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How to Determine REAL +12v Amperage on Your PSU

Discussion in 'General Hardware' started by boredgunner, Apr 14, 2009.

  1. boredgunner

    boredgunner

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    I'd like to mention an equation that would let you find how out much amperage your PSU has on the +12v rail(s). The +12v rails are probably the most important rails on a PSU since they supply the power to the CPU and GPU. People usually overlook this part of a PSU, in fact they tend to just buy cheaper PSUs in general which will cause many issues.

    If you look on the sticker, near the +12v area, you'll see the amperage of the +12v rail(s), and under that a power measurement in watts. Lets look at my PSU for example, the COOLER MASTER Real Power Pro 650W.

    [​IMG]

    See, three +12v rails with 19A on each and beneath it 540W. This means the +12v rails combined generate 540W of power at max load. Since 540W is the +12v power, divide 540 by 12 to get the real amperage, don't do 19 times 3 since that gives you 57.

    Correct Way:
    540W/12v = 45A - nice value by the way

    Incorrect Way:
    19A + 19A + 19A = 57A

    Why is that incorrect?
    57A x 12v = 684W - this is a 650W PSU

    NOTE: This does not have to be done for a PSU with a single +12v rail. Lets use the COOLER MASTER Silent Pro M 600W for example.

    [​IMG]

    It tells you right there it has 40A of +12v power. If you have any doubts...

    480W/12v = 40A

    I prefer multiple +12v rail power supplies since they are more stable (generally speaking). Here is what I recommend you look for in a PSU.
    • Below 400W: Above 20A on the +12v rail(s)
    • 400W-450W: Above 20A on the +12v rail(s)
    • 451W-500W: Above 25A on the +12v rail(s)
    • 501W-550W: Above 30A on the +12v rail(s)
    • 551W-600W: Above 34A on the +12v rail(s)
    • 601W-650W: Above 40A on the +12v rail(s)
    • 651W-700W: Above 45A on the +12v rail(s)
    • 701W-750W: Above 50A on the +12v rail(s)
    • 751W-800W: Above 50A on the +12v rail(s)
    • 801W-900W: Above 60A on the +12v rail(s)
    • 901W-1000W: Above 70A on the +12v rail(s)
    • 1001W-1200W: Above 80A on the +12v rail(s)
    Pretty much anything above 1200W has a solid +12v line, but that doesn't mean the unit is good.

    Not good at math? Neither am I. Use a calculator!

    http://www.calculator.com/pantaserv/makecalc

    Also check out my PSU guide, as well as the stickied guide.

    http://www.evga.com/forums/tm.asp?m=100557126
     
    Last edited: Apr 14, 2009
  2. Mussels

    Mussels Moderprator Staff Member

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    my new PSU has a single 60A 12V line :)

    cant wait for it to arrive

    Single rail PSU's have the advantage of avoiding imbalanced rails.

    My current PSU has 4x 18A rails... now what if i had two video cards share one rail, due to how i'd plugged it in? poof!
     
  3. Kursah

    Kursah

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    Yeah my PC P&C Silencer 750 Quad has a single 60A 12v rail, I've had it since last Sept and it's been a very solid PSU, great performer, runs cool, but it does have too many damn cables/connectors lol! I do miss the modularity of the Corsair HX520, but this PSU is more solid for my needs. I do prefer more powerful single rail PSU's for the reason Mussels stated above, definately a plus imo.

    :toast:
     
  4. alexp999

    alexp999 Staff

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    I reckon multi rail PSUs are just a fad, most are just single rails with load splitters anyway.

    Single Rail FTW! :rockout:
     
  5. Mussels

    Mussels Moderprator Staff Member

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    back when i did review work i definately got told most multi rail PSU's used load splitters. I cant specify brands or models however.
     
  6. alexp999

    alexp999 Staff

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    Classified or forgotten?

    Only the really expensive 1kw PSUs actually have more than one real 12v rail iirc.
     
  7. Mussels

    Mussels Moderprator Staff Member

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    classified.

    The real reason the 850-1200W PSU's have more than one rail, is cause they're more than one PSU! Go have a look at corsairs 550W and 1000W model, and look closely at the number and type of connectors, and the layout of the modular plugs. its two 550's reshaped and stuck together.
     
  8. alexp999

    alexp999 Staff

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    I know I noticed that, two sets of everything, lol.
     
  9. tong

    tong

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    I beleive you are incorrect because: (believe is the keyword)

    All multi-rail psu's are measured one rail at a time therefore (the loadsplitters don;t do a perfect job) you get bloated numbers for each rail and when you reverse the math you get incorrect interpretations like yours above. That PSU would be more likely to have 15 amps in each rail than having it's wattage rated 100w off.

    Again this is based on the fact that they measure each rail individually and the fact that load splitters don;t work perfectly, in fact they are meant to overcompensate not separate.

    The biggest argument of multi vs single rail is power draw: "multi's will give only "x" per rail and if you only have 3 hdd's and they only use 2 amps then you;re wasting the rest of it when it could be helping your gpu out, where as a single rail, everything uses what it needs and it gets shared". -- Unfortunately the splitter doesn't work perfectly for this argument to be 100% true, yes you can only get so much out of each rail but it's higher than you think due to the load-splitters ineffectiveness, as long as, one rail or more has less than rated load.

    Higher end psu's will rate for wattage and test each rail(s), however, they look at the overall picture to make sure that the PSU is within it's intended ability and are usually underrated by a few percent (very few)
     
  10. Mussels

    Mussels Moderprator Staff Member

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    well one simple way of looking at it is this.

    For a while, multi rail PSU's were all the rage. Yet now all the PSU's with the best reputations have only a single rail.

    That implies something, and to me that implication is that single rail PSU's are superior. (which may well change again in the future)
     
  11. tong

    tong

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    Not negating that, in my personal opinion I agree, was stating that the main logic for the single-vs-multi argument was slightly askew due to it's ability to slightly fluctuate between the rails (only a few)
     

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