So what's all this rubbish about multiple 12 volt rails?

If you've paid much attention to current (2006) power supplies then you've probably noticed that most of them have more than one 12 volt rail. A conventional dual rail ATX12V power supply has two 12 volt rails: 12V1 and 12V2. According to the ATX standard, 12V2 is the 12 volt rail which powers the CPU and is provided by the 4 pin 12 volt cable. 12V1 is the 12 volt rail used in all other power supply cables and powers everything but the CPU. A few motherboards don't follow the ATX standard on what is powered by 12V1 and 12V2. EPS power supplies can have as many as four 12 volt rails and have many combinations of which rails powers which devices.

If a PSU needs more 5 volt power then they just build a higher-capacity rail which can supply more current. So why do you see power supplies with two, three, or even four 12 volt rails? Why not just have one bigger 12 volt rail which can supply more power? Well, that's going to take some explanation.

I used to design embedded electronics which are small computers which control various kinds of machines. I still build hobby projects from time to time so I have loads of power supplies laying around. Of course, most of those are "real" power supplies - not PC power supplies. Okay, technically PC power supplies are actually real but since they come with such incomplete specifications it's hard to know what they can really do. Real power supplies tell you exactly what the PSU can do: input voltage range, minimum and maximum current, load regulation, output ripple, temperature derating curves, over voltage and current limits. You name it, they spec it. And when they say 12 volts at 40 amps at 50C, they're not kidding. At least as long as you avoid crappy ones. If the PSU has multiple outputs then they explain all the dependencies between them. So if you need to increase one rail to 10 amps to get 20 amps out of another rail, they always tell you in the specs. They tell you if there's a total wattage limit among combinations of rails. If it's a really nice PSU, there are no dependencies. They just operate as independent rails. The specs are very thorough because you need to know these things to select the right PSU.

And then, there are PC power supplies. Most PC power supplies, even many good ones, would be more truthful if they stopped refering to "specifications" and used the term "marketing hokum". I'm not going to delve into that subject here because it would involves pages and pages of cussing. And if you're looking for a PC PSU which doesn't have dependencies between it's rails, keep dreaming. They've got dependencies. They just rarely tell you what they are. If you get a good PSU then it may actually meet the vague and incomplete specifications on the label. If you get a bad PSU then the wattage ratings on the label can best be described as a work of fiction. PC power supplies actually do have real specifications. They just don't publish them. So when you buy a PC PSU, it's hard to know what you've really got. As a result, parts of the rest of this page have to be based on informed guesses. It would be nice to give you definitive answers but it's hard to do that when you don't know for sure what kind of PSU you're dealing with.

In order to understand the 12 volt rail mess, you first need to know about three different kinds of power supplies. Don't just read about the kind of PSU that you think you have. There's a pretty good chance that what you think you have and what you actually have are two different things.

Single 12 volt rail PSU

A single 12 volt rail PSU just has one output circuit which generates 12 volts. All the various connectors which supply 12 volts are hooked to that one output. This kind of PSU will work just fine with a modern computer as long as it can deliver the wattage. That's true even if the motherboard requires the extra 4 pin or 8 pin 12 volt CPU connector or if your video card requires the 6 pin PCI-Express connector. As long as your single 12 volt rail PSU has all of those extra connectors and sufficient wattage then things will work properly.

Multiple independent 12 volt rails PSU

A multiple independent 12 volt rails PSU has more than one 12 volt rail. Each of the 12 volt rails has its own separate circuitry. Each of the 12 volt power connectors on the PSU cables is hooked up to one of the 12 volt rails. Since this is merely a PC PSU rather than a "real" one, the manufacturers often don't feel obligated to tell you which connector is hooked to which rail.

One reason to have multiple separate 12 volt rails is to improve the load regulation and noise on the rail. When you connect an active load to a voltage rail you tend to end up with a noisy rail which jumps around a lot. It's not a nice flat voltage. It varies. The more active loads you hook to a rail the messier it gets. So building a PSU with independent 12 volt rails improves the "cleanness" of the power on each rail. Normally this is only done if you have some circuitry which is extremely picky about the quality of its voltage rails because separate rails cost more money than a single rail.

By the way, in case you're ever tempted to hook the independent 12 volt rails together (I've seen people on the Internet who think this is a good idea), don't do it. Your 12 volt rails may have different ideas about what voltage they should set their rails to. One may be quite a bit different than another. They're separate rails, after all, and they have their own circuitry which controls the voltage. They're bound to vary a bit. And if they're just a little different then you can draw lots of current when you connect them together because each of the output circuits try to force the voltage on the same wires to a different value. That causes either a nice orderly shutdown from the over-current protection or smoke and sparks. There are some power supplies which have switches which allow you to gang the rails together. Once you've set the switch properly it's okay to connect them.

Multiple current limited 12 volt rails derived from single rail PSU

This kind of PSU only has one set of circuitry inside the PSU which generates 12 volts. But it is split into separate 12 volt outputs each of which have their own current limit circuitry. If any one of the 12 volt outputs exceeds its current limit then the PSU shuts down. For example you could have a dual rail supply which has a single internal 12 volt rail which can deliver 30 amps. Then inside the PSU it's split into two separate rails each of which has a 20 amp limit. If you try to draw more than 20 amps from either of the 12 volt rails then the PSU with shut down. If you try to draw more than 30 amps of total current from both of the rails then it will also shut down (assuming that the internal 12 volt rail also has a current limiter).

This kind of PSU exists because of safety standards. The IED 60950 standard limits wiring to 240 VA (volt amps). At 12 volts that means that a wire is only allowed to carry a maximum of 20 amps. The standard exists to try to limit the amount of current which flows in a short circuit before the PSU shuts down. That can reduce the likelihood that a short will cause a fire or destroy something. So if your PSU needs to deliver more than 20 amps at 12 volts and obey the safety standard then it needs to have more than one 12 volt rail.

So what kind of PSU is it really?

You'd think the answer to that question would be simple. The names of the three kinds of power supplies are a bit long so let's shorten them to single 12, independent 12s, and current limited 12s. If your PSU specifications only claim that you have a single 12 volt rail then you know which of the three you've got. But if the specs claim that you have two or more 12 volt rails then things get more complicated.

If you look at the official ATX12V PSU design guide then you'll find language which states that no rail can deliver more than 240 VA. That means that a 12 volt rail is limited to 20 amps. It never says that the PSU must have independent 12 volt rails. Independent 12 volt rails would be legal as long as they're limited to 20 amps but they are not required. That's important because the independent 12s is the most expensive kind of PSU to build. A cheaper way to meet the ATX12V spec is to produce current limited 12s. It saves money by basing the separate rail outputs on a single internal 12 volt rail. And when it comes to PC components they try very hard to keep the costs to a minimum. As a result it's very unlikely that your multi 12 volt rail PSU is actually an independent 12s. The independent 12s design is the one with the cleanest 12 volt rails but PCs seem to work fine without them. Most of the loads on the 12 volt rails are either motors or DC/DC converters and neither of them are all that picky about the quality of their input voltages.

Some people who do power supply testing have reported consistent success at connecting the separate 12 volt rails together. As I mentioned earlier, it's highly likely that doing that with an independent 12s PSU will cause a short and shut the power supply down. But connecting the rails with a current limited 12s PSU will work just fine since there's really only one 12 volt regulator. The fact that there's a high success rate connecting the 12 volt rails strongly suggests that they're actually current limited 12s rather than independent 12s. Moreover, the PSU reviews at XbitLabs actually open up the PSUs to have a look at the internal designs. Virtually all of the PC PSUs that I've ever seen reviewed there have been single main transformer designs which means that they do not have independent 12 volt rails. In fact, I've seen a grand total of one power supply which actually had independent 12 volt rails. That power supply appears to really be a server power supply design which has been adapted for use in PCs. There may be other independent 12s PSUs out there but if they do exist, they're extremely uncommon. And given the price conscious nature of the PC market, you'll probably never run into one.

So now you may be assuming that your multi 12 volt rail PSU is a current limited 12s. If only things were that simple. Intel keeps a web page with a list of power supplies which meet their minimum requirements. Included on that list are a large number of power supplies described as "** Power supply did not meet 240 VA requirement during OCP test." OCP stands for over current protection. Intel considers these PSUs to have met their minimum requirements but they do not meet the 20 amp current limit on each 12 volt rail. Intel seems to be pretty lax about enforcing the 240 VA limit. If you check the manufacturer's specs on some of those PSUs you'll find that their claimed maximum currents on their 12 volt rails are well below 20 amps despite the fact that they delivered at least 20. So you can't trust the maximum current ratings on their 12 volt rails. Some can deliver more than their specs claim without tripping the overcurrent protection.

Current limited 12s are more expensive to produce than single 12s which provide the same total amount of 12 volt wattage. On top of that, many power supply folks are of the opinion that limiting the current on a rail to 240 VA hasn't resulted in any significant real-world improvement in PSU safety. Then you also have to take into account the load balancing complications caused by having current limited rails. All this adds up to the suspicion that many power supplies which claim to have multiple 12 volt rails are actually single 12 PSUs despite how they're marketed. It's clearly a fact according to Intel's testing that many PSUs can deliver far more current on a single 12 volt rail than their specifications claim and even more than the 20 amp limit. It's understandable that PSU makers would continue to market them as multi 12 volt rail PSUs since many people think that multi 12 volt rail PSUs are superior to single 12 volt rail PSUs.

People who do thorough tests on power supplies have written quite a bit about this subject. You can read their opinions on what kind of 12 volt rails are inside your power supply here, here, here, here, and at the bottom of this page. The kind of 12 volt rails which are in your power supply can have quite an affect on how it operates in high-powered computers so it's unfortunate that this subject is so obscure. The information is out there but it's not easy to find. It should be easy to know which kind of 12 volt rails you have but that's not going to happen until PSU makers start putting out real specifications.

So what kind of PSU is the best?

When building a high-powered machine with lots of hardware, people are often told that they should get a multi 12 volt rail PSU. The standard reasoning is that multi 12 rail PSUs provide more power at 12 volts than single 12 volt rail PSUs. But that's not very good advice. What they're trying to tell you is that newer computers put a large load on the 12 volt rail and that you should be sure to get a PSU which provides enough current at 12 volts. As you can see on this page, the biggest load on a PSU has shifted over time from 5 volts to 12 volts so you do need to be careful to get the right kind of PSU. But you don't need to get a multi 12 rail PSU in order to get lots of capacity at 12 volts. As you've seen above, many PSUs which claim to be multi 12 volt rail PSUs are actually single 12 rail PSUs. They're just marketed as multi 12s because people think that multi 12s are better. The real issue is whether the power supply provides enough total current at 12 volts (as well as the other rails) and not whether it has multiple 12 volt rails.

Remember that independent 12s power supplies are virtually impossible to find. So you only have two choices: a power supply with one internal 12 volt rail with current limiters for each external rail (a current limited 12s), or a power supply with one internal 12 volt rail without current limiters (a single 12). You're going to end up with a power supply with only one internal 12 volt rail. Your only real choice is whether to get current limited 12 volt rails. The bad news is that the marketing specifications for supposedly multi 12 volt rail power supplies don't tell you whether the current limiters are real or not.

The fact is that if you're building a high-end computer, power supplies with no current limiters are easier to deal with than power supplies with current limiters. Suppose that you are building a computer which when fully loaded has a CPU which draws 9 amps at 12 volts and two video cards which draw 10 amps apiece at 12. Those are among the most high-powered components in use as of 2006 but people definitely build such machines. Plus you also have some hard disks and other stuff which adds up to 4 more amps at 12 volts. If you have a single 12 rail PSU then you have to make sure that it can handle the 12 volt total of 33 amps. But if you have a dual 12 rail PSU which has 20 amp limits on each 12 volt rail, then you also have to make sure that you don't exceed 20 amps on each rail. If you exceed 20 amps on a rail then the PSU will shut down even if the PSU supports more than 33 amps total. You can see the complications of dealing with the "rail balancing" problem on this page. If you're building a non high-powered computer then it's unlikely that you will draw close to a total of 20 amps at 12 volts. In that case you don't really have to worry about the limits on individual rails. It's only high-powered computers which draw lots of 12 volt current that get into trouble.

Assuming that two PSUs have the same total 12 volt capacity, you're better off getting a single 12 volt rail PSU than a multi 12 rail PSU. The current limiters in the multi 12 rail PSUs apparently don't really improve safety but they can make your life miserable when building a high-powered computer. The single 12 volt rail PSUs cause less problems. Unfortunately, most PSUs with lots of 12 volt capacity are marketed as multi 12 rail PSUs even when they are actually single 12 rail PSUs. Intel's page can help identify PSUs which don't have the 240VA limit. Hopefully, in the future this whole daliance with 20 amp current limits will just disappear and 12 volt life will be simple again. But as for now, your best option is to try to find a PSU without current limiters if you intend to build a high-powered computer. If you can't avoid current limiters then be prepared to do some rail balancing.