On Figure 17 you can see NeoPower 550 labl stating all its power specs.

click to enlarge
Figure 17: Power supply label.

As you can see on the label this power supply has three +12V rails. These rails are “virtual” as all of them are connected together to the single +12 V “real” rail coming from the +12 V rectifiers thru a series of jumpers. You can see this on Figures 18 and 19.

click to enlarge
Figure 18: +12 V wires are separated into six wire groups and connected to the PCB as if there were three separated rails.

click to enlarge
Figure 19: However they are connected together on the PCB.

Each virtual rail, however, has its own over current protection (OCP) and that is why they are listed as individual rails even though they are all connected to the same place inside the power supply. From what the label says the over current protection is configured to shut down the power supply if you pull more than 18 A in any of the three rails (usually the over current protection circuit is set with a value a little bit higher than what is written on the label). At least in theory, as during our tests we pulled a lot more than 18 A on +12V1 rail and the power supply didn’t shut down (but let’s not get ahead of ourselves; more about this on the next page).

On power supplies with multiple rails it is very important to see how the available connectors are distributed among the available rails. On NeoPower 550 Antec distributed the connectors like this:

• +12V1: ATX12V, half of EPS12V.
• +12V2: All plugs from the modular cabling system.
• +12V3: Motherboard main connector, half of EPS12V.

One curious thing is the fact that half of the EPS12V plug is connected to the +12V1 rail (yellow with blue stripe wire) and half to the +12V3 rail (yellow with black stripe wire), as you can see on Figure 20.

click to enlarge
Figure 20: The EPS12V plug is connected to both +12V1 and +12V3 rails.

Putting all connectors from the modular cabling system on the same rail was something that we wouldn’t recommend because of the video card connectors (PCI Express auxiliary cables), which pull a lot of current (and thus power). This configuration, however, didn’t have any impact on the performance of this unit, as we will show on next page.

Another curiosity is that the plugs from the modular system are connected to the +12V2 rail thru two sets of two wires each. The first set is connected to the first two connectors and the second set is connected to the remaining connectors, as we clarify on Figure 21.

click to enlarge
Figure 21: How the plugs from the modular system are connected to the +12V2 rail.

Because each wire has its own current limit (which is a physical limit) if you are not going to use all the cables from the modular system, which is more likely, you will have a better power/current distribution if you balance the two groups of connectors. For example, if you are going to use just two cables, attach the first one to the first plug and the second one to the last plug (instead of using the second plug).