In a 30-foot run of conduit containing one conductor, the maximum cross-sectional area that can be occupied is what percentage?

The correct choice is based on the National Electrical Code (NEC) guidelines for conduit fill. For a single conductor in a conduit, the NEC allows a maximum fill percentage of 53%. This percentage is derived from the considerations of ensuring that conductors have adequate space for heat dissipation and to avoid damage during installation.

The 53% fill for one conductor in a conduit is a guideline meant to balance the need for effective conductor arrangement while ensuring safety and longevity of the electrical system. This is crucial for maintaining the integrity of electrical systems, especially in longer runs like the 30-foot scenario described in the question.

Other fill percentages apply to different configurations, such as grouping multiple conductors together, but in this case, for a single conductor, the 53% is specifically mandated to promote safe practices in electrical installations.