Which conductor is not permitted to be connected in parallel among the options provided?

When considering conductors connected in parallel, they must adhere to specific guidelines to ensure safety and proper functionality. The National Electrical Code (NEC) provides rules regarding the paralleling of conductors, primarily concerning their size and material type.

In this context, connecting different sizes or types of conductors in parallel can lead to unequal current sharing, potentially causing overheating and failure. The practice of paralleling is generally permissible for conductors of the same size and type. However, certain conductor sizes may not be allowed to be connected due to their ability to handle the distributed load efficiently and uniformly.

Among the given options, while both 2 AWG, 3 AWG, and 4 AWG conductors could be connected in parallel in compliance with NEC requirements, the 1 AWG conductor stands out as it does not meet the necessary criteria for this practice due to its relative size compared to the others mentioned. Therefore, the specific rules about paralleling prohibit connecting the 1 AWG conductor in this scenario, as it may lead to safety issues or inadequate performance. This understanding reinforces the importance of following NEC guidelines and ensuring all conductors in parallel installations are appropriately matched.