Some more details:
PDB-PT826-8824 is the "good" model of PDB.
It has thicker gauge wires than the "825" model. It has one 8-pin EPS plug and one 2x4-pin EPS plug. What I normally do for GPU servers, is a few things:
----------
First, I take the 2x4pin connector, and connect 4 pins to one motherboard 8 pin connector, and the other 4 pins to the second 8 pin motherboard connector.
Based on what I could find online, everything I read said this would definitely be fine for CPUs <100W TDP, probably OK for 115w TDP CPUs, and probably NOT OK for 130w+ CPUs or overclocked CPUs. I've had no issues with 115w TDP Intel chips (E5-2670v1 / E5-2680v2) with this configuration.
That leaves you with an available 8-pin EPS plug coming off the PDB. This can be converted to a 2x 8-pin PCIe cable.
Supermicro part number CBL-0424L -- I've bought these from wiredzone.com or superbiiz.com. I've used quite a few of these and they work great.
superbiiz.com:
CBL-0424L Supermicro 40cm 8Pin to Dual 6+2Pin GPU Power Extension Cable - Internal Power Cable - SuperBiiz.com
wiredzone.com:
Supermicro CBL-0424L Power Cables
Looks like both are out of stock right now, but they usually have them.
I would say, expect to power no more than 8+6 off the 8+8 cable. One exception is the Vega, as I draw quite a bit less than 200 watts on those, even though it has 2x 8-pin PCIe, this works fine. 1080ti has 1x 8-pin + 1x 6-pin. That has worked fine as well. However, powering 2 1080ti's, using both 8-pins on this to power the 8-pins on each of 2x 1080ti, and then connecting up something else for the 6-pins on those 1080ti's, I've not had that work out, so don't expect it to. I don't know if I've tried powering two cards that each need an 8-pin connector -- it might work, but don't count on it.
So that gets you part of the way there. Works fine for 1-gpu servers.
----------
Second, is make use of the molex plugs.
See this cable:
https://www.amazon.com/Athena-Power-4-pin-PCI-Express-Cable-PCIE4628/dp/B0032MT0OU
It takes 2x molex plugs, and converts to 2x 8-pin pcie. A bit on that in a minute.
As to the product itself, these are usually good, but I've had a few of them break. Make sure that the molex pins are securely seated inside the molex connector, that the wires are not damaged, and that the molex plug you are attaching it to is firmly seated. Finally, these aren't "delicate", but, I've had a couple break or not give a solid connection if you are too rough with your bend radius on these. If you keep those things in mind, the product itself is good.
Ok, so you can totally go and draw 300+ watts from one of these right? Maybe -- I don't recommend it. Try to stick to the rule for the EPS converter: -- 8 + 6 pcie is your target on these (i.e. 250w draw max), or a vega 8+8 so long as you're undervolting it. Also, whether this works for your use depends upon the PDB you're using, the wattage of the PSU, and the gauge of the wires you're connecting these to.
To keep things as simple as I can, here's how you wire it up:
----
Scenario 1 -- You have an old style "825" PDB that supports a maximum of 920w power supplies:
Connect one molex socket on this cable to a molex connector on one "molex string" on your PDB, and the other molex socket on this cable to a molex connector on a different molex string on your PDB. This advice assumes you plan to connect a 1080ti or other 250w TDP card (8+6 pin card) to this cable (or equivalent power draw). If you are connecting a lower wattage card (say, a 1070 or 1080 -- maximum of a single 8-pin pcie), then you can probably get away with powering both molex sockets from connectors on the same molex string.
But wait -- this uses up both strings! How can I connect 3+ GPUs to this PDB?
Answer: You can't. Sorry. It's a terrible PDB. You'll be lucky to get away with 2x 1080ti set to 200w TDP on this PDB. One GPU should definitely work, 2 if you're lucky, and not more than that.
What power supply to use?
1x 1080ti GPU -- Real draw is likely 300 - 500 watts, depending upon drives and cpu utilization -- 740w 80+ platinum is my recommendation (2 for redundancy)
2x 1080ti GPUs -- Real draw is likely 500 - 700 watts, depending upon drives and cpu utilization. 2x 920w 80+ platinum if you have them -- it's the highest wattage you can get into this PDB. Being over-specc'ed might give you a little more fudge factor due to the lousy PDB, but this might still not work.
----
Scenario 2 -- You have a newer style "826" PDB that supports 1600w power supplies:
This is a little more flexible. You have 3 molex strings to work with. You've also got that 8 pin EPS cable you can use. I've got 2 servers with 4x 1080ti powered by this PDB (plus a single CPU), but it was a little dicey. I have no problem suggesting that 3x 1080ti + dual cpus will work fine if your power supplies are beefy enough. Here's how you do that:
So, the first molex connector on each "molex string" has a thicker gauge wire than the second molex connector. If possible, wire up at least one of the two molex sockets on this cable to the "thicker wire" (first molex connector) on one of the strings. You can wire up the second molex socket on this cable to the "second molex connector" on a string -- that's fine. Looking at the cable I linked to, you'll notice that one molex connector has one yellow wire and two black wires. And the other molex connector has two yellow wires (on the same pin) and 1 black wire. My recommendation is to use the "2 yellow wires on one pin + 1 black wire on 1 pin" connector in combination with the "first molex connector on the string". Reason being, this connector is going to draw more power per wire than the other connector, so it makes sense to connect this one to the beefier molex connector.
From what I was reading online, the specs of the molex connector is such that the thicker gauge wire + 1 molex connector supports something like 125 watts, and the thinner gauge wire + 1 molex connector supports 100 watts. (If I am remembering correctly). So wiring up one molex to the thick wiring, and the other to the thin wiring, it should be within spec at 225w usage maximum. a 250w TDP card is supposed to draw 25w from the pcie slot and 225w from the pcie cables, so this should be within spec.
Anyway, this PDB has 3 strings, 6 molex connectors. 2 strings are medium length, and one is short. For a 3 GPU setup, I use the two longer strings to connect 2x 1080ti, and use the 8-pin EPS connector (with adapter cable) to power the third 1080ti. This leaves you with 2 molex connectors you can use for drives. The backplane on a 2u supermicro needs 3 molex connectors, so if you want to use that, you'll need a molex splitter / Y adapter. I recommend connecting the splitter to the first molex connector on the short molex string. Thicker gauge splitter cables are better -- I think 16 AWG is the best you'll find -- and this will work fine. Based on my prior math of "the better molex connector is good for 125w and the less-good-one is good for 100w", this means the backplane could hypothetically draw 225w. For a 12-drive backplane using 3.5" 7200rpm sata drives, 80-120w is the likely draw fully populated, so this is no issue.
What power supply to use?
1x 1080ti GPU -- 1x 740w platinum is fine, or anything higher. 2 PSUs for redundancy.
2x 1080ti GPUs -- 1x 740w platinum is probably fine, but you might want to go with 920w or higher to be on the safe side. 2 PSUs for redundancy.
3x 1080ti GPUs -- With 12 drives, 2 cpus at full utilization, and 3x 1080ti at 100% TDP, you could be drawing over 1100w from the PSU (roughly 1200w from the wall). 1280w and 1600w Supermicro PSUs are rated for 1000w on 120v power and only rated for their "name plate capacity" when operating on 208v or 240v. If you are using 120v power, get a pair of 740w or higher 80+ platinum PSUs and you should be fine (this will not provide redundancy under full load). If you are using 208v power, you want a 1280w or 1600w PSU, or two for redundancy, under full load. Or you can again get by with 2 smaller PSUs without redundancy.
----------
Ok, so that's what I've done for most of these. The "better 826 PDB", when you can find it, is between $30 and $100. It's a bit tricky to install into a 2u supermicro 825 chassis (requires use of a hammer in my experience) -- instructions are floating around this forum. If you don't want to use it with a chassis and just want to make use of a supermicro power supply, I guess that's an option. But the PDB's are hard to find, and the cables they give you aren't amazing, both in terms of the connectors you get and how much power you can draw from them.
If anyone wants 1280w 80+ supermicro power supplies, I've got a ton of those I could sell you, or you can get them on ebay easily enough for under $100. Then each 8+8 cable is easily another $10, whether you're using the EPS -> 8+8, or you're using the 2x molex -> 8+8. Either way you slice it, it's not "cheap" compared to the HP power supplies + sketchy-looking pcie breakout boards you see crypto miners using. But if you can find the PDB, it is a step up from using desktop power supplies, in terms of wattage and 80+ rating vs price.
==
One last thing:
Another option (I --don't-- recommend) is the Supermicro PDB-PT747-4648 power distribution board. This is an upgrade for 4u supermicro chassis. And it does technically qualify as an upgrade -- the PDB that comes stock with a lot of these 4u chassis is terrible -- good luck powering more than 1x 1080ti on it. However, this PDB is not much better. It does have a large number of PCIe power cables -- no adapters needed. But it can't supply much power for some reason.
If you use 2x 1600w 80+ platinum PSUs, both on 208v power, you can probably power 3 GPUs with this. That's 3200w of power supplies for a roughly 1000w maximum load!
On 120v power, 1000w+ PSUs are derated to 1000w each, which means 2x 1280 or 2x 1600 should be able to supply 2000 watts -- good luck with that. The ~1000w draw of a 3-GPU server will cause reboots as soon as you engage the GPUs, even with 2000w of power supply capacity attached and running. On 120v power, a pair of 1280w or higher PSUs will probably be reliable to power 2 GPUs with this PDB.
==
