Plenum-rated Cable Assembly: How is it Different From Standard Cables?

FACTS CHECKED BY  Jose George​

In the United States, if the quality assurance team suspects you aren’t using appropriate wires, you have heavy charges. Thus, commercial and residential buildings must conform to the NEC standards for safety purposes. All these rules call for one simple cable that greatly contributes to stopping fire hazards. It is a plenum-rated cable assembly with extra jacketing, so it does not catch fire in open-air spaces. Let’s dig into it. 

Table of Contents

What are Plenum-rated cables?

In the US, the NFPA or National Fire Protection Association dictates certain guidelines that the cables in closed spaces have to follow in NEC. As the NFPA controls fire hazards, all the government buildings and local bodies have to confine to these guidelines as their code of conduct. By abiding by it, you can rate your cables as either plenum or non-plenum wires.

This rating applies to any cable, Ethernet, Cat5, coax, or HDMI. The only difference the rating gives is the outer jacketing, which is usually fire-resistant.

Moreover, there are over 15 levels of ratings, of which the three most common are

  • Plenum Rated cables(CMP)
  • Riser-rated cables (CMR)
  • Standard cables (CM)
Plenum cable fitting

Caption: Plenum cable fitting

How do Plenum, and Non-Plenum Cables Differ?

As you can see, the CMP, CMR, and CM cables are not the same type of cable and can’t serve the same purposes. Here is a different chart between these three cables that you need to know.

CharacteristicsPlenum Cable (CMP)Riser-rated Cable (CMR)Standard Cable (CM)
Fire ResistanceHighLower than CMPLeast
UsesBest for plenum spaces, however, can be in use anywhereBest for vertical shafts and risers,  non-riser areas, as wellFor general purposes, where there are no fire hazards
JacketingFEP or PVC ( low-smoke) JacketingPVC JacketingPVC Jacketing
Burning PropertiesHas less toxic fumes while burningReleases high toxic fumes while burningReleases high toxic fumes while burning
CostExpensiveLess Expensive than plenum ratedCheap

When to use Plenum Cable?

According to the NEC Article 800, you must use a plenum cable in air ducts and open airspaces to prevent smoke, fire hazards, and toxic fumes. Also, due to its high safety rating, you don’t need a conduit for installation inside a plenum. They are also adaptable enough to be set up in areas that do not fit in plenum criteria.

Now, it only depends on your estimated cost, insurance policy, and code whether you choose to install plenum cables in open areas. For example, to comply with the code, many buildings, such as schools, healthcare facilities, and state houses, must use plenum-rated cables, even in non-plenum areas. However, you are not bound to such rules while installing the cables in your home or private area. Still, some people might use plenum-rated cables for extra safety in their residences or office buildings. 

In fact, for installing a signal booster, TV system, Antenna system, or a WIFI network, you can work with a coax cable in plenum spaces. Nevertheless, it has to be plenum rated.

Plenum-rated cable assembly: When to use Non-Plenum Cable?

Non-plenum cable is meant for open areas that are not plenum, like walls, open ceilings, etc. Now, there are different non-plenum cables, and each has its purpose. For instance, you must use CMR cables to connect the floor and vertical shafts. However, you cannot use it in the spaces that call for a riser cable.

Non-plenum cables are less resistant to fire, so it catches fire easily while giving out smoke and fumes. However, sometimes you can put the non-plenum cables inside a plenum-rated conduit and run it through closed spaces. 

The conduit helps to put out fires that start inside it and keep the smoke from spreading. Even though it saves money, using a non-plenum cable with a conduit is not always the best choice. For example, you might need to make a hole through a conduit to connect the cable with a signal booster or any other such system. Then there is no point in using the left conduit, and the cable is as vulnerable as before. 

Even though plenum airspaces are typically found above drop ceilings, this does not make all ceiling spaces plenum. Some spaces are closed-air spaces, and thus, you can easily use the non-plenum wires on that point. You need to look for any leaky ducts in that area first to prevent fire from happening.

Ventilation system and plenum wiring

Caption: Ventilation system and plenum wiring

Plenum-rated cable assembly: How do I Know if a Cable is Plenum Rated?

Typically, you can find all you need to understand about a cable—from its jacket rating and manufacturer to its verification status, temperature tolerance, and reference number—from the letters and numbers printed on its jacket. 

If the UL (Underwriters Laboratories) has tested and verified a cable as plenum rated, you can check the reference number on the UL website. However, if the cable doesn’t have a number printed on the jacket, you should verify its authenticity. You can either ask the vendor for the certification company and reference number. We advise looking elsewhere if the seller is unwilling to disclose the details.

Conclusion

So, Do I Need Plenum-Rated Cable in My Home? While it depends on your choice and budget, using plenum-rated wires in your residence is unnecessary. It is because most houses are not designed to have plenum spaces. Also, the purpose for which you need such cables can be fulfilled by coax cables solely. Plenum cables usually target commercial areas and not residential ones. While there are a bunch of category cables with different standards, you must know which goes where to make the right connection. The research will let you conform to the electrical safety codes and makes the wiring reliable.

Need a buying guide?