I'm using a ATX power supply as a unit for my project. The power supply is of the type "be-quiet" straight-power E8 480W with cable management. I cannot figure out how much current can I get from the 12V P8 connector, which has 4 12V poles and 4 ground poles. My need is 20A in total for the 4 couple. I read that this kind of ATX power supply has 4 12V rails, each one can give up to 18A. How are these rails physically placed and distributed among the different connectors?
According to the EPS12V standard section 6.1.2 Processor Power Connector, the requirement is that the connector uses two different power rails (12 V1 for pins 5 and 6 and 12 V2 for pins 7 and 8) if the 240 VA limiting is in place. So if nothing else is connected to those two power rails, you should b able to pull 20 A through the connector if it's divided correctly among the two "rails".
Going massively on a lark here, but I think you can start by measuring the resistance between different +12V terminals (obviously when the PSU is off and discharged).
If there are pairs that have significantly larger resistance than the rest, it usually means they are powered through different rails. At least you should see separate rails on P8 and on the main ATX(20+pin) connector.