Utilizing 12v PCIe Cables:
Requirements: 3 PCIe Y splitters and 12 barrel plugs per PCIe Cable
PSU -> PCIe cable -> PCIe Y Splitter -> Two PCIe Y Splitters -> Produces 12 hot and 12 ground wires -> 12 barrel plugs per PCIe cable
3 -> 6 -> 12
Technically, the 6 pin pci-e connector is only "rated" via PSU specifications for 75 watts. A 6+2 or 8 pin is rated for 150 watts. So, in theory, the PSU manufacturer could use higher gauge (lower amp) wire in a 6 pin only configuration. Also, if it is a multi-rail PSU, the 6-pin rail may not have enough amps. A PSU I have explicitly has 6 pin and 6+2 pin connectors. If you look at the amperage available on each rail, it explains why.
No... a PSU manufacturer could not use lower gauge wire for the 6 pin PCIe connector. Not legally anyways... 18 AWG is spec
That is correct a PCIe 6 pin connector is only rated for 75 watts, but if you breakdown what that 6 pin connector actually contains you will find it can handle more than 75 watts. A 8 pin PCIe connector doesn't contain any extra 12v lines and is rated at 150 watts. The 2 extra wires are black grounds for reduced resistance.
So... a PCIe 6 pin contains 3 +12v wires and 3 black ground wires. Spec for these wires is #18 AWG wire, so all PSUs should come with at least that. The original 6 pin 75 watt specification was for a minimum current of 2.08 amps for each +12 volt line, hence 75 watts.
3 +12 volt lines x 2.08 amps each = 75 watts.
However, The 6 pin PCI-e power connector is actually capable of handling more than 75 watts. You will see here an 18 AWG wire can run up to 110 watts at 12v depending on the length of the wire (distances in feet):
http://www.securitypower.com/AN2Wire.htmlUsing a safe distance of your 12v wires being shorter than 23 feet from the PSU to the Gridseeds (a very safe assumption IMO):
3 +12 volt wires x 4.16 amps each = 150 watts maximum wattage.
In summary, although a 6 pin PCIe connector is only specified for 75 watts in ATX PSUland specifications, by design it could safely handle at least double this amount... 150 Watts at 12 v. This is why Cablez chose to use 15 Gridseeds per PCIe cable max in Scrypt mode. From the man himself:
How many units can 1 pci-e cable power safely for scrypt mode? Also how many can you power from molex?
If you are looking to be conservative I would say 60w per wire pair within the PCIe connector. At roughly 10w per device that is 6 units per wire pair and 18 units total, conservatively. It is possible to go up to 80w per wire pair but that just stresses the housings, pins and wires and will yield lower MTBF rates.
Looking at a molex 'chain' there is only a single 12v wire pair to be utilized there but it is shared. This is going to limit you to 60w on the entire 'chain' (usually 3 molex plugs and a floppy).
Hope that helps.
