This is to clear up the enormous confusion I've noticed in the last few months about dual 12V line current specs on PSUs.SPECS
Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. Why? To abide by safety requirements of UL and EM 60950, which stipulates not more than 240VA on any wires or exposed traces. Intel's PSU Guide calls for a current limiter that keeps current to under 20A on each of the 12V rails: 12V x 20A = 240VA.
What is the safety reason for this 240VA maximum? It's the maximum recommended for an electronic device that a consumer will have reasonable likelihood of access. In plain terms, it might be to keep people from zapping themselves inside a PC, or more likely, accidentally creating a fire risk. This safety "rule" does not apply to any electronic or electrical devices where the chance of consumer exposure is low, such as a TV or CRT monitor, for example.
It's important to remember that even though there are two "independent" 12V lines, they still draw from the same main source. It's highly unlikely that there are two separate 120VAC:12VDC power conversion devices in a PSU; this would be much too costly and inefficient. There is only one 12VDC source, and the two lines draw from the same transformer. Each line is coming from the same 12VDC source, but through its own "controlled gateway".
PSU makers' specs are misleading in that thay rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line assuming the total 12V current capacity of the PSU is not exceed.
What the above means is that you don't need to worry about imbalances in power draw on the 12V lines —as long as no single rail is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis, probably more for marketing reasons than any other. A PSU rated for 32A max on the 12V lines can be labelled many different ways:
12V1: 18A, 12V2: 14A
12V1: 17A, 12V2: 15A
12V1: 16A, 12V2: 16A
12V1: 15A, 12V2: 17A
12V1: 14A, 12V2: 18A
It could be marked 20A + 12A, but being a cautious bunch, the engineers will probably not specify more than 18A on any one line. This gives 2A headroom to allow some room for error for the current limiting circuit.REALITIES
Note that 12V2 is supposed to supply only the AUX12V (2x12V) 4-pin plug, which feeds only the CPU. With PSUs that adhere strictly to the ATX 12V v2.xx Guide, 12V1 supplies 12V to all the other components that require it. This might lead to a problem with very high power gaming systems that utilize two high power video cards. Current high end VGA cards by themselves can draw >90VA each. Much of this comes from the 12V line via the 6-pin PCIe connector for the VGA card. If you add several hard drives and optical drives, the 240VA limit may be too low.
The current ATX12V v2.2 spec was created before dual VGA card gaming configurations for Intel boards were announced. SLI, being an AMD feature that came many months earlier, may have been ignored by Intel's PSU design guide team.
Not all PSUs with 6-pin PCIe connectors follow ATX12V v2.xx to the letter, as the guide does not cover the 6-pin 12V PCIe outputs. This connector and its current delivery capacity was specified by nVidia, the originator of the SLI concept. nVidia maintains a list of power supplies that they have certified as being suitable for SLI systems.
The question is, Where should this 12V come from?
I interviewed a number of engineers from several power supply manufacturers to pose this very question. The answers were surprising. All of the engineers I spoke with wished to remain anonymous. This is a summary of what they told me:
**Some PSU makers are using 12V2 to supply more than just the 2x12V or 4x12V connectors. It is often used to power the 6-pin 12V PCIe outputs.
**Many PSUs marked as having dual 12V lines actually have only a single 12V line — they do not feature two <240VA power limiters specified by ATX12V v2.xx; they have only one Over Current Protection (current limiter) for the single 12V line.
**The 240VA current limit is considered a high cost, useless annoyance by most PSU makers. If multiple 12V lines are used, because the vast majority of components now use mostly 12V, the 18~20A limit for any line means that the precise power distribution to the various 12V output connectors can become critically important in some cases.
**The engineers point to the many high power pre-V2.xx ATX12V PSUs that had as much as 30A on a single 12V line. As a product class, those have not proven to be any more dangerous in any way than other ATX12V PSUs.
What's really interesting is that Intel has tacitly waived the 240VA limit requirement in its PSU validation program
for the better part of a year.
Intel maintains a web page listing all the ATX12V they have tested that "meet MINIMUM electrical, mechanical fit and functional compatibility" with Intel desktop boards and processors: http://www.intel.com/cd/channel/reselle ... /35815.htm
For the 32 ATX12V v2.2 PSUs tested in 2005 that are on this list, 17 models are identified as having at least one output line that exceeds 240VA. And yet, these 17 models are on Intel's approved list. According to the engineers I spoke with, the majority of these 17 models have just one 12V line. They also point out that there are another 20 or so ATX12V v2.0 PSU models on the Intel list, and none of them were tested for the 240VA current limit conformance. My sources say that if these models had been tested, more than half would not conform to the 240VA current limit because they have only one 12V line.
Now, my sources say, in the last couple of months, Intel has notified the PSU makers verbally that the 240VA current limit has officially been removed. This means a single 12V line is the accepted norm, never mind ATX12V v2.xx.What does all this mean?
Essentially, the only potential benefit of dual 12V lines is improved safety, and this is disputed by the engineers I spoke with. There are many downsides to dual 12V lines, including higher cost and the extra worry of ensuring adequate 12V current for all the components in complex, high power systems. For the consumer who is trying to make a choice among the myriad of PSUs available on the retail market today, the most practical approach regarding dual 12V lines and power capacity is to consider only the combined 12V current capacity. EDITED Sept 17/05 with much more detailed info.