I have noticed recently, via SIW, EVEREST, etc that the +12v readings VARY between 11.902v and 11.908v, but never reach 12v. I have a 550w PSU, (cannot recall maker). My question is, need I be CONCERNED about the 'low' voltage? All other voltages are within the required parameters. Also, having looked at replacement PSUs, I see some advertised with 'Single 12v Rail', and others with multiple 12v rails. Can anyone advise which is better, and why? PC specs as per my profile. Regards to all, Alexma +12VDC ± 5%; +11.400 VDC  to  +12.600 VDCSo, nothing to worry about then! Many thanks CC.Forgot to answer your other question.  Many new computers no longer use the 3.3V, just a small amount of +5V & mostly +12V for everything.  You've seen motherboards with 24-pin connector and 4, 6, or 8-pin 12V CONNECTORS & all the +12V for the optical drives & hard drives & PCI-e graphics card, adds up to a lot of +12V.  One output is not enough, so ATX standard ATX12V 2.2 added multiple +12V outputs.
http://en.wikipedia.org/wiki/ATXAlso, just to ADD to the above - software voltage readings are not accurate.