Which of the following is a requirement for the installation of conductors under the NEC?

Conductors of 8 AWG and larger must be stranded when installed in raceways is a requirement that aligns with the NEC guidelines aimed at ensuring flexibility and ease of installation. Stranded conductors have multiple smaller wires twisted together, which enhances their flexibility, especially in tight spaces such as raceways. This helps prevent damage from bending and allows for easier handling during installation.

In contrast, while some conductors may indeed be insulated for safety and insulation properties, the necessity for insulation is not universally mandated for every installation scenario, especially if the conditions allow for it. The statement regarding conductors always needing to be solid doesn't account for the flexibility requirements stipulated by the NEC for larger conductors. Additionally, conductors being uninsulated in raceways goes against safety practices, as insulation is critical in preventing accidental contact and electrical faults. Hence, the requirement for stranded conductors for those larger sizes is designed to promote safer and more practical applications in electrical installations.