Which of the following is a general requirement of NEC Article 300 for conductors?

Insulation is a fundamental requirement for conductors as outlined in NEC Article 300. Insulated conductors provide essential protection against electrical shock and short circuits. The insulation material also helps prevent damage from environmental factors such as moisture, heat, and physical abrasion that could expose conductive elements. It's crucial that conductors are insulated to ensure they operate safely within electrical systems, promoting both the safety of individuals and the integrity of the installation.

The other options do not align with the general requirements set forth in NEC Article 300. For instance, there are no mandates on the maximum length of conductors in the code, making the option regarding length incorrect. Conductors are also permitted to be above ground, depending on the application and installation requirements. Additionally, while copper is a commonly used conductor material, the NEC allows for various materials, including aluminum, thus negating the idea that conductors can only be copper.