What is the minimum finish rating required for walls, floors, and ceilings where electrical nonmetallic tubing is concealed in buildings over three floors?

The minimum finish rating required for walls, floors, and ceilings where electrical nonmetallic tubing is concealed in buildings over three floors is a 30-minute finish rating. This is established to enhance fire safety and limit the spread of fire and smoke in multi-story buildings. Nonmetallic tubing, depending on the specific installation and building codes, can present a fire risk if not properly contained or if concealed in areas that could allow fires to travel between floors.

The building codes set the standard to ensure that additional fire protection measures are in place, which may involve the use of materials that can withstand exposure to fire for a minimum period. While the lower ratings like 15 minutes may provide some level of protection, they are inadequate for multi-story structures where the potential for fire spread is significant.

Thus, a 30-minute finish rating would be the minimum required to ensure appropriate safety measures are in place in these scenarios, emphasizing the importance of fire-resistance standards in building construction.