18 amps per rail

a good buddy is telling me theres only 2 power supplies that can handle a hd4850, because it requires 18 amps on a 12 volt rail? im sure i dont know squat about this, but building a new rig, so any advice would help
thanks in advance

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More about amps rail
  1. 18A on a 12V rail is a moderate figure that any modern mid-level PSU can accommodate. Only the cheapest PSUs or weakest (read less than 500W total power available) have less than 18A (typically 16A) available.

    But you are correct in using amperage on the 12V line as a metric for your new build. use 18A as the minimum figure (20A is the safety maximum in the USA, but some PSUs ignore that restriction) and stick to reviewed units.

    Here's a link to ATI's recommended PSUs:

  2. Your buddy is talking out of his rear end I'm afraid. The 4850 only requires a standard 6 pin connector which means that any decent quality PSU of around 450/500 watts and above will run it. Something 80+ ratted with around 26/28Amps.
    Like this inexpensive unit for example http://www.newegg.com/Product/Product.aspx?Item=N82E16817171028
    Mactronix :)
  3. Next time your buddy talks about computers,
    Just smile, nod your head...........and then walk away :>

    Come here for advice.
    You may want to suggest he come here and "help out".
    Maybe he will learn a thing or two, when he thinks he is helping.
  4. I would NOT get that PSU.


    I'm not sure how many amps a PSU needs to power the 4850, but I'd bet 30 or less is fine. 26A or more is probably enough. Stick with a 500W ish power supply from the quality list and I doubt you'd go wrong. (the list is PCP&C, SeaSonic, "modern" Antec lines, (Basiq, trio power, and Earthwatts) Corsair, FSP, and perhaps an OCZ or two. (Some are good, some of less so, look it up.) Any 500Wattish PSU from any of these companies should have no problem powering the 4850.
  5. fugfar said:
    a good buddy is telling me theres only 2 power supplies that can handle a hd4850, because it requires 18 amps on a 12 volt rail? im sure i dont know squat about this, but building a new rig, so any advice would help
    thanks in advance


    ATI Radeon™ HD 4850 System Requirements:
    450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)

    ATI Radeon™ HD 4870 System Requirements:
    500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)

    Neither takes a huge PSU to run and ATI's official documents don't list rails. When a company is stating a power requirement for their product they are talking about total power for the entire system, not just how much it takes to power their product.
  6. @ 4745454b
    Ok so can you answer me this then,
    The specks are as posted by ausch30. The Motherboard slot is rated to supply 75 Watts Standard. Now is that a set fact or is there deviation and if so is it enough to worry about ? Next the 6Pin Pcie cable. Now I'm assuming we are talking about a fitted cable from the +12V supply and not one cobbled together from a couple of Molex's. Same question, Is it standard and is there a deviation worth noting. And just to be clear here I'm asking if every 6 pin Pcie cable has to carry 75watts.
    Because if they are as given then that's 150 Watts for a card that draws 110. So if you have a PSU with a dedicated 6pin cable it has to run it. doesn't it?

  7. A 500W Raidmax with a 6-pin PCIE connector would probably choke, smoke, and croak on a 4850/4870. An Earthwatts 380 could probably run a 4850, although it would be right at its limits.
  8. The 4850 uses about 110w of power; that works out to 9.17 amps.

    GPU manufacturers recommends minimum Power Supply specs as a safety measure since everyone's PC is more or less different. That power requirement is for the entire PC.
  9. When a psu says it has 18 Amps per rail, thats AVAILABLE power, not constant, or full time power, its available. 6 pins are 75 watts, also available, not constant, just like the PCI slot, which also supplies 75 watts, its there if you need it. Thats why, one thing to check is total output vs available. Most psu ratings may say 18 amps per rail at 12volts each, and youd think thatd add up to be 36, but most the time it doesnt, as youll often see around 75%-80% of that number for combined total output, or actually 26-29 amps
  10. JAYDEEJOHN said:
    When a psu says it has 18 Amps per rail, thats AVAILABLE power, not constant, or full time power...

    Actually it depends on the PSU. The only way to determine if it is peak or constant amperage is to inspect the PSU's label.

    For example:

    This OCZ PSU lists 18a per 12v rail; or a total of 36a. To determine if it is peak or constant divide the total available power on the 12v by 12. As can be seen, this PSU can provide up to 432w on the 12v rails. So...

    432w / 12v = 36a

    This particular PSU lists constant amps.
  11. Thats why I said most, as some actually do what they infer, and some dont even, and cant even do what it "says", thats why I suggest to ALWAYS check whats available, as they love to play with these numbers to bump up an inferior psu to make them look better
  12. ^^^

    That's why I stick with Seasonic who also manufactures PSUs for Antec, Corsair, and PC Power & Cooling amongst others.
  13. Sorry Mactronix, I don't understand what your trying to ask. I simply provided a link to a review of the unit you suggested to try to convince people they shouldn't get that PSU. Read the link I provided. That PSU might be able to output "150W" but it was dirty as h3ll, inefficient, and not a good performer. Dirty power can kill devices early, poor efficiency will cost you money over time. Spend another $15-$20, and you can get something good.
  14. Well im sorry but i dont see how i can put it any more clearly. And to the OP just for clarity im sorry and should have been more clear i wasnt suggesting that PSU as a purchase i was just stating that it would run a 4850 which i know for a fact it can. I will agree that as far as buying a PSU goes it is always best to get something slightly over spec and to get as good a unit as you can afford. Again i appologise for not being clearer i was mearly adressing the issue of the friend saying only two psu's could run a 4850. For the record if i was to recomend a good PSU it would be this one http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003.

    However i still stand by what i said that psu may not be the best and as pointed out isnt the cleanest or most efficiant but at the end of the day it has run a friends rig with a 4850 in it since he upgraded when they first came out and there have been no issues.

  15. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006

    I would like to be more specific and if I were to recommend something it would be the link above. It's overkill for what your trying to power, but you will never have to worry about power again because you will know you have enough (unless you are looking at dual X2"s, in which case you will need a 1kw + PSU). And it's less than $100 for a very good quality 750w PSU. I always err on the side of more than enough power. You would be fine with a good 500-600w quality PSU
  16. honestly, this is exactly what i need, good opinions, good facts. thanks Toms Community.
    thanx to all who posted.

  17. and jdj, jag6, available vs. constant. was the foundation of his argument. glad to see it repeated.
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