Diagram of Plenum Spaces

One of the biggest sources of confusion for many people is where a plenum space might be located in a typical office building or other structure. The key is to determine the direction and extent of forced airflow; let's take a look at figure 1 and see how we would wire a building with completely vented and controlled airflow.

As you can see, since the airflow is totally contained within the vents from entry to exit, most of the ceiling space is actually not going to be considered a Plenum space. Unless we were planning to run cables inside the air vent itself, we wouldn't need Plenum cables for an installation here. Riser-rated (CMR) cables would be sufficient for the ceiling runs, and CL3 for the walls. Finally we would only need standard CM/CMG-grade cable within the living/work space itself.

While it would be nice if every building had such precision control over its airflow, the truth is that either due to design, damage, or decay, this is often not going to be the case. Let's take a look at a more common situation in figure 2.

As you can see in this diagram, the air intake brings the air up into the ceiling before another vent forces it out of the space. Because forced airflow could potentially run through any or all of this ceiling area, the entire region would be considered "plenum" and thus would require plenum (CMP) cable for any wiring in this area. Other than that, the wiring standards remain the same as in Figure 1.