What is "multiple +12V rails", really?
In most cases, multiple +12V rails are actually just a single +12V
source just split up into multiple +12V outputs each with a limited
There are a few units that actually have two +12V sources, but these
are typically very high output power supplies. And in most cases these
multiple +12V outputs are split up again to form a total of four, five
or six +12V rails for even better safety. To be clear: These REAL
multiple +12V rail units are very rare and are all 1000W+ units
(Enermax Galaxy, Topower/Tagan "Dual Engine", Thermaltake Tough Power
1000W & 1200W, for example.)
In some cases, the two +12V rail outputs are actually combined to
create one large +12V output (Ultra X3 1000W, PC Power & Cooling Turbo
Cool 1000W, for example.)
So why do they do they split up +12V rails??
Safety. It's done for the same reason that there's more than one
circuit breaker in your house's distribution panel. The goal is to
limit the current through each wire to what that wire can carry
without getting dangerously hot.
Short circuit protection only works if there's minimal to no
resistance in the short (like two wires touching or a hot lead
touching a ground like the chassis wall, etc.) If the short occurs on
a PCB, in a motor, etc. the resistance in this circuit will typically
NOT trip short circuit protection. What does happen is the short
essentially creates a load. Without an OCP the load just increases and
increases until the wire heats up and the insulation melts off and
there's a molten pile of flaming plastic at the bottom of the chassis.
This is why rails are split up and "capped off" in most power
supplies; there is a safety concern.
Is it true that some PSU's that claim to be multiple +12V rails don't have the +12V rail split at all?
Yes, this is true. But it's the exception and not the norm. It's
typically seen in Seasonic built units (like the Corsair HX and Antec
True Power Trio.) It's actually cheaper to make a single +12V rail PSU
because you forego all of the components used in splitting up and
limiting each rail and this may be one reason some OEM's will not
split the rails, but say they are split. Some system builders adhere
very closely to ATX12V specification for liability reasons, so a
company that wants to get that business but also save money and reduce
R&D costs will often "fib" and say the PSU has it's +12V split when it
Why don't those PSU companies get in trouble? Because Intel actually
lifted the split +12V rail requirement from spec, but they didn't
actually "announce" it. They just changed the verbiage from "required"
to "recommended" leaving system builders a bit confused as to what the
specification really is.
So does splitting the +12V rails provide "cleaner and more stable voltages" like I've been told in the past?
It is true that marketing folks have told us that multiple +12V rails
provides "cleaner and more stable voltages", but this is usually a
falsehood. Quite frankly, they use this explaination because "offers
stability and cleaner power" sounds much more palletable than "won't
necessarily catch fire". Like I said before, typically there is only
one +12V source and there is typically no additional filtering stage
added when the rails are split off that makes the rails any more
stable or cleaner than if they weren't split at all.
Why do some people FUD that single is better?
Because there are a few examples of companies that have produced power
supplies with four +12V rails, something that in theory should provide
MORE than ample power to a high end gaming rig, and screwed up. These
PSU companies followed EPS12V specifications, which is for servers,
not "gamers". they put ALL of the PCIe connectors on one of the +12V
rails instead of a separate +12V rail. The +12V rail was easily
overloaded and caused the PSU to shut down. Instead of correcting the
problem, they just did away with the splitting of +12V rails
altogether. Multiple +12V rail "enthusiast" PSU's today have a +12V
rail just for PCIe connectors or may even split four or six PCIe
connectors up across two different +12V rails. The rails themselves
are capable of far more power output than any PCIe graphics card would
ever need. In fact, Nvidia SLI certification these days REQUIRE that
the PCIe connectors be on their own +12V rail to avoid any problems
from running high end graphics cards on split +12V rail PSU's. There's
less components and less engineering to make a PSU that DOES NOT have
the +12V rail split up, so it's cheaper to manufacturer (about $1.50
less on the BOM, $2 to $3 at retail) and typically this cost savings
is NOT handed down to the consumer, so it actually behooves marketing
to convince you that you only need single +12V rails.
But some people claim they can overclock better, etc. with a single +12V rail PSU
B.S. It's a placebo effect. The reality is that their previous PSU was
defective or just wasn't as good as their current unit. If the old PSU
was a cheap-o unit with four +12V rails and the new one is a PCP&C
with one +12V rail, the new one isn't overclocking better because it's
a single +12V rail unit. It's overclocking better because the old PSU
was crap. It's only coincidental if the old PSU had multiple +12V
rails and the current one has just one.
The only "problem" the occurs with multiple +12V rails is that when a
+12V rail is overloaded (for example: more than 20A is being demanded
from a rail set to only deliver up to 20A), the PSU shuts down. Since
there are no "limits" on single +12V rail PSU's, you can not overload
the rails and cause them to shut down..... unless you're using a
"too-small" PSU in the first place. Single +12V rails do not have
better voltage regulation, do not have better ripple filtering, etc.
unless the PSU is better to begin with.
So there are no disadvantages to using a PSU with multiple +12V rails?
No! I wouldn't say that at all. To illustrate potential problems, I'll
use these two examples:
An FSP Epsilon 700W has ample power for any SLI rig out there, right?
But the unit only comes with two PCIe connectors. The two PCIe
connectors on the unit are each on their own +12V rail. Each of these
rails provides up to 18A which is almost three times more than what a
6-pin PCIe power connector is designed to deliver! What if I want to
run a pair of GTX cards? It would have been ideal if they could put
two PCIe connectors on each of those rails instead of just one, but
instead those with GTX SLI are forced to use Molex to PCIe adapters.
Here comes the problem: When you use the Molex to PCIe adapters, you
have now added the load from graphics cards onto the rail that's also
supplying power to all of your hard drives, optical drives, fans,
CCFL's, water pump.. you name it. Suddenly, during a game, the PC
shuts down completely.
Solution: To my knowledge, there aren't one-to-two PCIe adapters.
Ideally, you'd want to open that PSU up and solder down another pair
of PCIe connectors to the rails the existing PCIe connectors are on,
but alas... that is not practical. So even if your PSU has MORE than
ample power for your next graphics cards upgrade, if it doesn't come
with all of the appropriate connectors, it's time to buy another power
(edited out Example two)
The bottom line is, for 99% of the folks out there single vs.
multiple +12V rails is a NON ISSUE. (Editor: Bold emphasis is mine)
It's something that has been hyped up by marketing folks on BOTH SIDES
of the fence. Too often we see mis-prioritized requests for PSU
advice: Asking "what single +12V rail PSU should I get" when the
person isn't even running SLI! Unless you're running a plethora of
Peltiers in your machine, it should be a non-issue assuming that the
PSU has all of the connectors your machine requires and there are no
need for "splitters" (see Example 1 in the previous bullet point).