What characteristic defines luminaires used as raceways?

Luminaires used as raceways are defined by their need to be specifically labeled for such use. This labeling is crucial because it ensures that the luminaire meets the necessary safety and performance standards as outlined by the National Electrical Code (NEC). The luminaire must be designed and tested to function not only as a light source but also as a conduit for electrical conductors, allowing for safe wiring connections.

Being labeled for use as a raceway informs installers and inspectors that the luminaire can safely house wiring and that it complies with all relevant requirements for both lighting and electrical raceway functions. This characteristic helps maintain consistent safety and performance standards in installations, which is pivotal for preventing electrical hazards.

While the other characteristics mentioned might have their importance in specific contexts—such as energy efficiency being a goal for modern lighting solutions, waterproofing being necessary for certain outdoor or high-moisture applications, and glass being a common material for some types of luminaires—they do not inherently define a luminaire's capability or designation as a raceway. The clear requirement for labeling ensures that users can ascertain the intended use and safety compliance of the luminaire.