This twist-lock 30-amp part is probably the most common type of single-phase three-pole outlets found in most portable generators over 4,000 watt.
Theoretically, its maximum rating is 250Vx30A= 7,500 VA. In reality, the generator normally provides only 240V rather than 250, and in addition, UL and NEC® 2017 require ampacity derating to 80%. Therefore, practically you can safely get about 240x30x0.8= 5,760 VA continuously, assuming you managed to balance electrical loads on both 120V lines. The mating plug part number is L14-30P. To run your devices off such an outlet you can buy a so-called generator Y-adapter that splits the 30A output into two regular 15/20A outlets. If you want to assemble a generator cord by yourself, here is how to connect it to the plug. Hook up red and black leads to X and Y, white lead to the neutral and green one to the ground (refer to the wiring diagram above). Use a heavy-duty UL-rated cable suitable for outdoor with conductor gauges of #10 AWG. To temporarily power your entire house in case of an emergency without a transfer switch, see this page for information on using dryer outlets.