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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?

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  • \$\begingroup\$ As far as I know (and I did some research on that), there's no standard which specifies how should the "rails" be distributed. Also note that the "rails" only have separate current limiting circuitry and are shorted together internally. The 2x2 connecter should have separate current limit (according to ATX12V version 2.2) but right now I can't find out if the 2x4 connector has two separate current limiters or has a single one. Also note that while each rail can provide up to 18 A, not all rails can provide 18 A at the same time. \$\endgroup\$ – AndrejaKo Dec 18 '11 at 10:58
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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".

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  • \$\begingroup\$ Thanks. How do I know how do I divide the pinout? The connector has 4 ground wires on one side and 4 yellow wires on the other, can I simply mix up the ground ones and the yellow ones? \$\endgroup\$ – Francesco Dec 18 '11 at 11:20
  • \$\begingroup\$ Well the ground wires are all shorted together and the limiting is on the 12 V wires. As for the connection, I don't have an idea how you'd divide them in a precise and simple way. I think that you should be able to just short the 12 V wires and the ground wires and connect them to the device since the 12 V wires all have same voltage source and the length of the wires should be same so the current going through them should divide more or less qually. \$\endgroup\$ – AndrejaKo Dec 18 '11 at 11:25
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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.

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