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What's a Rail?
Been meaning to post this picture. It shows the various rails on a power supply. In this case, it's a theoretical 150 watt power supply. Note that the PSU breaks the 12 volt input into varying voltages for use by the computer.
These are known as RAILS.
Note also that each rail has a rating in amps. For example, the 12 volt rail in red shows 12 volts, 5 amps. Multiplying the volts (12) times the amps (5) tells us that this rail can supply 60 watts of power. In contrast, the 5 volt rail below it (in yellow) can only supply 10 watts of power (5v X 2A).
Consequently, putting a device that consumes 15 watts on the 5 volt rail would be likely to cause problems with powering your system, even if the total power draw of your system is under the 150 watt total of the power supply. MANY NEWBIES ARE BITTEN BY THIS PROBLEM.
Most often, it involves adding something like a PCI video card that draws too much for that rail. They are then confused about why their system won't start even though the power calculators show them under the total PSU wattage.
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