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Riser vs Plenum Cabling
Posted on Monday, March 22, 2021
Riser vs Plenum Cabling

March 22, 2021 - Cleerline has just announced additional availability of Duplex and Micro Distribution cables with a riser rated jacket.  What makes these cables different from plenum cabling, and what do you need for your installation?

The following article is for informational purposes only.  Please refer to local or national fire codes and the NFPA National Electric Code when making a final cable type determination. 

Fire Safety Standards

"Riser" and "plenum" refer to both cable ratings and locations. In either case, these terms relate to fire safety requirements. 

Plenum spaces allow air to move through a building.  Plenums are part of an HVAC system, often including open spaces above ceilings or under floors.  Because plenums are used for air return and circulation, they have strict fire safety requirements.  The jackets of plenum cables are designed and specially treated to be flame retardant, to self-extinguish, and to lower the emission of toxic fumes or smoke.  In a fire, while plenum cables will still burn, the cable's treatment helps to avoid toxic fumes spreading quickly throughout the airflow systems in a structure. 

Riser spaces are the vertical shafts, such as elevator shafts or vertical conduits, that run between the floors of multistory buildings.  These spaces are not part of the HVAC system, so the fire regulations are less stringent.  However, riser rated cabling is still designed to self-extinguish, helping to prevent the vertical spread of fire.

Plenum cables can be used in place of riser cabling in a "riser" space as plenum cables have a higher fire resistance. However, you cannot substitute a riser rated cable for plenum rated cable in a "plenum" space.

How to Tell the Difference

A riser-rated Duplex OM3 cable, for example, looks almost exactly the same as a plenum-rated Duplex OM3 cable.  They are also handled and terminated in the same way.  How do you tell the difference, given that lighting them on fire to check isn't a good move?  The print legend.

Fiber optic cable specifications and jacket print legends will utilize the following abbreviations to denote riser or plenum status:

  • OFNR FT4: Optical Fiber Nonconductive Riser (FT4 is the Canadian equivalent)
  • OFNP FT6: Optical Fiber Nonconductive Plenum (FT6 is the Canadian equivalent)
  • OFCR: Optical Fiber Conductive Riser
  • OFCP: Optical Fiber Conductive Plenum

A note on conductivity: Optical fibers by themselves are nonconductive, but sometimes conductive elements are included within the jacketing structure. Armored cables, for example, are conductive.  Take this into consideration when planning your system.

Does This Have Anything To Do with Indoor/Outdoor Rating?

Because many (but not all!) of our plenum cables are also indoor/outdoor rated, while most of our riser cables (but again, not all) are indoor rated, we sometimes get questions on whether all plenum cables are indoor/outdoor.  Short answer: no.  Riser and plenum ratings are related to fire code, not to a cable's indoor/outdoor rating status. Indoor/outdoor rating is based on water blocking agents and UV resistance.

Any Other Differences Between Riser and Plenum?

The other difference between riser and plenum cable types tends to be cost.  Unlike non-fiber cables, plenum fiber optic cables are often fairly close in price to riser cables.  Non-fiber cablers, like coaxial or category cables, require additional changes to insulation materials and other components to meet plenum standards.  As these component changes are generally not needed for fiber optic cables, the cost of plenum fiber cables tends to be closer to that of riser fiber optic cables.

Choosing riser vs plenum is a code compliance and safety issue, not a budget issue.  Always install plenum cabling in plenum spaces or where required by code. 

Our new and current riser-rated cables provide additional options for installations in trays and riser spaces.