A plenum space is a part of a building that can facilitate air circulation for HVAC systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum; however, some drop-ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space.Thomas, Timothy. "What is plenum? - Definition from WhatIs.com, TechTarget, September 21, 2005. Retrieved July 25, 2017.
For highest fire safety, it is best to assume all drop-ceiling and raised-floor airspaces are plenums, whether or not they are officially designated as such, and to always use plenum cable in these spaces.
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