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Are we supposed to add the current in all 12 volt rails if there are more than one 12 volt rail?
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I wish it was that simple! Unfortunately no.
Power (watts) = Current (amps) * Voltage (volts)
So for a single rail PSU you take the current on the +12V rail, multiply by 12, and you get the power available at +12V. So for my PSU, that would be 60*12 = 720W.
Unfortunately with multirail units there isn't an easy way to find out the
total amperage available unless the manufacturer provides it, or your find a review where they actually test or dissect the unit. The current ratings on multirail PSUs simply state that if the power draw on any individual rail goes above what it's rated for (usually between 16 and 24A) the whole PSU will shut down.
I've seen 550W PSUs with four 20A +12V rails, which if you just added it all up would make 960W at +12V, making it something like an 1100-1200W PSU. But in reality there are probably no more than 35A on the +12V rail, just four rails it can be delivered on.
The hardware team is trying to get around these confusing labels by only recommending PSUs that we know have lots of current on the +12V rail. Look for:
Corsair
SeaSonic
CWT
PC Power and Cooling
CoolerMaster
Real Power Pro (Not eXtreme Power)
Thermaltake
Toughpower (not Pure Power)