Which conductor size is prohibited for underground service conductors?

Underground service conductors must meet specific requirements laid out in the National Electrical Code (NEC) to ensure safety and sufficient capacity to handle electrical loads. The answer indicating that 14 AWG copper conductors are prohibited for underground service stems from their inability to carry sufficient current safely and comply with NEC standards.

The NEC specifies minimum size requirements based on the conductor material (copper or aluminum), installation conditions, and the expected load. For underground installations, it is crucial to use conductors that can reliably handle the potential loads without overheating or creating a safety hazard. In general, smaller wire sizes like 14 AWG do not have adequate ampacity for service conductors, which can lead to potential overheating or failure.

In contrast, the other conductor sizes listed (10 AWG copper, 12 AWG aluminum, and 6 AWG aluminum) are permissible options under NEC guidelines for various applications, including underground use, as they provide a higher ampacity suitable for the demands typically placed on service conductors. Thus, the correct answer highlights the importance of adhering to the minimum conductor size requirements to maintain safety and performance in electrical systems.