What is the required size for an ungrounded feeder conductor needed for a 150-ampere rated panel board in an individual dwelling unit?

The correct choice is 1 AWG THWN-2. This size is appropriate for a 150-ampere rated panel board in an individual dwelling unit due to the National Electrical Code (NEC) requirements for feeder conductors. According to NEC Table 310.16, for underground installation, conductors must be selected based on their ampacity—essentially the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration.

For copper conductors, 1 AWG is designated to carry up to 130 amperes under the standard condition. However, considering the need for adequate ampacity, particularly when factoring in factors like temperature rating and installation conditions, the THWN-2 designation ensures that the conductor is rated for wet locations as well as high temperatures.

The other sizes do not meet the requirements for a 150-ampere load. Although smaller gauge conductors might physically fit, they would not safely handle the intended current without risking overheating or failure. Therefore, 1 AWG THWN-2 is the ideal choice, combining both sufficient current-carrying capacity and appropriate installation characteristics for the application.