Which two sizes are used to express conductor sizes in the NEC?

The correct answer highlights that American Wire Gauge (AWG) and circular mils are the two standard sizes used to express conductor sizes in the National Electrical Code (NEC). AWG is a standardized wire gauge system used primarily in the United States, where specific sizes correspond to certain gauges that reflect the wire's diameter and current-carrying capacity. Each size in the AWG system has a defined cross-sectional area, which is crucial for determining how safely a conductor can carry current without overheating.

Circular mils, on the other hand, is a unit of area used to describe the cross-sectional area of a wire, specifically in terms of the circular cross-section. One circular mil is defined as the area of a circle that is one mil (one-thousandth of an inch) in diameter. This measurement is particularly useful in electrical engineering applications as it provides a straightforward understanding of the wire's capacity to carry electrical current based on its cross-sectional area.

The other choices do not provide the correct or applicable standards used in the NEC for conductor sizes. While metric measurements or other systems like cubic centimeters are used in various contexts, they are not standard for electrical conductor sizing in the NEC.