On Figure 13 you can see ST-420BKV label stating all its power specs.

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Figure 13: Power supply label.

As you can see on the label, there are two +12 V virtual rails, each one able to deliver up to 16 A, but with a combined 26 A maximum. Inside the power supply these two rails are connected to the same place – the +12 V rail coming from the +12 V rectifiers. What happens is that each virtual rail is connected to its own over current protection (OCP) circuit, which shuts down the power supply if you pull more than 16 A on each rail or 26 A total, in the case of this power supply. Usually the OCP circuit is configured with a value a little bit above from what is printed on the label. During our tests we checked whether the OCP was really active, as you will see.

All +12 V (yellow) wires are connected to the first rail (+12V1) but the wires from the ATX12V connector, which are connected to the +12V2 rail, using yellow with black stripe wires. This distribution is correct for a power supply that doesn’t provide cables for video cards or lots of peripheral plugs.

Let’s now see whether this power supply could deliver its rated power or not.