For Class 2 and Class 3 circuits, what type of cable must be installed in open spaces above ceilings or below floors?

Plenum-rated cable is specifically designed for installation in spaces that are used for air circulation, such as above ceilings or below floors. These areas often serve as pathways for heating, ventilation, and air conditioning (HVAC) systems. Plenum-rated cable is constructed with materials that have a low flame spread and low smoke-producing characteristics, which are critical for maintaining safety in case of a fire. The use of this type of cable minimizes the risk of fire spreading through air ducts, as the cables can potentially ignite and release toxic smoke.

In contrast, non-plenum rated cables do not possess these fire-resistant characteristics and are unsuitable for installation in such critical areas. Standard-rated and low-voltage-rated cables also do not meet the specific safety requirements necessary for plenum spaces. Therefore, the requirement to use plenum-rated cable in these scenarios is in line with safety codes and standards that emphasize the importance of fire safety in building design.