Usually applied to cabling, plenum rating is a special flame retardant rating applied to materials, indicating that they are safe to run through a plenum. Since fresh air from ventilation or air conditioning systems flows through plenums, fire rating is important; a fire would spread very quickly if started in an area of high airflow, such as under a raised floor or through a duct.

By definition, the plenum space (where plenum rated cable is used) usually includes the space above a drop or false ceiling and below a structural ceiling or below a raised floor, depending upon where HVAC duct is located.

Given this definition, some inspectors will say that plenum rated cable is requires ANYWHERE within the above-named spaces. Other inspectors insist that installers may use PVC cable as long as they are not entering an area of moving air.

Plenum Cable is often manufactured with Teflon, while the non-plenum varieties are most often found to use PVC and other toxic components. The composition of the plenum rated cable allows it to produce less smoke than non-plenum cable when subjected to heat and fire.

The plenum rating requirement is intended to reduce the potential smoke inhalation risk and toxicity to building occupants during a fire situation.

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