Originally posted by: BassBomb
you dont get losses on two rails , youd get less on two than you would on one
They key is to look at "combined" Amperage. Generally speaking you want over 26A on a high quality PSU (Enermax, OCZ, Antec, etc) or 28A+ for lower quality brands.
For Example, my Enermax 485W Noisetaker has two +12V rails: 18A on +12V1 and 18A on +12V2. However the combined rating is 32A for the two rails. This is written plainly on the PSU, but some manufacturers hide the combined rating and instead only write the individual ratings.
Just remember you can't just add up the voltage rails to get total +12V amperage.
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However, if they give you the total
watts on the 12V rail, you can obtain amperage by dividing it by 12. For example, let's say your PSU says the following:
+12V1 17A
+12V2 16A
Total power for +12V rails: 312W
Divide that by 12 to get the amps: 312/12= 26A combined power.