Question regarding multi rail PSU

[jrivers010]

I often go on reddit /r/buildapc subreddit for build info. common theme i see is regarding single rail vs multi rail.

 

they elude that multi rail power draw is "risky". they will say comments like this

 

"It should be enough cuz its a single rail psu. It is only dangerous in case the rails like (2x25 amps) isn't able to handle the gpu and causes voltage peaks."

 

if a multi rail PSU has specific rails that supply the power to, lets say the 12v rail for GPU, how would it ensure it gives the full power draw required?

 

a single rail PSU on the 12v rail that says 650w, will have something like 12v rail at 52A on the 12 rail. saying max power draw on the 12v is essentially 650w. 

 

but a multi rail will distribute that among all its rail. (atleast intuitively thats what I understand it as). so it wont be able to get the full 650w.

[SavageNeo]

11 minutes ago, jrivers010 said:

so it wont be able to get the full 650w.

it will get the full 650w (depends on the specifig psu), but its splitted across multiple 12V rails. for rtx 3090 or 3090ti you want a single rail psu, so the transient spikes wont trip the OCP.

[mariushm]

Single rail ... all 12v wires coming out the 650w power supply are connected to that 52A "bucket" / "pool"  of current.  Any component could break or cause a short circuit and take all those 52 A for itself and damage the component or the power supply cables. 

 

Multiple rail .... the 12v wires are grouped into rails, and all rails are connected to the "bucket"/"pool" of current. 

On each rail there's a monitor measuring the current going to the wires in that rail.  If the monitor detects more current than the set limit flows through, the power supply shuts down 

So a multi-rail 650w power supply could provide 624w on 12v (52A)  but could have 3 rails, each with 25A limit - you don't have 3 x 25A = 75A of current capability, but any of the 3 rails could go up to 25A before the power supply's protection trips and shuts down. 

 

A power supply with multiple rails will arrange the connectors so that you don't have a single big consumer on a rail .. for example 1 rail gets assigned the CPU 8 pin connectors, one rail the 24pin and peripherals , another rail pci-e, a 4th rail another pci-e cable. 

So you could have a video card with 2  8 pin connectors be powered partially from 1 rail (the one that has the 24 pin mobo connector on it, because the pci-e slot gives up to 65w to the video card through the slot) , one rail where one cable with 2 pci-e 8 pin connectors is assigned, and a third rail where the second cable with 2 pci-e 8 pin connectors is assigned. 

 

If you use a single cable and use both 8 pin connectors on that single cable to power the video card,  the video card may briefly pull more than 250-300w through those two connectors from a single cable, which may be higher than the rail's limit of 25A (25a x 12v = 300w) and that could make the power supply shut down as it thinks there's a hardware fault. 

If you use two separate cables and one connector from each cable, and the power supply has different rails for each cable, then transients shouldn't be an issue. Even if the video card briefly pulls 400-500w from the power supply, those are split across the two cables, resulting in around 200w per rail, which should not trip the protections. 

 

[suedseefrucht]

26 minutes ago, jrivers010 said:

I often go on reddit /r/buildapc subreddit for build info. common theme i see is regarding single rail vs multi rail.

You didn't really ask a question.

 

What I think, what you would like to know:

Multi rail power supplies are more safe in a bad situation than single rail ones, because if there is a short circuit, single rail power supplies will deliver more current, which creates more damage.

If there is a 750 W power supply recommended for a graphics card, it doesn't mean, it needs the whole 750 W of power.

A RTX 3080 will "only" need 350 W most of the time under full load. A problem can occur because of peaks, when the graphics card needs more power for a very short time.

That's why you should add a little headroom to your calculation and calculate maybe 450 W for the graphics card.

 

Personally I look at, which power supply power is recommended and then I pick the power supply by the lowest noise.

 

P.S. Take a look at our 

It helps a lot.

[jrivers010]

12 minutes ago, mariushm said:

Single rail ... all 12v wires coming out the 650w power supply are connected to that 52A "bucket" / "pool"  of current.  Any component could break or cause a short circuit and take all those 52 A for itself and damage the component or the power supply cables. 

 

Multiple rail .... the 12v wires are grouped into rails, and all rails are connected to the "bucket"/"pool" of current. 

On each rail there's a monitor measuring the current going to the wires in that rail.  If the monitor detects more current than the set limit flows through, the power supply shuts down 

So a multi-rail 650w power supply could provide 624w on 12v (52A)  but could have 3 rails, each with 25A limit - you don't have 3 x 25A = 75A of current capability, but any of the 3 rails could go up to 25A before the power supply's protection trips and shuts down. 

 

16 minutes ago, SavageNeo said:

it will get the full 650w (depends on the specifig psu), but its splitted across multiple 12V rails. for rtx 3090 or 3090ti you want a single rail psu, so the transient spikes wont trip the OCP.

gotcha that makes sense. 

 

so the purpose of why on heavily overclocked CPU/GPU it recommeneds to use multi rail? for the second layer of protection? (which would also mean a naturally higher wattage PSU due to lower individual limits?)