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# What exactly does 30 amps on the 12v rail mean?

I want to go SLI with my system with two 7600GT cards and I want to make sure that I get the right power supply. Can someone explain to me in detail how the whole amp/rail thing works? How do you calculate it? Also can someone translate all this for me?

Output: +3.3V@30A, +5V@28A, +12V1@18A, +12V2@18A, -12V@0.5A, +5VSB@2A

How do you use that to tell what the amps are on the 12V rail?
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Amperes (A) is the amount of power suplied by a PSU or needed by any part in your PC. Modern PC's draw most power from the 12V rails, and thus these are the most important.

The relationship between Volts (V), Ampere (A) and Watt (W) is easy: W = V x A.

In the example you supplied, the PSU has two 12V rails, denoted by 12V1 and 12V2. Each rail can supply 18A. However, the total output on the dual 12V rails is limited, often to an amount less than the sum of the dual 12V rails.

Some examples (see images below):
The ENERMAX Liberty ELT500AWT ATX12V 500W has dual 12V rails of 22A each. However, the total output on the 12V rails is limited to 32A (384W).

The Antec TRUEPOWERII TPII-550 ATX12V 550W has dual 12V rails of 19A each, but a total output on the 12V rails of 36A.

Thus, at first glance it seems that the Enermax has more power on the 12V rails (higher amps per rail), but when you look closer at the specs, the Antec can deliver more total power on the 12V rails.

Finally, take a look at http://www.slizone.com/object/slizon..._powersupplies . You'll find a list of SLI certified PSU's. Go with any well known brand name. Both PSUs I meantioned are SLI certified, but if you plan to overclock, you should get something a bit heavier than the minimal requirements (i.e. I'd pick the Antec over the Enermax.)

Do your research, since there are other good PSUs out there..these two are just examples. Let us know what you pick, and we can advice you further.
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Yep, that's a popular one also, and a good PSU. It's has a little less power on the 12A rails (30A), and personally, I don't like the low efficiency.

However, many members here love it, so it is a proven solid PSU
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[QUOTE=Chozart]Some examples (see images below):
The ENERMAX Liberty ELT500AWT ATX12V 500W has dual 12V rails of 22A each. However, the total output on the 12V rails is limited to 32A (384W).

The Antec TRUEPOWERII TPII-550 ATX12V 550W has dual 12V rails of 19A each, but a total output on the 12V rails of 36A.

How did you come up with the 32A and the 36A? With the formula you listed?
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Sorry. My bad. I forgot these pictures. These are the actual labes on the PSUs I mentioned.

On the Antec picture you see +12V1 @19A and +12V2 @19A
Underneath you will see written +12V1 & +12V2 max. load 36A.

On the Enermax picture you see +12V1 @22A and +12V2 @22A.
Right underneath you see a number of 384W (32A) for the two 12V rails combined. and of course, 384W = 32A x 12V.

When looking online, make sure you get information on the amps on the combined 12V rail. Most PSUs either mention a max Amperage (A) or a max wattage for the 12V rails. If wattage is mentioned, divide by 12 to get the amperes.

Hope this makes it more clear.

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sweet, thanks guys!
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Glad we could be of help
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Have fun with those cards I know i do!!!
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