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Apr 14, 2009, 12:49 AM   #9
tong

Join Date: Oct 2006
Location: MA, USA
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System Specs

Quote:
 Originally Posted by boredgunner http://www.cluboc.net/reviews/power/...ps_sticker.jpg See, three +12v rails with 19A on each and beneath it 540W. This means the +12v rails combined generate 540W of power at max load. Since 540W is the +12v power, divide 540 by 12 to get the real amperage, don't do 19 times 3 since that gives you 57. Correct Way: 540W/12v = 45A - nice value by the way Incorrect Way: 19A + 19A + 19A = 57A Why is that incorrect? 57A x 12v = 684W - this is a 650W PSU
I beleive you are incorrect because: (believe is the keyword)

All multi-rail psu's are measured one rail at a time therefore (the loadsplitters don;t do a perfect job) you get bloated numbers for each rail and when you reverse the math you get incorrect interpretations like yours above. That PSU would be more likely to have 15 amps in each rail than having it's wattage rated 100w off.

Again this is based on the fact that they measure each rail individually and the fact that load splitters don;t work perfectly, in fact they are meant to overcompensate not separate.

The biggest argument of multi vs single rail is power draw: "multi's will give only "x" per rail and if you only have 3 hdd's and they only use 2 amps then you;re wasting the rest of it when it could be helping your gpu out, where as a single rail, everything uses what it needs and it gets shared". -- Unfortunately the splitter doesn't work perfectly for this argument to be 100% true, yes you can only get so much out of each rail but it's higher than you think due to the load-splitters ineffectiveness, as long as, one rail or more has less than rated load.

Higher end psu's will rate for wattage and test each rail(s), however, they look at the overall picture to make sure that the PSU is within it's intended ability and are usually underrated by a few percent (very few)
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