Single 12V rail vs split 12v rail

[Aggravated Salmon]

My motherboard has 6 EPS 12V 8 pin connectors with maximum load exceeding ~700W

 

I have two power supplies on hand, Cooler Master Silent Pro Gold 1200W and Enermax MaxRevo 1500W.

 

The CM has a single 12V rail and the Enermax has split 12V rails.

Since the Enermax only has 30A Max on the 12V2 rail which powers the EPS power, does it mean it will overload the power supply, considering I plan to just get a 8 pin EPS -> 2 x 8 pin EPS splitter?

 

so by this knowledge, is the CM a much better choice despite the lower max rated wattage simply due to the single 12V rail?

 

or should I run two power supplies because the main modular EPS 12V wire is not rated for such high power?

[OrionFOTL]

Hi there.

 

First of all, why would you plan to buy an 8pin EPS -> 2x8pin EPS splitter, when your PSU already comes with two 8pin EPS connectors?

 

Second of all, a single 8pin EPS connector is fine to provide over 300 watts. If you aren't 100% sure your CPU comes anywhere close that number, you simply don't need to populate both 8pin EPS connectors of your motherboard.

 

40 minutes ago, Aggravated Salmon said:

Since the Enermax only has 30A Max on the 12V2 rail which powers the EPS power, does it mean it will overload the power supply, considering I plan to just get a 8 pin EPS -> 2 x 8 pin EPS splitter?

Splitting an 8pin EPS into 2x8pin EPS doesn't make any sense. If your CPU actually consumes so much power that it really needs to be powered by two EPS, then if you use a splitter it is STILL being powered by just a single EPS in the end. And flowing "two EPS worth of power" through a single EPS will overheat the connector.

 

The 12V2 rail which powers the single EPS connector is rated for 30A, which is 360W. If you split that connector into two EPS and draw more than 360W from them, then it will trigger the rail's OCP. As intended, since a single EPS connector is only good for around 300W or slightly more, no matter if you split it into 2x8pin EPS, into three, or not split at all.

 

What you are doing is exactly the thing multirail is designed to protect you from: trying to overload and/or overheat a single connector by drawing too much power or attaching too many splitters to it. You're inclined to use a non-protected power supply because it would allow you to intentionally do a critical mistake the other PSU is protected from.

 

 

Your PSU comes with two 8pin EPS cables, as said. One of them is attached to the main 24pin cable, and the other is separate. You can connect the second 8pin EPS cable to one of the 12-pin modular sockets on your PSU. I see you've read the manual of your unit since you knew which rail powers the first EPS connector, so you'll also know where to exactly connect the second EPS cable so it draws power from a rail separate from the PCIe cables' rails.

 

However, if you don't think your overclocked CPU even draws anywhere close to 200W, let alone over 300W, then just use a single 8pin EPS and don't care about the other socket.

[Bombastinator]

Random Old fashioned ugly hackery thought from the days before eps:

can You make any hay by putting together two atx 12v to make 1 eps 12v?  It’s hideous visually.  No idea which PSU has what where.  I just like electrical tape

Might not even work.  Might not be necessary if it did.  If you really want that second eps port populated it could get you off the eps rail.  Maybe I’m just making more trouble.  I dunno.

[Mezoxin]

what is the system components that you plan using with this PSU 

by Intel Specifications the 2x4 8pin cpu shouldnt handle anything more than 32A

https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf

 

so if you are running some sort of cray cpu setup that needs more than 30A  then using a splitter to power two 8 pins from a single cable is very dangerous 

[Mezoxin]

anyway for high wattage power supplies as you see the need for mulltirail is increased for safety purposes as it would prevent foul play by the end user to create a dangerous scenario that would melt wires and connectors , and you should never use extensions and adaptors with a power hungry setup that bypasses the inherently safe design of the psu , the number of connectors are made for a reason 

[Aggravated Salmon]

ok cheers guys, now i know that its the limitations of overpowering a cable and overheating rather than the rails and i understand the protection now.

to clear it up, there are 6 (SIX) 8-pin EPS12V inputs on the motherboard and the maximum load is at least 700w.

4 EPS 12V cables are for FOUR individual CPUs with a TDP of 155W each and the remaining 2 EPS 12V cable is powering the motherboard

 

so theoretically can i swap it by using 2x molex to 8pin eps cables instead (using maxrevo 1500w)?

 

so the CPU0 EPS 8 pin will be powered by the included 650mm 16pin modular to 8 pin EPS cable using the 12V2 rail

the CPU1 EPS 8 pin will be powered by the included 600mm 12pin modular to 8 pin EPS cable using the 12V3 rail

the CPU2 EPS 8 pin will be powered by two 5pin modular to molex cable with a molex to 8pin eps converter each. (12V4 rail)

the CPU3 EPS 8 pin will be powered by two 5pin modular to molex cable with a molex to 8pin eps converter each. (12V6 rail)

the motherboard0 EPS 8 pin will be powered by two 5pin modular to molex cable with a molex to 8pin eps converter each. (12V4 and 12V6 rail)

lastly, the motherboard1 EPS 8 pin will simply be connected to he included 600mm 12pin modular to 4 pin ATX cable using the 12V3 rail

 

also ill probably connect a Titan Xp to the power supply using 2 of the 500mm 12pin modular to PCI-E 6+2 pin cables (12V5 rail)

 

The max load on the 12V1 rail (24 pin) will be 10A??

The max load on the 12V2 rail will be 13A

max load on the 12V3 rail will be 13A + however much the motherboard uses (23A max?)

max load on the 12V4 rail will be 13A + half of motherboard1 usage (20A max?)

max load on the 12V5 rail will be 25A? after overclocking

max load on the 12V6 rail will be 13A + half of motherboard1 usage (20A max?)

 

the max loads of each rail will be far below the maximum 30A for each.

this should be good right?

[OrionFOTL]

Why not just say what is the motherboard and CPUs? Those are pretty basic details about your use case. Without that, we were left in the dark and had to blindly guess what would be your power usage.

 

Unfortunately, Molex to EPS converters are entirely inappropriate and are a big risk to your computer and your own safety. Molex connectors usually have terrible connection quality and absolutely aren't made to handle the kind of load of a full 8pin EPS connector, which makes them a risk of fire.

 

I'm not sure what you can do, since there are no single PSUs with six EPS connectors. You could use two PSUs for a total of four 8pin EPS connectors, and I think it would be enough. That allows for over 1200W pulled from four EPS alone. Unfortunately I don't know what motherboard, processors and hardware you have. If only we knew what parts in particular you have, then we could google for their power consumption details.

[Aggravated Salmon]

4 minutes ago, OrionFOTL said:

Why not just say what is the motherboard and CPUs? Those are pretty basic details about your use case. Without that, we were left in the dark and had to blindly guess what would be your power usage.

 

Unfortunately, Molex to EPS converters are entirely inappropriate and are a big risk to your computer and your own safety. Molex connectors usually have terrible connection quality and absolutely aren't made to handle the kind of load of a full 8pin EPS connector, which makes them a risk of fire.

 

I'm not sure what you can do, since there are no single PSUs with six EPS connectors. You could use two PSUs for a total of four 8pin EPS connectors, and I think it would be enough. That allows for over 1200W pulled from four EPS alone. Unfortunately I don't know what motherboard, processors and hardware you have. If only we knew what parts in particular you have, then we could google for their power consumption details.

4 x E7-8895 v2

Supermicro X10QBI

320GB RAM

Titan Xp

 

i am making a custom case so i can accommodate any reasonable form factor psu

[OrionFOTL]

I couldn't find how much power the motherboard takes, but the manual insists on connecting all six EPS connectors. I'm not sure what consumer power supply you could use with it, I don't think there was ever any desktop PSU with that many EPS connectors. I suspect you would have to use a dedicated server form factor PSU to use this properly.

[Spotty]

Don't use Molex to EPS12V adapters. That's a horrible idea.

 

I would recommend looking at buying additional EPS12V cables, but looking at the diagram you posted above even then with the graphics card there's not enough PCIe/EPS12V connectors available on the PSU, and I'm also not sure how easily available the modular cables compatible with those power supplies are.

 

7 minutes ago, Aggravated Salmon said:

4 x E7-8895 v2

Supermicro X10QBI

320GB RAM

Titan Xp

Corsair AX1600i? It has 10x PCIe/EPS12V connectors on the power supply that are interchangeable which would be enough for the 6x EPS12V and the PCIe cable(s) for the Titan Xp graphics card. You would need to buy additional EPS12V cables (2 included) but they're only $4 each from the Corsair website.

https://www.corsair.com/us/en/Categories/Products/Accessories-|-Parts/PC-Components/Power-Supplies/Type-4-Sleeved-black-8-pin-(4%2B4)-ATX12V-EPS12V-Cable%2C-compatible-with-all-CORSAIR-type-4-pin-out-PSU/p/CP-8920141

[Aggravated Salmon]

okay guys how about this.

if molex is so unsafe

can i use 6 pin PCI-E to 8 pin EPS connectors instead?

https://www.amazon.com/inch-6PIN-PCIE-P4-12V-Adapter/dp/B01EEOW3CA

[OrionFOTL]

Of course not, since a 6pin PCIE is rated at only 75W, which is so far below the over 300W an EPS is rated at. And that's not even mentioning the potential point of failure or a short in place where the adapters connect. 

 

Spotty posted a perfect solution - Corsair PSUs have modular sockets which can be used PCIe cables but also for EPS cables simultaneously, so if you buy additional EPS cables you'll be able to plug them in the PSU. Some other PSUs from other brands have interchangeable PCIe/EPS sockets as well. 

[Mezoxin]

30 minutes ago, Aggravated Salmon said:

okay guys how about this.

if molex is so unsafe

can i use 6 pin PCI-E to 8 pin EPS connectors instead?

https://www.amazon.com/inch-6PIN-PCIE-P4-12V-Adapter/dp/B01EEOW3CA

The 8 pin eps has 4 12v cables , while the 6pin or even the 8pin  pcie has only 3 12v cables 

 

It could work but it’s risky as you risk overloading your cables 

[Mezoxin]

7 minutes ago, OrionFOTL said:

Of course not, since a 6pin PCIE is rated at only 75W, which is so far below the over 300W an EPS is rated at. 

 

Spotty posted a perfect solution - Corsair PSUs have modular sockets which can be used PCIe cables but also for EPS cables simultaneously, so if you buy additional EPS cables you'll be able to plug them in the PSU. Some other PSUs from other brands have interchangeable PCIe/EPS sockets as well. 

The 75w is only a pci-sig requirement to prevent user error from under powering their discrete power gpu’s , but the cable itself could handle much more than that , that’s why you see 2 8 pin pcie connectors daisy chained on a single cable 

[Aggravated Salmon]

8 minutes ago, OrionFOTL said:

Of course not, since a 6pin PCIE is rated at only 75W, which is so far below the over 300W an EPS is rated at. And that's not even mentioning the potential point of failure or a short in place where the adapters connect. 

 

Spotty posted a perfect solution - Corsair PSUs have modular sockets which can be used PCIe cables but also for EPS cables simultaneously, so if you buy additional EPS cables you'll be able to plug them in the PSU. Some other PSUs from other brands have interchangeable PCIe/EPS sockets as well. 

really?

my titan x consumes >400W and it only has 1 8 pin + 1 6 pin (running stable 24/7 for a year, measured with wall wattage tester), pretty sure since PCI-E 2.0 standards its 150W for 6 pin and 300W for 6+2 pin PCI-E

[Spotty]

20 minutes ago, Aggravated Salmon said:

okay guys how about this.

if molex is so unsafe

can i use 6 pin PCI-E to 8 pin EPS connectors instead?

https://www.amazon.com/inch-6PIN-PCIE-P4-12V-Adapter/dp/B01EEOW3CA

Avoid any type of adapters. They add additional resistance both in the extra cable length and the additional connectors. They can also be of questionable quality and/or use insufficient wire gauge. There's a possibility that the cables/connector/socket will melt, especially under high loads like you are planning.

While an improvement over molex, the 6pin PCIe still has less 12V/Ground cables than the 8pin EPS12V, so you could be running more current through them than what they're designed for. Coupled with the extra resistance the adapters add it's not advisable. Plus I imagine you'd be using a pigtail PCIe connector, so you'd have 2 8pin EPS12V cables plugged in to the one PCIe cable coming from the power supply. Couple that with the diagram you posted showing the EPS12V/PCIe sharing rails, you would be having up to 4 EPS12V cables running off the one 30A rail. You'd potentially trip OCP when the CPUs take on a load, assuming it doesn't melt first.
 

Instead of looking at adapters, see if you can find EPS12V cables compatible with your power supply. That way you are running a EPS12V cable designed for that type of load directly from the power supply to the motherboard. The power supply manufacturer might sell them as replacements/accessory parts, or you can try places that make aftermarket/custom cables. It needs to be compatible with the pinout on your power supply model.

Though, with your current Enermax power supply which has 6 EPS12V/PCIe connectors on the power supply that still wouldn't be enough for each of the EPS12V cables you need as well as the 2x8pin PCIe connectors needed for the Titan XP graphics card.

The problem is that almost all consumer power supplies simply aren't designed for what you're doing. Quad socket server boards are usually run with power supplies purpose built for servers. Price wise you'd probably be better off going with the AX1600i + extra cables.

[Mezoxin]

1 minute ago, Aggravated Salmon said:

really?

my titan x consumes >400W and it only has 1 8 pin + 1 6 pin (running stable 24/7 for a year, measured with wall wattage tester), pretty sure since PCI-E 2.0 standards its 150W for 6 pin and 300W for 6+2 pin PCI-E

No it’s 75w for 6 and 150w for 8 

but you already have 75w from your gpu slot which makes it 150 and 225

[Spotty]

4 minutes ago, Aggravated Salmon said:

really?

my titan x consumes >400W and it only has 1 8 pin + 1 6 pin (running stable 24/7 for a year, measured with wall wattage tester), pretty sure since PCI-E 2.0 standards its 150W for 6 pin and 300W for 6+2 pin PCI-E

Titan Xp is 250W, not >400W.

https://www.techpowerup.com/gpu-specs/titan-xp.c2948

 

https://www.tomshardware.com/reviews/nvidia-titan-x-12gb,4700-7.html

 

 

How did you measure with a wall tester? That would measure the total system usage, after efficiency loss of the power supply, not just the GPU usage?

 

6pin PCIe is rated for 75W
8pin PCIe is rated for 150W
PCIe slot on the motherboard can deliver up to another 75W.

It's recommended not to use pigtail connectors (2 PCIe connectors on 1 cable) when exceeding 225W

[Aggravated Salmon]

1 minute ago, Spotty said:

Titan Xp is 250W, not >400W.

https://www.techpowerup.com/gpu-specs/titan-xp.c2948

 

How did you measure with a wall tester? That would measure the total system usage, after efficiency loss of the power supply, not just the GPU usage?

 

6pin PCIe is rated for 75W
8pin PCIe is rated for 150W
PCIe slot on the motherboard can deliver up to another 75W.

It's recommended not to use pigtail connectors (2 PCIe connectors on 1 cable) when exceeding 225W

i overclocked my other Titan X (not Xp), the original X and bios modded it and ramped up the core voltage for a +630 core overclock. Power consumption rose like mad.

 

 

[Aggravated Salmon]

10 minutes ago, Spotty said:

Avoid any type of adapters. They add additional resistance both in the extra cable length and the additional connectors. They can also be of questionable quality and/or use insufficient wire gauge. There's a possibility that the cables/connector/socket will melt, especially under high loads like you are planning.

While an improvement over molex, the 6pin PCIe still has less 12V/Ground cables than the 8pin EPS12V, so you could be running more current through them than what they're designed for. Coupled with the extra resistance the adapters add it's not advisable. Plus I imagine you'd be using a pigtail PCIe connector, so you'd have 2 8pin EPS12V cables plugged in to the one PCIe cable coming from the power supply. Couple that with the diagram you posted showing the EPS12V/PCIe sharing rails, you would be having up to 4 EPS12V cables running off the one 30A rail. You'd potentially trip OCP when the CPUs take on a load, assuming it doesn't melt first.
 

Instead of looking at adapters, see if you can find EPS12V cables compatible with your power supply. That way you are running a EPS12V cable designed for that type of load directly from the power supply to the motherboard. The power supply manufacturer might sell them as replacements/accessory parts, or you can try places that make aftermarket/custom cables. It needs to be compatible with the pinout on your power supply model.

Though, with your current Enermax power supply which has 6 EPS12V/PCIe connectors on the power supply that still wouldn't be enough for each of the EPS12V cables you need as well as the 2x8pin PCIe connectors needed for the Titan XP graphics card.

The problem is that almost all consumer power supplies simply aren't designed for what you're doing. Quad socket server boards are usually run with power supplies purpose built for servers. Price wise you'd probably be better off going with the AX1600i + extra cables.

so should this work: https://www.moddiy.com/products/ENERMAX-Revolution-12-Pin-to-8{47}4%2dPin-CPU{47}RAM-Modular-Sleeved-Cable.html

i can get like 2 of these

so i'll be 16pin modular to cpu0

included 12pin modular to cpu1 + mbd0 (4pin)

first purchased 12pin modular to cpu2 + mbd1 (4pin)

second purchased 12pin modular to cpu3

[Mezoxin]

17 minutes ago, Spotty said:

It's recommended not to use pigtail connectors (2 PCIe connectors on 1 cable) when exceeding 225W

you can see here it taking much more than double the 225w it went up to over 550w

but corsair has decided to set the OCP at 40A for thes 12v connections , i am almost sure that your  cables will be fine as long as you dont exceed 40A

[Spotty]

1 minute ago, Mezoxin said:

you can see here it taking much more than double the 225w it went up to over 550w

Yeah, the PCIe cable can definitely handle more than 225W. Same as how the 6pin/8pin cables themselves can do much more than the 75W/150W specifications recommend.

Some people & manufacturers just recommend not to use a single cable for high powered (>225W) cards.

For example this leaflet that came with some Seasonic power supplies:

 

I wouldn't have any concerns using a single cable for a 250W GPU. Though if you're overclocking and pushing it to 400W+ then I would definitely recommend using two separate cables from the PSU.

Using a single cable with 2 connectors on the end can be an issue in some cases with some cards like the HD7990 or R9 295X2 which have 2 GPU dies that can draw 400W+

[Mezoxin]

5 minutes ago, Spotty said:

 

For example this leaflet that came with some Seasonic power supplies:

 

these guys just dont care about cable management 

yeah ofcourse if i am runnung anywhere near 400w i would use two cables , but that would mean the graphics card manufacterer would be required  by pci-sig standards to provide  atleast 2x8pin + 6 pin connectors 

[Spotty]

1 hour ago, Aggravated Salmon said:

so should this work: https://www.moddiy.com/products/ENERMAX-Revolution-12-Pin-to-8{47}4%2dPin-CPU{47}RAM-Modular-Sleeved-Cable.html

Those cables are listed as compatible with the Enermax Revolution, which is not the same as the Enermax MaxRevo. I don't know if those cables are compatible with your power supply.

 

1 hour ago, Aggravated Salmon said:

so i'll be 16pin modular to cpu0

included 12pin modular to cpu1 + mbd0 (4pin)

first purchased 12pin modular to cpu2 + mbd1 (4pin)

second purchased 12pin modular to cpu3

I didn't even realise the 24pin motherboard cable also had an 8pin CPU connector attached to it with your power supply. That's... Odd.

 

12V2 = CPU0
12V3 = CPU1 & MB1
12V4 = CPU2
12V5 = CPU3 & MB2
12V6 = Graphics card

That might work I think? @OrionFOTL thoughts?

Far better option than using adapter cables at least. You'd need to buy an extra few EPS12V cables though.
You can possibly share the rails with the CPU & MB without worrying about tripping OCP for the rail. I have no idea how much power will be drawn through those connectors or what they're used for (Providing additional power to all the PCIe slots maybe?) maybe see if you can find more information in the manual/specs for the board. Each rail allows up to 30A, or 360W. With your estimate of 150W per CPU I think you should be fine as that still allows up to 200W for the motherboard connectors. Beyond that you're going to be exceeding what your power supply can output anyway.

 

46 minutes ago, Mezoxin said:

yeah ofcourse if i am runnung anywhere near 400w i would use two cables , but that would mean the graphics card manufacterer would be required  by pci-sig standards to provide  atleast 2x8pin + 6 pin connectors 

At stock that would probably be true for most...

The HD7990 came with 2x8pin connectors and was spec'd at "375W" stock, and while it stuck to that limit in most gaming loads it could actually draw much more than that in synthetic compute workloads. Then there's overclocking as well (not that you'd be able to keep it cool enough to overclock it).

https://www.tomshardware.com/reviews/radeon-hd-7990-devil13-7970-x2,3329-12.html

 

At least the R9 295X2 had 3x 8pin.

With overclocking you can easily exceed the recommendations for what each cable is rated for. A card that is normally 250W power draw can be overclocked with a +50% power limit set, effectively allowing it to draw up to 375W. Plus there were people who ran their cards full throttle 24/7 for mining which only made things worse.
Besides, the cable itself might be rated for that sort of load, but the connector between the cable and the modular panel on the power supply might not be. AFAIK that was typically where you would see the cable melt in those situations.

 

Those are really edge cases though. For most people it probably won't matter. Like I said if you run a 250W GPU on a single cable with pigtail connectors then it'll probably be fine 99.9% of the time. You'd be more likely to suffer failure due to other reasons, like using a crappy power supply.

Edited December 21, 2019 by Spotty

[mariushm]

You can use pci-e 6/8 pin to EPS adapter cables.  Use higher quality ones and you'll be fine

 

For future reference, the miniFit Jr. series connectors (the ones used for pci-e 6/8 pin, eps, 24pin atx) are rated for 9A of current per pair of pins.

So the EPS connector could do at least 4 pairs x 9A x 12v = 430w

For safety and other reasons, the ATX standard limits to 7A per pair, so they say one EPS connector for 4 pairs x 7a x 12v = 330 watts.

 

Same, pci-e standards limit the pci-e 6pin to 75w and pci-e 8 pin to 150w even though both have the same number of current pairs, 3.  So the connector can actually safely carry 3 pairs x 9A x 12v = 324 watts, though you shouldn't use the connectors to their maximum.

Ideally, you'd get an adapter cable that converts 2 pci-e 6pin connectors to a single EPS, by taking two pairs of wires from each pci-e 6pin, so you'd have the current spread across 4 pairs.

Or you could make your own ... buy two adapter cables and a pin extraction tool, and move one pci-e connector to the other adapter cable.

 

However, my advice would be to buy a power supply that has the same pinout for pci-e and eps headers on its case, like for example Seasonic Prime 1300w : https://www.newegg.com/seasonic-prime-gold-ssr-1300gd-1300w/p/N82E16817151208

It comes with 8 pci-e / eps connectors on the case, in which you can plug either EPS or pci-e , the psu doesn't care.

You can go on Cablemod and order EPS cables for 10$ each: https://store.cablemod.com/configurator/

 

The 1000w version has only 6 connectors, so if you can manage without pci-e connectors, you can have your 6 EPS connectors with that as well.