Rail load balancing and identifying rails

[Tristor]

So the PSU I wanted was a single rail design, unfortunately it's out of stock everywhere so I ended up picking up my next choice which is the Enermax Platimax 1350. This PSU has 6 12V rails, with 4 of them being 30A rails intended for the PCI-E modular connector banks. I'm going to have two GTX 780s in SLI and will be overvolting. My understanding is that with volt modifications you can see peak load of around 440W per GPU which is 36A@12V.

What would be the best way to go about identifying which modular connector is on which rail so that I can ensure I dedicate a separate rail to each individual 8-pin PCI-E connector connected to the GPUs? The remainder of my system should be easily handled within the other 2 20A rails, so I'm not really concerned with that. I just want to ensure I have enough power to reliably feed the GPUs at peak load while overvolted and overclocked.

I'd prefer to not have to disassemble the PSU if possible, but I'm not entirely opposed to it either. I'm hoping perhaps the manual that comes with it notates the rail locations by connector, but I'm not holding my breath.

 

[theAnimal]

There are only 4 PCI-E cables, with 2 connectors on each.

 

[Deders]

Remember that up to 75w per card will be supplied by the rail that powers the motherboard via the slots themselves.

Also I doubt that the 440W figure is for each card, I expect it's the entire system load with one card overvolted.

 

[Tristor]

440W is the figure per card, at stock speeds and without overvolting the cards draw 300W per card. As to the PCI-E cables, each card requires 2 8-pin PCI-E power connectors, if there are 4 cables with 2 connectors each and 4 30A rails for PCI-E, it would stand to reason that each cable has a spot to connect which is on an independent rail, so I could run two cables and make use of only one connector on each for each card. I do not know this for a fact and would like some way to verify, however.

 

[Deders]

Even Nvidia say the max it should use at stock is 250W
http://www.geforce.co.uk/hardware/desktop-gpus/geforce-gtx-780/specifications

Where did you get the 300/440 figures from?

 

[Tristor]

http://forums.anandtech.com/showthread.php?t=2341641 -This is just an example, but if you just look around at any of the threads where people have volt modded and done power readouts off their cards (most of the folks in that thread are actually referencing threads on the various overclocking forums) you can go well over 500W peak power draw. With what I am planning, the peak people have seen is around 440W before the card throttles.

As to the TDP at stock speaks, the factory reference TDP for both the Titan and the 780 is 250W, but in the existing BIOS on the Classified cards the maximum TDP you can set is 300W before it throttles. You have to flash a BIOS that disabled GPU Boost and VDroop and increases the settable voltage limit in order to exceed this.

Per the PCI-E specs, 2x8-pin PEG connectors is 300W (150W per) + 75W from the slot for a total of 375W able to be delivered in that scenario. 300W is 25A, so a single set of connectors would be capable of delivery adequate power at maximum voltage without flashing the BIOS. But with the modified BIOS we are exceeding the PCI-E specifications as well as the specifications of the card. Obviously there is risk entailed, but when is there not when overclocking?

At any rate, I'm pretty sure each modular connector for the PEG cables is a separate rail, so I'll see if I can find out in the manual but otherwise move forward with that assumption. It would be very helpful though to know how to test for this. This is why I'm annoyed they didn't have my first pick PSU available. It was single-rail so I didn't have to worry/think about any of this.

 

[nwo]

2x 150W 8 pin PCI-e = 300W + 75W from the motherboard = 375W per card.

Since you have 4 PCI-e 12v rails available, are you able to put an 8 pin PCI-e power connector cable in each rail? Or would you have to use 2 per rail?

Ideally, you will want to use 1 PCI-e power connector per rail, if possible. But, you should also not have much trouble using one rail per card. Since if each rail is 30A x 12v = 360W and you're looking at about 300W power draw from two 8 pin PCI-e power connectors which leaves you with 60W or 20% room to spare for overvolting. Up to an additional 75W will be provided from the motherboard/PCI-e slot which is 3.3 V/3 A + 12 V/5.5 A from the motherboard.

 

[AntonioHG]

https://www.youtube.com/watch?v=ajiN9aVOv4A

 

[Blastman]

While the 12V rails are rated 20A and 30A (360W), the OCP (overcurrent protection) trip point is 40-50A (480-600W) on all the 12V rails on that Enermax 1350. So, you should be able to pull up to 480W on any individual rail without issue.