In order to fulfill the requirements of UL 1950, CSA 950, EN 60950 and IEC 950 specifications, the ATX12V specification states that no output can deliver more than 240 VA continuously (240 VA is the same thing as 240 W in a DC circuit). One thing that is frequently misunderstood is that this limit is PER WIRE.
To correctly fulfill these standards, manufacturers would need to add an over current protection (OCP) circuit to each voltage output wire of the power supply, cutting the current flow in that wire if the circuit connected to it is pulling more than 240 W.
This would mean that power supplies would need to add an OCP circuit to each +12 V, +5 V, +3.3 V, +5VSB and -12 V wire coming out from the power supply. A low-end power supply has at least 20 wires coming out of it, with high-end models reaching double this. Think about not only the cost of doing this but also the space that this huge circuit would take inside the power supply.
Manufacturers decided to play with the fact that current is almost never pulled from a single wire alone. For example, current to the system CPU is split in two (ATX12V) or four (EPS12V) +12 V wires, current to video cards is split into three (6-pin PEG) or four (8-pin PEG) +12 V wires, ect. In other words, you would need a CPU pulling 480 W from an ATX12V connector or 960 W from an EPS12V connector to reach the 240 VA limit. You would need a video card pulling 720 W from a 6-pin PEG connector or 960 W from an 8-pin PEG connector to reach the 240 VA limit, and so on.
Some manufacturers decided to implement one over current protection (OCP) circuit for all +12 V wires, simply trusting the fact that it is highly unlikely that at any given time a single +12 V wire would be delivering more than 240 W on a PC power supply, because of what we explained in the previous paragraph. This approach is called single-rail design. In fact, some power supplies, especially very low-end ones, don’t have any OCP circuit at all. (Protection circuits are optional, which we will talk more about on the next page).
Other manufacturers, believing that some wires can actually deliver more than 240 W during normal PC operation, decided to add more than one over current protection circuit (OCP). Each group of wires that is connected to a single OCP circuit is called, in this context, a “rail.” The OCP circuit will kick in if this group of wires (or “rail”) pulls more current than its trigger point (e.g., if the OCP circuit is configured at 20 A it will shut down current from flowing on a group of wires if they together pull more than 20 A).
They aren’t “real rails” because almost always the power supply has internally only one circuit to generate the +12 V outputs, and that is why we frequently call these rails “virtual rails.”
This second approach is called multiple-rail design and is the most popular design nowadays. On power supplies using this design, you will see more than one +12 V rail being listed on their labels (e.g., +12V1, +12V2, +12V3, etc.) See Figure 29 for a real example.
One side effect of the multiple-rail design is that you need to worry about power distribution. If you pull too much current/power from a given rail, it will shut down if you reach the rail’s OCP trigger current, even if your PC is working under normal circumstances. An example is, if you have your CPU and two video cards connected to the same rail. (The solution is to move at least one of these components to a different rail.) This occurs because the OCP trigger current on the multiple-rail design is set to a lower value compared to a single-rail design.
But pay close attention, because several power supplies are advertised as using multiple-rail design, but their over current protection is set at a value that is so high that it works just like a single-rail design. Some units don’t have any over current protection at all and are actually single-rail units.
In summary, single-rail design is used by power supplies with just one or no OCP circuit, while multiple-rail design is used by power supplies with more than one OCP circuit.