Choose Appropriate Cables
Charles "Buc" Fitch and I have been trying to coordinate our columns in RW. He writes about the National Electrical Code, while I’m talking about wire and cable, which is supposed to meet the NEC.
The code is a moving target. The authors regularly come out with a revised version. Sometimes these revisions affect your choice of wire or cable. If you save copies of magazine articles about the NEC, or old copies of the code itself, make sure to buy an up-to-date copy.
You can order one through bookstores that specialize in technical publications, online at most of the larger discount booksellers or call (800) 344-3555.
Looking for ratings
One of the most interesting changes in the last few years is the issue of non-rated cable. For a while the rule was that unrated was fine as long as you put it in conduit.
Not anymore – now the code has been changed so that it can be interpreted to state that all cable in conduit must be rated. The cable doesn’t say what rating, just that it must have some rating.
And that is a problem with any code, especially a "voluntary" code like the NEC. You can talk to the association that publishes the code. But your local fire marshal, building inspector, planning commission or board of permit appeals – not to mention your architect, contractor or system integrator – may choose to interpret the code in a completely different way.
Arguing with any of the above will be almost as nice as beating your head against a wall, and about as constructive. The key is to confirm that all of these people agree about which standard, such as the NEC, is to be recognized and that all parties agree to the interpretation of the code.
More than a few engineers live dangerously and scoff at the mention of the NEC or rated cables.
"They’ll never inspect me," they say. "My station is in (insert name of any city with less than 50,000 population), and our fire marshal doesn’t have a clue."
I usually smile and nod, and hope s/he doesn’t come back to me when someone with the power tells her/him to rip out all the nice wiring he just did.
Sorting through spaghetti
Lots of cables are unrated. As a rule of thumb, I would say an unrated cable made today probably is not intended for installation.
Consider microphone cables. These cables are made to be flexible, rugged and low self-noise (they don’t make electrical noise when being flexed). Why waste those features by installing such a cable in a conduit?
Much of the cost of these cables was spent giving you those features. So don’t waste them. Use microphone cables where you can see them, where you need the flexibility, ruggedness and low noise. If you intend to put that signal in a conduit, make a transition from the microphone cable to an install version.
The install version is most often smaller, lighter and cheaper. It might even beat the performance of the mic cable. Of course, it won’t be as rugged or flexible, or deliver the same low-noise performance. But that’s okay; it will be in a conduit where it is protected. It won’t move once it is installed, so self-noise isn’t a factor and flexibility may even be a hindrance to installation.
Flexible cables often have soft matte finish jackets. These plastics have a tendency to "grab" the inside of a conduit, causing the cable to bunch up and making it much harder to pull.
What is needed is a hard and shiny jacket. This will slide through the conduit and the stiffness resists bunching up. The trick is to give the cable enough flexibility to be pulled through some 90-degree turns, but not so much that it grabs and bunches up.
The exceptions to unrated cables are those old cables from the Dark Ages – anything invented in the 1960s or before.
Many inspectors have allowed such cables to be "grandfathered" in installations, especially when the station has been using these cables as their standard for decades. Soon, however, they will no longer look the other way.
Go look at your cable now. The fire rating should be clearly marked on the jacket.
The most common ratings are CL2, CL3, CM, CL2R, CL3R, CMR, CL2P, CL3P or CMP. There are other ratings, but these cover most of the cable installed in broadcast applications.
If there is another rating, or what you think is a rating, check the NEC book or drop me an e-mail. If it doesn’t have a rating, it is unrated.
The land of plenum
Every so often, I see a piece of plenum cable running in a conduit. Now you might think this is a big mistake. After all, plenum cable is intended for use where the cable is not in a conduit. So why would anyone put a plenum cable in a conduit?
The answer is simple. Part of the run of this cable will not be in a conduit but will run through a plenum area. Then it is easier to make the whole cable plenum-rated, instead of making a transition from one kind of cable to another.
Of course, you could have avoided the conduit altogether. Usually I see "plenum in a conduit" in stadium projects, in which a contractor is doing the audio and video wiring. A different contractor installs the conduit, and the issue of plenum or non-plenum spaces may not have even been considered. There’s a technical term for this. It is called "wasting money."
There is also a recent increase in the introduction of riser-rated cable. Riser rating is only one step below plenum.
Riser-rated cables can go between floors vertically, without a conduit. Many video cables are now available in riser ratings. Even multipair analog audio snakes are manufactured in riser ratings.
Misinformation is floating around about riser-rated cables, though. They can go between floors without a conduit, but they don’t stop the requirement for fire blocking between floors. The fire blocking can go right around the cables, so you still have to put that in.
Next month we will look at starquad cable designs.