Okay, so i've been trying to work out the most feasible way to power say, 16 boards on a single 750w PSU (estimating ~35w / board, for 560w total and leaving ~20% for breathing room).
A single PCI-E 6 pin plug can handle 75w, 8 pin plugs can handle 150w. the 6+2pins can handle 150w, but its unclear as to how they are rated (gauge). If the +2 is daisy chained off the 6, then the 6 is rated for 150w. if the +2 has its own dedicated wires, then you're still limited to 75w / connector (on the 6 pin).
Safely, lets assume we can split the 75w into two 6 pin connectors, allowing 2 boards / connector. Lets use this PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E168171390068 x Peripheral
8 x SATA
4 x PCI-E (6+2)
In PCI-E alone, that nets us 4 connections, split each, 8 boards. However, if these are wired to support 150w each, then that is all 16 boards. The caveat with this is you'd need to make sure your initial splits are able to handle 150w, or it wont work. Again, lets be safe and just stick to 75w/2 connections. So 8 boards.
The best way to connect the rest?
Each molex is able to handle 11A @ 12v for 135w per chain. (3 boards effective)
Sata power is 9A @ 12v, for 108w per chain. (3 boards max, 2 boards effective)
It seems at first we can safely attach the remaining 8 boards via molex->PCI-E 6 pin adapters, however that is likely 4 molex connectors / chain, so two chains (really 6 boards). We'd need to utilize one of the sata power chains to get our last 2 boards up and running. After all is said and done, we're utilizing an estimated ~46.66A of the available 62A on the 12v rail.
That leaves a grand total of:
4x PCI-E splitters [8 boards] (
http://www.newegg.com/Product/Product.aspx?Item=N82E16812198016 - $4.31 ea.)
3x (2x)Molex->(2x)PCI-E [6 boards] (
http://www.newegg.com/Product/Product.aspx?Item=N82E16812198022 - $5.27 ea.)
2x SATAp->PCI-E [2 boards] (
http://www.amazon.com/Monoprice-8-inch-15pin-Express-Power/dp/B009GUP6O0/ref=cm_cr_pr_product_top - $2.80 ea.)
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$99.99 PSU (after MIR)
$38.65 Adapters
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$138.64 Total
Not bad for cost to power these guys. Please critique this freely, I'm going to be implementing this and I'd hate to be wrong.
I'm thinking you might be ahead to just run a a couple of beefed up lines straight off the PSU board, rather than messing around with existing wiring. You could run a pair of 12 gauge lines, which is acceptable for chassis wiring up to 41A @ 12v, giving you 984w in total between with two lines.
You're going to be overhauling a lot of connections anyway, so IMHO, you're better off eliminating everything you don't need. You could even remove the ATX header, short the appropriate power sense wiring, and run the PSU off of the main power switch on the back of the unit.
It'd be a mighty clean little setup. This is most likely the route I'll be taking. I'll only have a 10 board cluster, starting out, so I'll easily be able to get away with the 450w PSUs I already have on hand.
If your PSU doesn't have a main switch, for some inexplicable reason (or even if it does, and you just want to get extra fancy), you could put in a toggle switch across the power sense circuit. Hell, you could get REALLY bold and wire it in with a relay or opto-isolators and remotely control it for doing a power cycle, if you needed to do a reset... You could get pretty out of control with it, if you wanted.