The cabling world is full of acronyms and "computer-ese" that can often be confusing.  In effort to clarify these terms, Computer Input has compiled an on-going list of user-friendly answers to our customer's Frequently Asked Questions.  We hope you find them helpful and informative. 

Help us improve this resource - Please feel free to submit your questions, and we will be happy to answer them here. 

What is plenum-rated cable? Why do I need it?

To plenum or not to plenum; that is the question that architects ask themselves when designing a new building. Increasingly, the answer is plenum. Why? What is the plenum anyway?

A plenum is a space within a building that is used for the movement of environmental air. This typically refers to the air space above a suspended ceiling, but it can also be the area beneath a raised floor in a computer room. In effect, these spaces act as ducts for the air conditioning system. Since a return-air duct no longer needs to be installed, the building costs go down. And that is exactly why they appear in the majority of offices built today.

Savings to a building’s owner doesn’t translate into savings for the tenant, however, because plenum-rated cable costs more. The plenum space is part of the air conditioning system, so anything inside it must exhibit low flame spread and low smoke producing characteristics. Unfortunately, this is not optional. The National Electrical Code mandates that all cables run in an air plenum must be plenum-rated.

Coincidentally, the plenum above an office ceiling also represents the most practical route for running cables in most buildings. So special plenum-rated cables have been developed with insulation and jacketing that utilize Teflon and other compounds which can be run safely in air plenums.

How do you know if you need plenum cable? Ask your building engineer. Or call your local electrical code inspector to come by and check it for you. As a rule, however, most offices built since the Eighties require plenum cabling, for the obvious reason of cost.

Identifying plenum-rated cables is much easier. Simply look for the UL markings on the cable jacket. Amongst the hieroglyphic part numbers and manufacturer’s codes will be the letters CMP or MPP. This designation assures you that the cable is indeed fire resistant and safe for use in your building.