Smart Building

Fiber Cable Ratings: Selecting Riser vs. Plenum

Jim Zimnicki

A big factor impacting fiber cable selection is the installation space: Different environments call for different types of cables in order to ensure performance, installation ease and safety.


Inside buildings of all types – from schools and hospitals to corporate and hospitality venues – cables are often deployed in plenum and riser spaces. Find out what that means for cable selection!


Inside buildings of all types – from schools and hospitals to corporate and hospitality venues – cables are often deployed in plenum and riser spaces. Find out what that means for cable selection!


What is Riser-Rated (OFxR) Cable?

A riser is the vertical shaft, area or series of rooms between two floors inside a building. These spaces allow primary utilities to be distributed vertically (elevator shafts and conduits that run from one floor to another are risers, for example).


Fiber cables with this fire rating are self-extinguishing to prevent flames from traveling up the cable. Although cables installed in risers need to offer fire resistance to help prevent travel up the cable, the fire ratings for these cables aren’t as strict as those for plenum spaces.


What is Plenum-Rated (OFxP) Cable?

A plenum is the pathway or space used to circulate air as part of a building’s HVAC system (the space is often located between the structural ceiling and drop-down ceiling, but plenum spaces are also found underneath a raised floor). Because plenum spaces contain few fire barriers and forced-air circulation, the cables deployed here need a higher fire resistance than cables used in riser spaces.


Fiber cables with an OFxP fire rating (plenum cables) can safely be installed in plenum areas. They have fire-retardant plastic jackets, such as PVC or FEP, so that, if a fire were to ignite, fewer toxic fumes are emitted as the jacket melts.


Because they’re more flame retardant (less smoke and flame propagation are required and the test conditions are more severe in order to achieve the OFxP rating), more expensive jacketing and subunit compounds are used to manufacture plenum cables. In recent years, the cost of plenum cable has come down, but they’re still more expensive than equivalent riser-rated cables.


If you’re comparing apples to apples (like 12f riser vs. plenum cables), the riser cable will typically be more flexible and a little easier to handle during installation (they’re less likely to kink). 


If you’re dealing with a long cable run between different areas in a building, using a plenum cable means you don’t have to re-terminate it into a junction box to switch between riser and plenum cables. Instead, you can run that same cable through multiple areas using the same product, which is shown in the NEC substation chart for fiber cables.


A few important notes about riser vs. plenum cables:

  • A plenum cable can be used in a riser space, but a riser cable generally cannot be used in a plenum space. For this reason, many contractors and installers choose to keep only plenum cable on hand to reduce storage requirements and streamline inventory.

  • Using the wrong cable type can result in legal issues if a fire were to break out. For example: If riser-rated cables are used in a plenum space where CMP-rated cables should’ve been installed, smoke and fumes from burning cables could be swept into the HVAC system and spread throughout the building.

  • When in doubt, consult the NFPA National Electrical Code. It will tell you which cable type can be used where. 

If making a decision about riser vs. plenum cables is still weighing on you, it might help to think about it like this: When it comes to buying a car for your family, safety is likely top of mind. Even though they might be more expensive, people often choose to invest in larger vehicles with better crash ratings. Although a car accident may never occur, you’ll rest easy knowing it offers better protection if something does happen.


When it comes to selecting the right cable rating, you really can’t go wrong when you put tenant and occupant safety first. Not only will the right solution keep people safe, but it will also reduce building damage and protect against liability. 


Because of its versatility and fire resistance, Belden keeps more than 40 plenum-rated fiber solutions in stock at all times. Our fiber products are faster, easier and better to use. Have questions about choosing riser vs. plenum fiber cable? We can help you find the answer you need! 


Note: This article is meant to serve as an informative piece. The information included here should not be used in place of information provided by national or local fire codes.