I have a non-modular 600W PC PSU which has one 8-wire (3 yellow, 5 black) cable split and connected to two 6+2 connectors (see the image below). I've read that according to the ATX standard, an 8 pin is restricted to 150W. So what maximum power can the two 6+2 connectors transfer together within the standard? 150W or 300W?
The only source for more or less authoritative information is your PSU specs. As far as I can tell, it doesn't specify it, so fhe usual assumption is that if it fits, plug it.
On the other hand, these 550W on the 12V rail should be somehow distributed between 12V outputs and there is no much of them.
A reasonable design would back the split cable with tick enough wires to compensate for the double load. 300W at 12V are ~25A distributed between the 3 yellow wires, so ~8.3A per wire. Not much of an issue if the wire is at least 18AWG or 0.85mm2.
The wire is probably labeled with its gauge, temperature range, manufacturer name and other useful information, so you can see yourself.
Single 6-pin connector allows 75W power delivery, and single 8-pin connector allows 150W power delivery.
Since you need to power two 150W connectors it can draw up to 300W.
If you have split a single 8-pin 150W connector into two 8-pin connectors, it means that there can be double the expected load, so there can be double the expected current in the wiring and voltage at the connectors can be less than expected. Wiring meant to power a single 150W load may not be enough to power a 300W load.
Since the wiring which comes out of the power supply is factory-made with two 8-pin connectors that are 6-pin compatible, there is no doubt that it can handle two 150W loads or 300W in total.
People sometimes overlook that the GPU has an additional power supply - up to 75W - from the PCIe slot. So, a card with 2 x 8 pin sockets can draw 2 x 150W directly from PSU, plus 75W via PCIe - total 375W.
