I'm not sure what the wire gauge number is on the Bitmain power supplies, someone will have to see if there is some printing on the wires and post the AWG size.
BUT, for reference, when I made the cables for the DPS-1200FB power supplies I have, I used 20ga wire.... its what I had laying around that would fit the pins I have.... Calculating the load per wire, I divided the maximum amperage the power supply could produce across the 6 Molex plugs, the amperage per pin/wire was 5.5 amp.... WELL within the specification for the wire size and insulation type using an NEC wire chart.
YES, insulation type is also a factor. You can have anything from varnish coated wires, plastic insulated, silicone insulated, or even teflon insulated. Some insulation melts at lower temps, so if you have a Molex connector that heating up because of old age or you have been plugging and unplugging a lot and you have cheap ass plastic insulated wires, more likely you will have a meltdown from insulation melting then wires shorting out than if you were running a smaller wire gauge, same amperage, and teflon insulation.
You might think silicone insulated wires will handle the heat... somewhat. Silicone insulation is used when maximum flexability is required, or is in a higher heat environment, or is carrying higher voltage (think 1000V or more). Silicone insulation is not very good when its in direct contact with a high temperature material such as a wire which has a contact that is micro arcing causing the wire to heat up massively.... it will melt with relative ease.
Typically I would recommend against 20ga wire for mining application, with 16awg a minimum for many of the miners out there, but you are spot on with your numbers for ampacity. As for insulators, the type it is isn't quite as important as the rating on it. For example, the cables that I typically order have UL 1007 or better (UL1015 is made for higher temperature, and would be a bit better for mining, though I haven't had problems with the 1007).
Point being, you could have a better type of insulator than PVC, but if the temperature rating isn't high enough you could end up in trouble. My only experience is from mining/supplying cables, so it is far from professional advice.
