Originally Posted by redsunx
PSU's; when one of the rails is being loaded too much it draws amps from the other rails right?
Wrong. Therefore there is no proof.
If a connector could take power from another rail once it's rail is overloaded, it would completely defeat the purpose of splitting the rails up in the first place.
The fact is, the capability of all of the connectors on any one particular rail should come no where near what that rail's OCP is set at. That is, unless you use adapters and what not to defeat the design of the PSU.
For example: If a PSU has two 6-pin PCIe connectors on it's own rail rated at 20A, that's fine. But if you go adding a couple more 6-pin PCIe power connectors using adapters to the rail that's feeding power to all of the other Molexes powering your hard drives, fans, lights, pump, what have you, you've defeated the initial design of the PSU and run the risk of overloading a rail (causing the PSU to shut down under heavy load.)
I've seen this happen with someone (maybe it was here?) with a OCZ 700W and a pair of GTX cards. The OCZ GameXstream 700W has four +12V rails, but only two PCIe connectors. Each PCIe connector is on it's own 18A rail. That's three times as much power as a 6-pin PCIe power connector is designed to deliver. Therefore, the OCZ GameXstream 700W is capable of powering virtually any machine, as long as you don't need more than two PCIe connectors. GTX cards require two PCIe connectors each, so this particular user used Molex to PCIe adapters for the second power connector on each of his GTX cards. The end result was he overloaded the rail that fed power to all of the Molexes.
If you're up against a "trapped power" argument, don't take the "shared power" argument because in reality it lacks any fact. Just give them an example of any PSU with four +12V rails. Have them add up the value of the four rails. Show them that the sum of those four rails greatly exceeds the actual +12V capability of the PSU... in some cases exceeds the total capability of the PSU... and then have them stew it over for a while.
MOST of the time a PSU only has one +12V rail. This rail is then split up into two, four, five, whatever. The Amp rating under +12V1, +12V2, etc. is not a measure of capability. It's a measure of at what point this PSU is going to shut off if this much current is going to try to go through these connectors. The number found underneath is the actual capability number.