What is the name of the wiring rating when passing through ducts?

The correct choice is that wiring passing through ducts is referred to as plenum rated. This designation is crucial because it indicates that the wiring has been tested and deemed safe for use in air handling spaces, such as ducts, where combustion or any failure could pose a significant risk to health and safety.

Plenum rated wiring is designed to have low smoke production and to resist flames, thus making it suitable for use in locations where it may be exposed to airflow that circulates through a building's heating and cooling system. This ensures that any potential toxins or harmful substances are minimized, protecting occupants from risks associated with poor air quality.

In contrast, insulation rated, fire rated, and ventilation rated are terms that do not specifically address the safety and suitability of wiring in plenum spaces. Insulation rated could refer to the thermal resistance of materials used within electrical installations, fire rated generally pertains to materials that resist fire but not specifically for wiring in air ducts, and ventilation rated does not correlate directly with electrical wiring standards in a plenum context. Therefore, plenum rated is the appropriate term that highlights the safety regulations necessary for functioning within ducts.