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PSU ratings clarification

post #1 of 5
Thread Starter 
OK...I've been doing a little research out of the curiosity of xfire 7850s on my rig. I currently have a Antec neo he550. According to the specs there are three 12v rails @ 18A each. Total power 504w for the 12volt rails. Can someone help me understand this? According to my math(504w/12v=42A) I have 42A available on the 12v rails? But the 18A rating is suggested max amperage per rail? Am I wrong to think that these 3 are the same as a single rail @ 42A?

Thanks for reading
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CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
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Home PC
(12 items)
 
   
CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
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post #2 of 5
Yes, those three rails are the same as a single 42A rail, it's just that they have protection circuits on different groups of cables to prevent overloading or to stop a short circuit from blowing up the PSU.
post #3 of 5
Thread Starter 
So according to http://forum-en.msi.com/faq/article/printer/power-requirements-for-graphics-cards one 7850 requires 21A and a 500W psu minimum. Does that mean 2 would be 42A? There is no doubt that 550W is fine for my rig...but the amperage is sketchy to me because it is hard to find specific date on GPU amperage requirements, and how that increases with load and xfire etc. Sorry to drag this out if the answer is right in front of me. But I like to learn things and not just accept an answer all the time wink.gif Even if 42A was required by Xfire 7850s then 42A on my 12V rails isn't enough technically, correct?

I can't believe I took out my TP3-650 for this HE550 because I wanted modular PSU. doh.gif
Home PC
(12 items)
 
   
CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
  hide details  
Reply
Home PC
(12 items)
 
   
CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
  hide details  
Reply
post #4 of 5
21A is for the entire system... CPU and graphics card and drives and fans and everything else. 21A is an estimate for an average system that would use that PSU, plus a bit of margin.

To figure out how much power the GPU uses, look for the "TDP" rating. Divide that by 12 and that tells you the amperage.
post #5 of 5
Thread Starter 
This makes much more sense. Thanks for your help
Home PC
(12 items)
 
   
CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
  hide details  
Reply
Home PC
(12 items)
 
   
CPUMotherboardGraphicsRAM
Intel i7 - 4770k MSI Z87 Mpower Max AMD R9 290x G.Skill Trident X Series 16GB (4 x 4GB) DDR3-2400 
Hard DriveHard DriveOptical DriveCooling
Seagate Desktop HDD 4TB 3.5" 5900RPM Seagate Desktop HDD 4TB 3.5" 5900RPM LG WH14NS40 Blu-Ray/DVD/CD Writer NZXT Kraken x60 
MonitorPower
Asus PB278Q 27.0" Monitor Rosewill Capstone 750 modular 
  hide details  
Reply
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