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Amps on +12v requirements - XFX 7970 Black DD + Xilence XP700

Tags:
  • Xilence
  • Power Supplies
  • XFX
  • Graphics Cards
  • Components
  • 7970
Last response: in Components
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January 20, 2014 5:45:07 PM

I have been using this for over a year now. What I wonder is how to calculate amp requirements for my GPU when it comes to more than one rails like my graphics card which needs two +12v cables to be connected.

http://www.xilence.de/en/products/power-supplies/gaming...
My PSU is listed as having 4 +12v rails. The first two are 18amps and the last two are 20amps.

http://forum-en.msi.com/faq/article/power-requirements-...
Here it says that my graphics card needs around 31A and a 550W PSU.

I have the following hardware:

Gigabyte X79S UP5
5 sata harddrives, 1 dvd writer
1 ssd
Intel i7 3820 running at 4.3Ghz
A PCI wireless card.

How do I calculate how much amps are needed on the two +12 rails for my graphics card?

So if the card requires 550w, and my PSU is 80% efficient:

550 / 12 = my graphics card needs 45 amps
45 / 0.8 = 56 amps ?

Is this how it's calculated? There are two power connectors on the graphics card and I guess it also draws power from the PCI Express slot as well. So how do I calculate this when I'm feeding the card with two power cables?

More about : amps 12v requirements xfx 7970 black xilence xp700

a b U Graphics card
January 20, 2014 5:55:50 PM

You don't need to calculate anything. Your graphics card like you said needs 31A on the 12v rail. Your PSU has 18x2+20x2 12v rails so you've got 76A available. You're good bro, even for Crossfire possibly if your CPU doesn't take too much juice.
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a c 297 ) Power supply
a c 208 U Graphics card
January 20, 2014 6:10:00 PM

I cannot find the combined 12V power rating for that PSU anywhere, they don't admit to it on their load sticker, but because you have a 700W PSU you won't have more than 58A available, I found one spot that said it had a combined power of 56A but I can't find anything to corroborate that.

You'll be fine, the power ratings are for the system as a whole not just for the card, the 7970 has a power consumption of about 250W(~21A), even if your PSU is grossly over rated it will handle that. The card will pull 75W from the PCI-e slot and the rest from the two cables feeding it, your PSU will have the cables distributed so it is difficult to over draw any single rail without trying.


@mf red, you cannot just sum up the rails to find the combined source power, the rails are simply circuit breakers but there is a main source feeding them. Think of it like the breaker box in your house, you have a 200A main feed, but 15 20A feeds leading out, if you exceed 20A on any single feed you will trip it, but if you exceed 200A distributed across the feeds you will trip the main while being under the limit for any single rail.

@OP, efficiency is used to figure out how much power your PSU will draw from the wall, if your PSU is 80% efficient and has a 500W DC load on it, it will draw 625W from the wall (500/0.8)
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a b U Graphics card
January 21, 2014 4:03:51 PM

Thanks hunter, some one explained it to me like that and I took them at their word. I should know better than to trust people darn it!
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a c 297 ) Power supply
a c 208 U Graphics card
January 21, 2014 4:17:57 PM

A lot of people think it works that way and that each rail is its own source so don't feel too bad about it! Just trying to share knowledge so you can share it with others in the future!
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