Ok, ordinarily ATX psus tend to have 18 AWG wires, but many wires per rail (e.g. more than one molex connector, and they all run separately back to the board). In the linked example, there will be many +12V connections in the modular connectors, all connected to that big 62A rail internally.
18 AWG should be fine for up to 15A over short distances (e.g. your printer!). If you want to pull more, just connect more than one wire, and they'll share pretty happily!
For 30A in one cable, 14 AWG or below should be fine.
If this was a Rumba, there are multiple power inputs (separate one for the bed heater!) so you get to split up the load a bit better.
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi