A plenum is the space between a dropped ceiling and the structural floor above, or between a raised floor and the structural floor below. Plenums house mechanical, electrical, and communication systems including HVAC ductwork, electrical wiring, plumbing, and data cables. Building codes require special fire-rated materials in plenums since they can allow fire and smoke to spread throughout buildings. Plenum-rated cables have fire-resistant jackets required for installation in these spaces. Building inspectors verify proper materials and installations in plenum spaces. Understanding plenum requirements helps building owners and contractors meet code requirements for commercial and institutional construction.