While browsing the available bulk Ethernet Cable at my local home improvement store I noticed that they did not do a very good job of showing why some cables of the same category rating were almost twice as expensive as others. The single feature that accounted for the increased cost was the plenum rating of the cable but the feature was not even listed on most of the store’s signage. I’m sure that this situation causes a lot of confusion and leads many people to wonder if they need plenum or riser cable for home use.
One of the best ways to save money when installing a physical home network is to use riser-rated cable rather than plenum-rated cable. In most homes, there is no reason to use anything but standard riser cable as long as it is installed correctly. There are however some situations where plenum is not just recommended but required by building fire code.
What Makes a Cable plenum-rated?
Plenum-rated cable has a specially formulated outer jacket that makes it perform better in a variety of different ways in the event of a fire. Not only does plenum cable burn less readily but it also produces much less smoke and is far safer if it does eventually lite. On the other hand, most non-plenum cables burn well. In some cases, fire can even travel along a cable from one part of a structure to another. Riser cable can also produce noxious or even toxic smoke as it burns. You can get almost any type of cable with a plenum rating, including most networking cables as well as many kinds of audio and video cables, but in the context of this article, I will be focusing on Ethernet.
How to identify plenum-rated cable
When buying a box of cable the plenum rating is normally listed as a feature right on the box. Most of the time you will be able to see the actual word plenum somewhere in the feature list. If you do not see the word plenum anywhere the cable jacket grade may be identified as an abbreviation. The table below shows some of the most common abbreviations that I have seen.
| Abbreviation | Meaning of Abbreviation | Fire Resistance Level |
|---|---|---|
| CM | Communication Cable | No fire resistance |
| CMG | Commercial Communication Cable | No fire resistance |
| CMX | Residential Communication Cable | No fire resistance |
| CMR | Communication Riser Cable | Keeps fire from spreading through the cables |
| CMP | Communication Plenum Cable | Works as riser cable and reduces dangerous fumes while burning |
There are many more Abbreviations that you will see depending on the kind of cable that you are using but these are the most common ones out there for Ethernet cable.
If you are looking for the cable jacket rating for an Ethernet cable that is already installed somewhere you can usually either find one of the above abbreviations, the word riser, or the word plenum printed directly on the jacket of the cable. If you do not see a rating anywhere on the cable it likely does not have any fire rating at all and would be considered CM or generic communication cable.
When plenum rated cable is required
The requirements for what type of cable must be used for a given application can vary a little bit from place to place because local building codes can be more strict than the NEC (National Electrical Code) standards. In most places, however, you only need to use plenum-rated cables in plenum spaces. There is some confusion online about what exactly constitutes a plenum space because it is normally defined in the HVAC context. A great rule of thumb is to think about the air in the space. If a person will be breathing the air in a space or the air in the space will be mixed directly with air in the HVAC system then the space is a plenum.
Here are some examples
| Space Description | Is Plenum-Cable Required? |
|---|---|
| Forced air duct | Yes |
| Cold air return | Yes |
| An attic or crawl space being used as an air return | Yes |
| Any room designed for human occupancy | Yes |
| An attic or crawl space not being utilized by the HVAC system | No |
| Utility spaces not designed for human occupancy | No |
| Inside of walls, floors, or ceilings | No |
For residential networking projects, I recommend that you try to keep all of your cabling out of plenum spaces if at all possible. This allows you to use the much cheaper riser-rated cable. I do not recommend using cables that do not at least have a riser rating as the cost savings are not that large and I like the extra safety that comes along with it. Riser cable also has an additional strength member in the center that helps keep the cable from stretching so it is less likely that the cable will get kinked or otherwise damaged while it is being installed. If you would like to save the money and go with a cable that does not have any fire rating at all make sure you check with your local building codes first. Generally, riser cable is only required when you are running the cable between floors or firewalls but I have seen jurisdictions that are more strict about this.
Additional Requirements for Using Riser Cable in a Home
Because riser cable cannot be used inside a space that is designed for human occupancy it is important to terminate the riser cable inside of the non-plenum space (usually the wall) and seal it off from the plenum space with a faceplate. You can also use things like surface mount raceway, conduit, network cabinets, and junction boxes to isolate non-plenum cables from a plenum space.
There are some situations, such as surface mount access points, where many people use an RJ-45 connector and plug the cable directly in the access point but this can only be done with a plenum cable. In these situations, you either need to use a plenum cable or you need to terminate the riser cable behind a faceplate, or some other device that will keep the riser cable isolated from the plenum space, and connect a plenum patch cord to be used inside of the plenum space.
Conclusion
Most people can use riser cable for the majority of their residential network construction. This can be a great way to save some money on materials. Just make sure that you will not need to install any of your cables inside of a plenum space and that you will be able to terminate all of the riser cables in such a way that it will not need to enter any plenum spaces to connect to the end device.