For commercial buildings, the decision to use plenum cable materials is driven by the mechanical HVAC design, the architectural design, and construction costs. Per the National Electrical Code (NEC®), the exposed data cabling installed in “other space used for environmental air” must be plenum rated (e.g. OFNP, CL2P, CMP)1. One example is the space between the building’s structure and a drop ceiling that is used as a return air plenum in a typical office space.
Although the occupied space for an office may be in the same airflow, the plenum cable requirement only applies to the non-occupied space above the drop ceiling; it does not apply to the cables in the occupied space. Therefore, patch cords extending from a data outlet to a workstation do not require plenum-rated cable. For a computer room, which is defined as an Information Technology Room by the NEC, there is a provision that allows for a non-plenum cable in a plenum space. This unique provision applies when a raised access floor space (RAF) is used to transport air and certain conditions outlined in Article 645 are met2.
Not having to use plenum-rated cable in a data center is good news. Plenum cables typically cost 20 percent to 50 percent more than non-plenum cables. Data centers are cable-intense environments. In addition, it may be difficult to get some specialty cable harnesses in a plenum rating. However, if your data center is very large, you may have to install plenum cable anyway and not just in the RAF area but even in the occupied space areas. The need to use all plenum cable materials may be driven by FM Global engineering guidelines.
FM Global is a property insurance company underwriting institutions, hospitals, industrial facilities, manufacturers, and large data centers. They publish engineering guidelines on a variety of subjects to help their clients mitigate risk. FM Global has published Property Loss Prevention Data Sheet 5-32 for “Data Centers and Related Facilities.” This document takes a holistic view of the data center facility and identifies recommendations to reduce property loss. One item specific to plenum-rated cables states:
The recommendation by FM Global for all the data cables to be plenum rated applies to all the cables in the data center. The text does not limit the recommendation to only plenum spaces. Even cables in non-plenum occupied spaces require a plenum-rated cable.
The data center plenum cabling paradox is that even though you can design a data center per the NEC that would require no plenum-rated cables, if you are required to follow FM Global recommendations, all the data cabling would need to be plenum rated. The takeaway for the engineer designing a data center is to ask your client/owner whether the facility will be underwritten by FM Global and, if so, what are the expectations for following the FM Global engineering guidelines?
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1 NEC 2017 Articles 725.3 (C), 300.22 (C)
2 NEC 2017 Articles 300.22(D), 645.5 (E) (2)