Deciphering the Cable Code
I can remember at an early age the mystery in learning a secret code. It was a way to feel important because only you and a select group really understood how to communicate with this order of the secret code. Now that we are older and either have, or are working on, a profession the fancy of childhood codes has passed. But as some have learned this adventure has been replaced with another mysterious code — the National Electrical Code™ (NEC™).
NEC is designed to cover all aspects of applying electricity to our everyday environment. It is typically broken down into high voltage (>70V) and low voltage (<70V). (Did you know the term “low voltage” is not specifically defined in NEC?) If you do not already have a copy, the first thing to do is get a current copy of NFPA 70. What? I thought we were talking about NEC. We are; they are one and the same.
While self-accomplishment might be at the top of your list, some of the key reasons we learn NEC is public safety and being able to comply with your local authority having jurisdiction (AHJ) requirements. This is their bible, too. The organization behind NEC2008, which is the most current version, is the National Fire Protection Association (NFPA).
The complete NEC is not simple to master, or should I say even satisfactorily understood. I will not lie to you; it takes some serious commitment and even some investment in order to be a respected technical professional in your community. However, once you get into the code you might find yourself starting to actually enjoy it.
Understanding NEC and Your AHJ
Did I say get a current copy? Well be careful on this; you see NEC is upgraded by NFPA about every three years and NEC2010 is already in the works. Since we are talking about NEC and your local AHJ, I would like to offer a couple of words of observation and possible caution.
First, since NEC changes so often, there is a high likelihood your AHJ is enforcing an older version, such as NEC2002 or NEC2005, maybe even NEC1996. Next, you must understand a very large part of the code enforcement work an AHJ does has to do with high voltage; you know, things like lights, outlets and feeder panels. This keeps them very busy.
More than likely the AHJ inspecting your security or automation installation is not as well versed on low-voltage NEC requirements as we would like. Just the other day I saw where one AHJ was having a dealer redo his wiring because low- and high-voltage cables where sharing the same junction box. In another case, a dealer was informed that plenum cable was in a plenum duct. Plenum cable not allowed in a plenum duct? How confusing. There’s more to learn here for sure.
In the first example, if the dealer was up to speed on the low-voltage aspects of NEC he might have been able to point out to the AHJ that under certain conditions the junction box automation installation did meet NEC. This, if nothing else, is financial incentive to learn more about the code.
Some Key Low-Voltage Areas
NEC is broken down into sections, or “articles.” There are certain articles that deal more with low-voltage cabling requirements for areas such as CCTV, fire and security, communications, and other power-limited cabling. Some of these key areas are:
Article 830 – Broadband Installations
How Code Dictates Cable Types
Now let’s learn some NEC cable codes. Manufacturers follow codes to produce and identify particular cable types. The cables are identified for their fire-resistance level by testing and listing in Underwriters Laboratories (UL).
The cable might have designations such as MPP, OFNP and CL3 to indicate their listed fire-resistance levels. The cable type codes begin with two-four prefix characters that represent a group. These groups are: MP – Multipurpose; CM – Communications; CL – Class (power limiting and signal); FPL – Fire Power Limited: CATV – Community Antenna Television; OF – Optical Fiber; and B – Broadband/Network.
The middle characters in the code better define the application group. These would be 2, 3 for Class 2 and Class 3 remote-control / signaling / power limiting (e.g. CL3R, Class 3 Riser); N – Nonconductive (e.g. OFNP, Nonconductive Optical Fiber Plenum); C – Conductive (e.g. OFCR, Conductive Optical Fiber Riser); M or L – Medium Power (<150V) or Low Power (<100V) (e.g. BMR, Medium Power Broadband Riser).
The last character in the code is an indicator of where the cable can be used. This would be P – Return-Air Plenum (e.g. CATVP, Community Antenna Television Plenum); R – Vertical Riser (e.g. OFCR, Conductive Optical Fiber Riser); G – General Purpose (e.g. CMG, Communications General Purpose); X – Limited Use / Residential (e.g. BLX, General Purpose Broadband Low Power).
Using the Substitution Chart
Have you committed all of this to memory? In an effort to pull all this together take a look at the Cable Substitution Chart in this article. You may find similar charts in the back of most cable distributors’ catalogs. I especially like the detail and information on this particular diagram.
You will notice across the top of the chart are the NEC Articles each cable type is referencing. Down the left side are the applications areas listed with the highest fire rating at the top. Next you will see a bunch of arrows referencing one cable type to another. These are the substitution arrows that let you know how a higher fire resistance cable may be substituted for a lower type.
An example would be that, in many instances, I can use a Plenum type, such as CATVP, in place of a CATVR in a vertical riser. This may help in specifying fewer types of cable and dealing in more quantity. And, one higher type can allow for more overall flexibility in running cable in uncertain areas. Also understand that the charts are guidelines and one should in the end reference the NEC sections pertaining to a particular cable application.
OK, so now you can tell your friends you understand some of the secret (NEC) cable code!
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