How would one calculate total amp rating on Dual +12V rails?

I'm looking to upgrade to a new graphics card and know very little about PSUs and wanted to know if my current power supply unit will support the card I'm considering purchasing.

The card I'd like to buy requires a "Minimum of a 350 Watt power supply.(Minimum recommended power supply with +12 Volt current rating of 18 Amp Amps.)" (as listed in its specs)
Here's a link to the card on newegg:

My current power supply is an LC-A420ATX. If the model name is any indication, I have a 420 watt supply. Is that correct?

So far as the voltage rating, my stats are as follows:


There appear to be dual +12V rails but I don't know how to calculate the total Amps. Do I just add the total Amp rating of both +12V lines, meaning I'd have an Amp rating of 28?

Thanks for your thoughts :-)
8 answers Last reply
More about calculate total rating dual rails
  1. Your PSU satisfies the Requirements, is > 350 Wats & 12 V rail => 28 Amps (Yes you add them).

    Does that mean it will work, good question. Your Power Supply CAN NOT provide all the listed individual powers at the same time. Add up the individual rails:
    3.3 x 17 = 56 W + 5 x 25 = 125 Watts + 12 x (14 + 14) = 336 W.
    56 + 125 + 336 = 517 W. OOPS It's only a 420 Watt PSU

    You also must take in to account all of what makes up your compute. You need to also take into account the Quality of the PSU. Low quality PSU may not provide their rate value as temperature goes above ambient.

    You should "google" PSU calculator and google PSU reviews ( or evaluation) for a better understanding.
  2. RetiredChief said:
    Your PSU satisfies the Requirements, is > 350 Wats & 12 V rail => 28 Amps (Yes you add them).

    Not always. Usually the specs say what the maximum current each rail can provide. That is not the same as simply adding them up and arriving at the total 12 volt output.

    Example: I have an Antec 650 watt TP3 PSU - three rails, each providing a max of 19 amps. But the total 12 volt output is 52 amps. This is very common. You need to read the PSU label very carefully.

    This is a very inexpensive PSU. I doubt if it can provide 28 amps.
  3. Jsc is correct, you can't just add them. Its possible you'll get the correct amount, but the odds are higher that you won't. My EA500 has dual rails, each individual rail is rated for 22A. I don't have 44A available however, its only 34A. My wifes EA500 is the older model which SeaSonic made. It two had dual rails but each rail is limited to 17A. In this case if you did add both rails together you'd get the correct amount.

    The correct way to see how many Amps you have is to read the line under the chart which says how much power is available. There should be something that says 12v1 + 12v2 not to exceed XXXW. Divide that number by 12 to get the number of amps.

    I haven't yahoo'd the model number, so I have no idea if thats a good PSU or not. I do know the 9500GT isn't a good card for gaming, more so the model your looking at. It might have 1GB of memory, but its slow DDR2 memory. The low end for good gaming is the 4670 for AMD, or the good 9600GSO. If your at 1680x1050, you'll be disappointed if you get anything else.
  4. jsc said:

    This is a very inexpensive PSU.

    It's a Deer, the worst of the worst.
  5. Thanks, all, for your time and thoughts. I really appreciate the input. I checked my PSU sticker and it actually doesn't provide any "not to exceed max of XXXW" information. It just has the voltage, current & frequency statistics and that's all.

    I did run my PSU components through a PSU calculator, adding the video card. My components would be as follows:

    Intel Core Duo 3Ghz
    Regular Motherboard
    NVidia GeForce 9500 GT
    Memory -- 4GB DDR2
    HD 7200 RPM

    According to the newegg calculator, I'd need a 282 Watt PSU. Unfortunately that doesn't answer my amp question. So based on jsc's post, I need to find the maximum wattage allowance on the 12V rails and divide it by 12 for the amp rating? I can't seem to find that particular statistic in any search engines. I guess I'll drop my computer manufacturer a line?

    Thank you all for your thoughts. They've been really helpful and I appreciate it.
  6. From my experience, if that info isn't on the sticker, the PSU is junk and should be tossed ASAP. I'm not joking. PSUs that can't/won't supply you with this detail aren't worth running. Get a new PSU. Deer? I agree, one of the worst out there.
  7. I guess I was not to clear. as even jsc missed the OOPs

    Your wattage is their. you just have to calculate it
    BUT YOU CAN NOT TAKE IT to the bank

    3.3V x 17A = 56 W
    5V x 25A = 125 Watts
    12V x (14 + 14)A = 336 W.

    56 + 125 + 336 = 517 W. OOPS It's only a 420 Watt PSUWhat this says is that as long as your 5 V and 3.3 V Power is less than 84 Watts you will have up to 336 w avail on the +12 volt rails. As the power on the +3.3 and + 5 go above 84 Watts the Power (and ampreage availble on the +12 V DECREASES.

    Not above is for a PSU of GOOD quality, as quality comes down so does power output.
  8. I didn't miss what you wrote RC, you were still wrong.

    Your PSU satisfies the Requirements, is > 350 Wats & 12 V rail => 28 Amps (Yes you add them).

    Again, no you don't. I caught that with whats written its can't supply full voltage to all rails. If you read any sticker however, even Corsairs, this is always the case.
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