ATX power supply max current 12V P8 connector
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".