Plenum is an air-filled space contained within a structure. Drop ceilings or below floor spaces that contain lighting fixtures, heating and air-conditioning ducts, wiring for electrical communication, alarm and security circuits are plenum spaces. This space is commonly used for return air and ventilation and can thus be a high oxygen content space during a fire. Current Electrical and Building Codes mandate that wires installed in this space must be plenum rated or inside a metal conduit. Plenum rated, FT6 cables are low smoke and fire resistant to meet these codes.