Throughout the NEC, conductor sizes are expressed in which two units?

Conductor sizes in the National Electrical Code (NEC) are primarily expressed in two units: American Wire Gauge (AWG) and circular mils. American Wire Gauge is a standardized system used to denote the diameter of round, solid, nonferrous electrical conductors, where a lower gauge number represents a larger conductor size. This unit is crucial for ensuring safe and effective electrical installations, as it helps determine the conductor's current-carrying capacity.

Circular mils, on the other hand, is another unit of measurement that quantifies the cross-sectional area of conductors. This measurement is particularly useful for understanding the conductor's ability to handle electrical current without overheating, as it factors in the conductor's material as well.

Although amperes measure electrical current and are essential in discussing the electrical system's capacity, they do not directly relate to the sizing of conductors themselves. Similarly, feet is a unit of length and does not apply to the sizing of conductors. Therefore, the correct response highlights the primary units used in the NEC to specify conductor sizes, focusing on their importance for safety and functionality in electrical installations.