This is part two of a column written in response to requests to Donna Ballast to unravel the mysteries of the NEC 2005 and acceptable conduits for extending unlisted optical fiber and unlisted communications cable inside the building beyond the exterior wall.
Here’s a summary of Donna’s comments in part one, which appeared in the August issue:
So far, there are at least three strong opinions [for acceptable conduits]:
1. Only rigid conduit is acceptable;
2. EMT (electrical metallic tubing) is also acceptable; and
3. The NEC is so loosely written that you can read anything you want into it, and interpret it any number of ways.
Each group, of course, cites sections of the Code to support their arguments.
Before we can begin to unravel this web of code, you will need to read (at a minimum) the following sections in NEC 2005:
• 770.2 Point of entrance;
• 800.2 Point of entrance;
• and 300.22.
Bottom line-the definitions for “point of entrance” are pretty much the same, and both 770 and 800 reference 300.22. The real fork in the road appears between 770.113 and 800.113
To explain how we got to this fork, we need to go back to NEC 2002.
NEC 2002 770.50 Listing, Marking, and Installation of Optical Fiber Cables states that “Optical fiber cables in a building shall be listed…” …and gives three exceptions to that requirement. During the process of reviewing the proposals for the NEC2005, many of the articles were renumbered, hence 770.50 (2002) is now 770.113 (2005).
And there was the proposal to narrow the list of choices in Exception No. 3 (2002). Exception No. 2 (2002) disappeared altogether and Exception No. 3 (2002) morphed into Exception No. 2 (2005).
While Exception No. 3 (2002) accepted anything mentioned in Chapter 3, Exception No. 2 (2005) narrows the field by specifically listing IMC (intermediate metal conduit), RMC (rigid metal conduit), and EMT as raceway types that are accepted for use with unlisted non-conductive optical-fiber cable that “enters the building from the outside.”
Notice that the “enters the building from the outside” and not “point of entrance” language is used here … re-read NEC 2005 Section 770.2, Point of entrance. Notice that the only way to extend the point of entrance beyond the building wall is by choosing RMC or IMC grounded to an electrode in accordance with 800.100(B).
NEC 2002 800.50 Listing, Marking, and Installation of Communications Wires and Cables states that “Communications wires and cables installed as wiring within buildings shall be listed…” …and gives four exceptions to that requirement.
Just to keep things interesting, 800.50 (2002) is now 800.113 (2005). Exception No. 2 and Exception No. 4 (2002) disappeared altogether, and Exception No. 3 (2002) morphed into Exception No. 2 (2005).
Come on, guys! Having the type of raceway be determined by whether a term (or point of entrance, in this case) is used in an exception to a listing requirement is just too tortuous, even for the NEC.
... and now, for a story
In the early 1860s, while looking out of the palace window, the Prussian ambassador to the court of Czar Alexander II of Russia, Prince Otto Edward Leopold Von Bismarck, saw a sentry stationed in the middle of the palace lawn.
The Prince asked the Czar, “Why is a sentry stationed in the middle of the palace lawn?” The Czar summoned his Chief of Staff and asked why a sentry is stationed in the middle of the palace lawn. The Chief of Staff lamented that there had always been a sentry posted there for as long as anyone could remember. He summoned the Commander General of the Army, who explained that a sentry is stationed in the middle of the palace lawn in accordance with ancient custom, but he was not sure just how that custom originated.
The General was instructed to find out why and report. After three days, the General reported that sentry records indicate that, early one spring morning in 1780 (that would be 80 years earlier), Catherine the Great was looking out of the palace window when, in the middle of the palace lawn, she saw the first daffodil of spring. Inspired by the beauty of this lone flower protruding through the frozen and snow-covered ground, she decreed that a sentry be posted to prevent anyone from picking that flower…and 80 years later at the very spot where the flower once bloomed, a sentry was still standing guard.
The Czar ordered that the sentry position be recalled, because it was a waste of time, energy, and resources…and to this day, that spot has been left unguarded.
Fast-forward 150 years. While I have no real historical proof that this story is true, I am reasonably certain that such events have happened throughout history, begging the question, “Was this custom, habit, or just a case of ‘well, because we have always done it this way!’?”
Moral to the story
Remember, when someone asks you why, “Because we have always done it that way” is always the wrong answer. No custom, habit, procedure or policy should be maintained simply because it is the way things have always been done.
I was trained to design building service entrance facilities to accommodate the “worst case” cable placement-that would be large pair-count copper conductor cables. This means a system, of trade size 4 conduit with large sweeps, that’s designed to last 50 years.
RMC or IMC is used because it can survive an impact of 350 foot-pounds, and the ends are threaded. RMC or IMC pullout strength depends on the mechanical interface of the metal threads and the tensile strength of the steel.
For example, according to a Steel Tube Institute of North America white paper, on a trade size 1 conduit, these treaded connections can withstand around 18,000 pounds of joint pull-out force. It is tough stuff!
But, do we need to continue this practice? Is EMT, with its compression-type connectors “good enough” for building service entrance facilities dedicated to optical-fiber cabling? As for me, my sentries (RMC and IMC) are still standing guard, and I know why they are there.
While NEC 2005 Section 770.113 allows you to make that decision for your clients, I strongly suggest that you include the AHJ in your decision-making process. Cheaper is never better if it will not pass the ever-watchful eye of the inspector.