What is a single rail +12V power supply?

For power users and overclockers, multi +12V rails design from ATX 12V v2.0 is just not flexible enough to deliver the power they demand.  Many multi +12V rails PSUs have maximum output limit of 20A for each rail.  If the consumption of CPU is only 10A, then the remaining 8A of power cannot be used for other components such as hard drives, fans, graphics cards, and system lighting etc…  This undesirable situation does not occur when using a single +12V rail power supply, where the power is shared with all components.

The SilverStone ST56ZF, with a single +12V rail output of 38A is a great example of a power user PSU.  A top performing single +12V rail PSU such as the ST56ZF requires more high quality and reliable components to achieve optimal and tightly regulated output at nearly any loading conditions and environment.

The following graphs illustrate the difference between single +12V and multi +12V output in
situations where higher than usual power draw is required such as overclocking.
Multi +12V rails PSU


Single +12V rail PSU
 
According to FCC regulation, if a power supply does not comply with the 240VA safety regulation, then a licensed electrician is required for assembling or installing the product. However, the 240VA is not a state-regulated certificate and TUV/UL do not have the same regulation so a single +12V rail power supply that exceeds 240VA can still be installed by computer builders.

Although Intel indicated that the power supply have to acquire the 240VA certificate to fit in with its ATX 12V v2.0 standard, this does not mean that the newer Intel motherboards have to work with 240VA compliant PSUs.  Power supplies that are compliant with previous version, the ATX 12V v1.3, can still operate with the newer motherboards perfectly. The only requirements for any motherboard to operate correctly is that the power supply has to achieve the need of entire system’s +12V output in addition to other rails such as +5V, +3.3V, etc...