The term “multiple rails” is commonly used when discussing PC power supplies – and thanks to vigorous marketing, consumers often go out and buy whatever match the adverts’ description of a good power supply.

The problem comes in when adverts extol the various virtues of competing power supply technologies, and confusion creeps in.

What are “Multiple” 12v rails?

Most power supplies feature one 12v source, which is then split up into multiple 12v outputs known as rails. Each of these rails has a limited output capacity to prevent cables from carrying too much current (measured in Amperes).

Power supplies with multiple 12v sources do exist, though these are relatively rare (especially in South Africa). These are often called “real” multiple rail power supplies; though that is technically incorrect. A source has its own circuitry to provide 12v power, while a rail uses circuitry to limit the amount of amperes that can be drawn from a 12v source by each rail. Power supplies with multiple 12v sources are relatively rare and have high wattage (1000w+), such as the original Enermax Galaxy 1000w.

Enermax Galaxy 1000w

Why rails are split up

A 12v source in a PSU is split into multiple rails for safety reasons. They are meant to limit the amount of current that can be drawn on specific cables, to prevent cable burn and shorting – similar to how a circuit breaker in a house works. The original guideline in the Intel ATX standard required that no more than 20 amperes can be provided over one rail in a PSU. However, Intel changed the wording of the ATX standard in 2007 from required to recommended, meaning power supplies can now sport multiple rails rated at over 20 amperes each.

Marketing myths

“Multiple rails provide cleaner, more stable voltage.”

This is not the case, as the circuitry used to split the rails has no form of conditioning or filtering stages to improve “cleaner” power. ‘More stable’ is subjective – in most cases, splitting load from various sources such as the GPU, CPU and hard drives across multiple rails will lessen the risk of supply instability thanks to limits set by the rail circuitry.

“Single rail PSU’s are more powerful.”

This is not the case. A single rail PSU simply negates the circuitry needed to split rails. The benefit of this is that balancing load across rails is not an issue, as all 12v devices plugged into the rail can draw as much power as they need (until the single rail is overloaded). That being said, single 12v rail PSU’s do not offer better voltage regulation, nor do they offer better ripple filtering unless the single rail power supply is a higher quality unit than the multi rail PSU it is being compared with.

Antec PSU rails

So which is better?

There is no clear-cut winner from a consumer viewpoint. Power supplies with multiple rails offer the benefit of an additional layer of protection when it comes to the amount of amperes drawn by a specific device – though at the cost of a slightly trickier install procedure (where the various components need to be split up correctly over multiple rails). Single rail PSUs do not need components split across multiple rails, but do lack the added protection layer found on multiple rail PSUs.

All of this remains moot for most users however, as a good quality 1000w power supply from a well know brand such as Corsair or Antec is more than capable of powering even the most high end tri SLI hexa core systems with a mild overclock. The rail debate becomes more pertinent when you start to run very high draw components such as multiple thermoelectric “peltiers”, cooling every other heat source in your PC.

If you find yourself in that position, you probably know all of the above already.