What is the primary difference between Riser (CMR) and Plenum (CMP) cabling?

• A. Plenum is vertical between floors; Riser is for air-handling spaces
• B. Riser is for air-handling spaces; Plenum is vertical between floors
• C. Riser is rated for vertical runs between floors; Plenum is rated for air-handling spaces with higher flame/smoke requirements
• D. Riser is for outdoor use; Plenum is for indoor

Answer: (C)

The key idea is that these two cable ratings reflect different fire-safety requirements based on where the cable is installed. Riser (CMR) cables are designed for vertical runs between floors, where a fire could climb a shaft, so they’re rated to limit flame propagation in that scenario. Plenum (CMP) cables are required for air-handling spaces (plenum spaces with airflow in ceilings or ducts) and must meet stricter flame and smoke standards because smoke movement is a major concern in those areas. Therefore, the main difference is the installation location and the corresponding higher flame/smoke performance needed for plenum spaces.