Plenum vs. Riser Cabling
Planning a networking infrastructure for a building requires a thoughtful approach to the choice of cabling. Understanding where to deploy fire-resistant cables such as Communication Multipurpose Plenum (CMP) and Communication Multipurpose Riser (CMR), will have a substantial impact on your building’s safety.
Unrated PVC-type cable jackets are flammable, potentially acting as conduits for fire to spread rapidly within a building. In oxygen-rich environments like plenum or ductwork spaces, the airflow can escalate a minor spark into a major spread. These cable jackets not only fuel the fire but also serve as a pathway for flames.
In contrast, riser and plenum cables have fire-resistant jackets that are either self-extinguishing or inherently non-flammable. This design helps contain fires and prevent the cables from contributing to the spread.
Jump to each section to learn more
• What is a Plenum Space
• What is Plenum Cable
• What is Riser Cable and Where to Use it
• Where to use Non-rated Cables
What is a Plenum Space?
A Plenum space is a space above a building’s dropped ceilings or below a raised flooring system that is set aside for air circulation. It is designed to house HVAC equipment and serve as a receiving chamber for air that has been heated or cooled, before distributing the air throughout the building.
In addition to being important for air circulation, plenum is also a common place to run network cables. The open airflow in the space can quickly spread a fire throughout a building and non-plenum cables can be a catalyst for a faster spread. Due to this risk, any cable installed in plenum spaces must be plenum-rated.
What is Plenum Cable?
Plenum cables can refer to any cable type (coaxial, Ethernet, fiber optic, etc). The plenum rating of the cable refers to the outer jacket of the cable.
Since plenum cables are routed through air circulation spaces which often contain very few fire barriers, they need to be coated in material that won’t contribute to the spreading of flames. As network cabling is typically spread throughout buildings, that increases the chances that if one ignited, flames and smoke could spread throughout the building.
Plenum cables boast more stringent fire ratings compared to riser cables and non-rated cables. The raw materials necessary to achieve plenum ratings, such as FEP (Fluorinated Ethylene Polymer) has a high melting point of 500°F as well as excellent electrical insulating properties.
When to use Plenum cables
Plenum cables are required whenever you are running cable through HVAC ductwork or in air space that is an active part of the HVAC air flow balance. They may also be used in non-plenum spaces to increase building safety, and in some cases, plenum cables are required in non-plenum spaces to meet building code requirements.
How do you know if a cable is Plenum Rated?
The information printed on the cable’s outer jacket typically contains all the essential information about the cable, including the jacket rating and who verified it.
Two main laboratories conduct the majority of plenum cable testing: Underwriters' Laboratories (UL) and the Electrical Testing Laboratories (ETL). If the cables successfully meet the required testing criteria, the cable manufacturer receives a certification indicating compliance with testing standards.
You can verify product numbers on the UL or ETL websites. If a product appears on either of these sites, it is categorized as “UL listed” or “ETL listed,” signifying it successfully passed the tests conducted by the respective laboratories.
What is Riser Cable and Where to Use it
Riser cables are designed for vertical applications within a building and help prevent the spread of flames from one floor to another in the event of a fire. They are intended to connect different floors of a building, such as from the basement to higher levels or between floors in a multi-story structure.
Riser cables use the same copper conductor and have the same electrical features as plenum cables. Their distinction lies in the fire rating of the outer cable jacket. Riser cables are flame-retardant, however, they do not have the same fire rating as plenum cables and should not be used in plenum spaces.
The cost disparity between plenum and riser cables can be substantial with plenum cables costing double or more.
Where to use Non-rated Cables
Non-rated cables, referred to as Communication Multipurpose (CM), are appropriate for areas that neither involve plenum spaces nor vertical runs between floors. They are often used to connect IP security cameras to the network PoE switch.
They are the least fire resistant of the three categories and will release a higher level of toxic fumes if burned. You can always opt to use riser cables or plenum cables in these general use scenarios for an added layer of protection against spreading fire.