The Plenum is the open space between a ceiling and the floor above it.
Generally, Plenum-rated cable has the same data transmission performance specifications as regular CAT5e. However, there are some differences in the wire pair coating, and the outer jacket of the cable itself.
Plenum-rated CAT5e cable has a jacket that meets specific ANSI requirements, such as a flame retardant and low smoke production. It is often coated in a Teflon cladding. This is so that it will not give off toxic fumes if it is burned.
In Canada, code requires that plenum-rated cable must be installed through any plenum air return space.
Deciding if you need plenum-rated cable will come down to what the code requirements are for any given job.
How do I determine if my building uses Plenum Air Return?
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If you have a big ceiling or wall return air grate; take it off and see if there the air is channeled through sheet metal ducting. If a sheet metal duct is not there; only an open ceiling or wall space, then you are required by law to use Plenum-rated cable.
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If you don't find a ceiling or wall return air grate; look carefully at your fluorescent lighting fixtures to see if there is space around the edge of the fixtures. Put your hand over the spaces so you can feel air movement. Your air conditioner should be running while you do this. If you do not have wall or ceiling return air grates, it is normally a sign that you require plenum-rated cable.
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Computer Rooms usually use a raised flooring system for the server racks. This enables intensive cooling to be pumped through the space under the floor to cool the servers, routers, switches, etc. Cabling between the many devices and to the users runs through this open floor space. This by definition is a plenum air space and requires Plenum-rated cable.
Using or not using Plenum-rated cable is an important issue depending on your building's construction. It is necessary if your building does have plenum, which is common in high rise construction and newer construction. Plenum-rated cable is necessary because standard cable is jacketed in PVC, (Polyvinyl Chloride). The Chloride causes a problem. When the PVC smolders or burns it releases toxic fumes into the air. A plenum air return could circulate toxic air throughout your office.
Plenum-rated cable is more expensive, and most users and installers will not need to use this type of cable. However, be sure to take the time to find out if the construction of the building you are making an installation in requires Plenum-rated cabling or not.