably under £40.
  2. Homeless

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    his computer should be fine
    crazy pyro says thanks.
  3. cdawall where the hell are my stars

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    yea when they say 20A they mean total system so dual 18A is more than enough
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  4. Black Panther

    Black Panther Senior Moderatorâ„¢ Staff Member

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    It's definitely more than enough to power his system.

    I've been running my 8800GT that way for the past year, gaming and oc'ing with no problems.

    The 8800GT needs 22A on the +12V. Anyway, my PSU's got two +12V rails, one at 16A and the other at 18A. Lol that's less than your friend is running his 7900GT on so quite very surely he should be fine 100% because the amperage requirements for a 7900GT are less than that of the 8800GT.

    To be precise, the 7900GT needs 20A on the +12V rail.
    Now the amperage on the +12V rails doesn't simply add up so it doesn't mean that since he has 2x18A he has a total of 36A because he got much less actually.

    In case you want to calculate the exact amount of +12v amperage being delivered - you need to divide the combined power with 12, then you will have the amps...

    W=V.A
    A=W/V
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  5. crazy pyro

    crazy pyro New Member

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    Ok then, cheers guys. I just wanted to make sure since I don't want his rig to blow up and him blame me.
  6. kiriakost

    kiriakost New Member

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    The Black Panther answer , was a bit confusing to me .

    There are on the net specifically info , about components and energy requirements .

    For instance all the CPUs from 2005 until now, they use from the 12V line 6-12A .
    The PCIe and AGP , have both specific limits in wattage.

    So the sub total of energy needs, are some how easy to calculate.

    CPU=10A + VGA =20 + HDD,DVD,Fan,lights=4A generic total 34A as Peak value.

    Logically , systems with multiple VGAs will demand more.

    The only question are ... Does the PSU delivers truly the power that is written on the label.

    No one can do that in praxis ... with out special tools as induction amperage clamp .

    I like to believe, that the "non that detailed rule" of 34A or 30A as efficient wattage for single VGA systems , are more easy as description for wider understanding.

    I have read lots of technical white papers, and the only that i gained from it,
    are to evaluate the PSU wattage by more realistic factors, but still in theory.

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