The impedance of NM cables is based on the blank degree column of table 310.15 B 16.

The correct choice is based on the specific temperature ratings given for non-metallic (NM) cables in the National Electrical Code (NEC), particularly referenced in Table 310.15(B)(16). NM cables are typically rated for use in residential wiring and have insulation that can accommodate various temperature ratings.

In this context, the 60 degree Celsius column specifies the standard conductor ratings for cables, which is particularly relevant for NM cables. The 60 degree Celsius rating implies that the conductors in NM cable can safely operate at that temperature without exceeding their ampacity limits. This is critical for ensuring the safe and effective performance of electrical systems, especially under load over extended periods.

While the other temperature degree columns represent different ratings pertinent to other types of conductors, NM cables specifically align with the standards set in the 60 degree Celsius category for safe operation and compliance with the code. Therefore, referencing the 60 degree column is crucial for determining the appropriate impedance and ensuring the safety and effectiveness of electrical installations using NM cables.