Not enough Amps because it is split in a Multiple rail power supply

The terminology "rail" often leads people to confusion in the wonderful world of power supplies. People get this idea that, in a multi-rail design, there are two of these "rails" from which the 12V is derived. To understand multi-rail designs, first we have to understand wires. Yes, wires are conuctors, such as copper, that go from the power supply to the hardware of the computer. These wires have insulation around them (the rubber). In a most power supplies, the wires are color-coded to indicate their purpose. 12V wires are always yellow, so you know that any yellow wire is somehow derived from a 12V source. Obviously, a rail is not a wire, because that would mean a single-rail design has a single 12V wire, which is not true, as all power supplies have multiple 12V wires. So what is a rail!

A rail is related to the protection circuitry of a power supply. Imagine that there are ten 12V wires in a power supply. Overcurrent protection is the protection circuit of the power supply that checks if any rail of the power supply is demanding too high of power at the moment, and if so, it'll shut down the power supply as a safety measure. So imagine for these ten 12V wires that there is a 12V overcurrent protection trigger point of 250W. If you take those ten wires combined, add up the current wattages (rate at which energy is transferred) of each wire, that would be how much load the 12V rail has on it. If that sum of the wattages is greater than or equal to 250W, the unit will shut off, because there was an overload on the 12V rail. It shuts off so nothing sets on fire, as the 12V transistors in the power supply can only handle so much current and energy, which is why the overcurrent protection limit should be set lower than the point of fire.

That is an example of a single-rail power supply, where every single 12V wire, every yellow wire, is linked to the same overcurrent protection. To keep this example going, let me show you the potential dangers that can still happen. If all ten 12V wires have equal wattage and make up 250W, that would be 25W each wire at that moment; no hazard at all. Now, take into consideration a scenario where one single 12V wire has 150W of load on it, and the other nine wires have 90W combined load on them. In total that is 240W, so overcurrent protection does not go off, but that 150W on that one single wire is enough to physically melt that wire, meaning bye-bye wire, and a potential fire hazard. A multi-rail design seeks to solve this potential problem by separating different wires into clusters.

In a multi-rail design, using that same example, five wires would be dedicated to their own overcurrent protection, and the other five wires would be dedicated to their own overcurrent protection. In this instance, the overcurrent trigger point would be reduced to 125W instead of 250W to take into account that only five wires are linked each instead of all ten. Now, in that same scenario, if that one 12V wire has the absurd 150W of load on it and the other nine wires have 90W combined, the unit will shut off. Why? This is because the 12V wire with 150W of load surpassed the overcurrent protection trigger point of 125W. By dividing up wires into multiple overcurrent protections, a multi-rail design is much safer than a single-rail design. Ideally, every wire should have its own single overcurrent protection, but this unfortunately is not the case, as it'd be too costly.

Some people are under the impression that multi-rail designs are bad. It depends on how it's designed. What is important is what wires go to the same protection monitor. For example, a poor multi-rail design would put the CPU cable with six PCIe video card cables, and leave the molex and SATA chains with the motherboard adapter. This would be a poor design because the CPU and video card cables have so much more potential to handle a larger amount of current, meaning that the protection trigger point may be set too low for them. A design could go well like this, though, if they set the overcurrent protection higher for the CPU and PCIe cables than the motherboard, SATA, and molex cables. It all depends on how well the configuration is. A single-rail design is a lot easier for engineers to not screw up, but a multi-rail design is safer, but is easier to screw up.

The importance of a multi-rail design is all about what wires are grouped as clusters. A single cluster of wires would be linked to its own independent overcurrent protection. In a single-rail design, all the same-voltage wires are linked to one single overcurrent protection monitor. The minor rails are never split up into multiple rails, primarily because they are so unimportant these days compared to the 12V rail. A lot of power supply units seem to be fading away from multi-rail designs, making them a thing of the past. But when there are power supplies capable of handling a higher enough current on the output wires to merit 1600W, it is scary when the potential hazards of a single-rail design are taken into thorough consideration. Some companies are advertising single-rail as a marketing point even; don't listen to this. Just understand the benefits of both, but know that multi-rail in the end is safer.

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