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Dual vs single rail

I know this is an old topic and i´m not asking about which one is better, I want to know if i have a dual rail Psu delivering 18 amp each rail. What´s the limit for my GPU? I mean 18 or 36 amps.
19 answers Last reply Best Answer
More about dual single rail
  1. What's the brand and model number of the PSU?
  2. Dual rail of 18 amps does not always mean 36 amps total more likely 30 amps. The PCIe connector is only of one of the rails so 18 would be the limit.
  3. ko888 said:
    What's the brand and model number of the PSU?



    OCZ ModXStream Pro 500 W Power Supply, i bought it because it has a golden medal at secretshardware and it got a good price.

  4. Best answer
    It is probably wired so that each PCIe cable is off an 18A rail. If you use a card that requires 2 x 6-pin connections, you will have 36A (432W) available minus what is used by the CPU and other +12V devices/components.
  5. ^+1 really depends on if you need both connectors.
  6. clutchc said:
    It is probably wired so that each PCIe cable is off an 18A rail. If you use a card that requires 2 x 6-pin connections, you will have 36A (432W) available minus what is used by the CPU and other +12V devices/components.


    ok so if i got it rigth you´re telling me if i use the second 6+8 cable i have the complete 36 amp at my disposal except for the amp needed by the cpu and other stuffs and that psu can handle an hd 7950?
  7. 36amps is what your PSU can handle max on the +12 volts. Yes that PSU can handle HD7950.
  8. rolli59 said:
    36amps is what your PSU can handle max on the +12 volts. Yes that PSU can handle HD7950.


    can you tell me the maximun gpu that psu can handle, i just wanna make sure of it limits because that issue of the single vs double rail got me confused. Thanks
  9. Go here: http://www.hwcompare.com/
    Compare the cards you are interested in for wattage. Divide by 12 (v) to get the amps the +12v rail should be required to handle for the respective card(s).
  10. This can be useful http://www.realhardtechx.com/index_archivos/Page362.htm
    remember the card gets 75watts through the slot itself a 6pin is additional 75 watts and the 8pin 150watts. (watts devided by volts (12) gives amps).
  11. clutchc said:
    Go here: http://www.hwcompare.com/
    Compare the cards you are interested in for wattage. Divide by 12 (v) to get the amps the +12v rail should be required to handle for the respective card(s).


    ok the hd 7970 takes 20 amps to be powered up and as you tell me earlier the combine 12v rail amps of my psu is 36 and minus cpu and the others there´s plenty of power then.
  12. pit_1209 said:
    ok so if i got it rigth you´re telling me if i use the second 6+8 cable i have the complete 36 amp at my disposal except for the amp needed by the cpu and other stuffs and that psu can handle an hd 7950?

    Yes, assuming the PSU is wired so that the 2nd PCIe cable is on the 2nd rail. Which normally would be the case.
    The HD 7950 3GB requires 200W at max. That's apprx. 16.7A on the +12v rail, or 8A+ per rail. A 125W CPU (for example) will require 10.4A from those rails also. 200W + 125W = 325W just for the CPU and GPU. Other +12V devices won't be as demanding. If possible, you should try to keep your PSU at 80% or less. But not carved in stone. Remember, your GFX card will rarely reach its maximum demand of that 200W.

    (most CPUs require less than 125W, but I used that example as a max)
  13. pit_1209 said:
    ok the hd 7970 takes 20 amps to be powered up and as you tell me earlier the combine 12v rail amps of my psu is 36 and minus cpu and the others there´s plenty of power then.

    HD 7970? Are you changing your GFX card choice now?
  14. clutchc said:
    Yes, assuming the PSU is wired so that the 2nd PCIe cable is on the 2nd rail. Which normally would be the case.
    The HD 7950 3GB requires 200W at max. That's apprx. 16.7A on the +12v rail, or 8A+ per rail. A 125W CPU (for example) will require 10.4A from those rails also. 200W + 125W = 325W just for the CPU and GPU. Other +12V devices won't be as demanding. If possible, you should try to keep your PSU at 80% or less. But not carved in stone. Remember, your GFX card will rarely reach its maximum demand of that 200W.

    (most CPUs require less than 125W, but I used that example as a max)


    As it seems in the image the second cable goes on the second trail (i think so), the red one so this is wired rigth?

  15. clutchc said:
    HD 7970? Are you changing your GFX card choice now?


    No, it´s just hypotetically speaking taking one of the most power hungry gpu into the play.
  16. pit_1209 said:
    As it seems in the image the second cable goes on the second trail (i think so), the red one so this is wired rigth?

    http://i1340.photobucket.com/albums/o738/pit1209/17-341-016-15_zpsd2a0654b.jpg

    The colors mean nothing. But, myself, I would naturally assume a name-brand 2 rail PSU will have each of the 2 PCIe cables on different rails. You would need a schematic of the PSU to verify for sure, but it is almost a given.
  17. clutchc said:
    The colors mean nothing. But, myself, I would naturally assume a name-brand 2 rail PSU will have each of the 2 PCIe cables on different rails. You would need a schematic of the PSU to verify for sure, but it is almost a given.



    Ok thanks to all for bringing light to the matter.
  18. Best answer selected by pit_1209.
  19. http://www.hardwaresecrets.com/article/OCZ-ModXStream-Pro-500-W-Power-Supply-Review/973/6

    +12V1: The cables that are permanently attached to the power supply.

    +12V2: The cables from the modular cabling system. <--- Video card is all on this rail.
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