Actually the limiting factor is the amperage rating of the molex terminals ("pins") that make the physical contact. I have expertise in this area back from the

Actually the limiting factor is the amperage rating of the molex terminals ("pins") that make the physical contact. I have expertise in this area back from the days of Bitcoin GPU mining where I built custom cables that I crimped myself after buying the molex housings, molex terminals, and wire. Each individual terminal, formally called a Molex mini-fit junior terminal, is rated either 7.0A (typical for those with 2 contact springs) or 9.0A or 13.0A (for those with 4 contact springs.) As you can imagine, most cheap PSUs and wire harnesses are made of terminals with 2 contact springs, so the three 12V wires on the ATX12VO spec allow a max of 3×7.0A = 21.0A or 252 watts. As to voltage drop caused by resistance in the wire, it will be sufficiently small with 16AWG wire which will easily carry 7A over the short distances in a PC case.