the last resort :
sometimes manufacturers will split the rails in a very bad way, so the user can not receive anywhere near the stated power. Also, if you have 1 rail, as long as you don't exceed that amount you will be good. But, if on a multirail system, you have say a graphics card or something that pulls more current than what is stated for the rail, you can't run it.
Correct. I bought a PNY GTX 460 OC card for 110 CAN $, and PNY claimed that it required a 38 AMP rail when configured with a 130 watt CPU. Well I realized when looking at Canada Computers and Newegg, that no other competitor than Corsair had PSU as such for less than a 100$, in fact only the corsair builder series 600 watt, and more expensive variants, met the spec at 40 amps single rail, but those were the requirements for 130 CPU.
Let's do some math,
10-50 watts maximum for motherboard and ram
95 watts for modern 4 core Intel CPU
160 watts TDP for GTX 460 V2
25 watts for other components
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330 watts safe, not agressive.
Now if you look at, the 430 watt Corsair builder series, its single rail of 28 amps outs 336 watts.
Fuckin PNY playing it really safe; 38 amps lmao.
All other PSUs i saw under 80 bucks used multi rail systems.
If you think of a rail as an outlet, then on a multi rail psu the problem how i do know the outlet has enough power. To eliminate that headache just stick with single rail psu. 34 amps is ample for ANY single GPU configuration save dual GPU card; 500 watt builder series. For multi GPU systems you need 200 to 250 more watts from the rail. If you look at builders series 600 watts, it only provides 40 amps or 480 watts which is insufficient, take a look at the corsair gaming variant rated at 48 amps or 576 watts and your gold.