I have been working with and searching all aspects of the National Electrical Safety Code (NESC®) for almost 30 years. I refer to the book at least twice a day. Engineering, construction, claims and legal personnel in my company call or e-mail me regularly to ask questions about the NESC. Five years ago, I thought I had a very good understanding of the NESC . The first interpretation I had an opportunity to work on was what I thought was a “no brainer.” It seemed to me that the rule we were asked to look at had to be one of the best written, easy to understand rules in the entire book. The chairman of the interpretations subcommittee receives the interpretation requests and sends them to the members of the committee who are “experts” on that part of the NESC in question. Each member responds independently back to the chairman and sends copies to the other members. I sent out my response and then I started receiving copies of the responses from the other members. My first reaction to the other views on the subject was, “Are they looking at the same rule I looked at?” The nine members of the interpretations subcommittee who were asked to respond to this interpretation request, came up with what appeared to be nine different responses. After studying the other responses, I could partially understand some of the reasoning and I realized that the rule was not as simple as I thought it was. That’s probably why someone asked for an interpretation. The chairman of the subcommittee gleaned the commonality from the nine responses and drafted a proposed response. The proposed response went back to the members for voting and comment. The drafts and discussion went back and forth many times over the next seven months it took to develop a consensus report. And this one was my “no brainer.” Over the past five years, I have worked on 26 interpretations and gained tremendous insight in understanding the NESC. Studying the interpretations is very important in understanding the NESC.
As the interpretations are developed, they are published in interim collections about once each year. Every five or so years, all the interpretations are published. The last full publication covered all the interpretations from 1943 to 1997, over 500 interpretations.
Some Hints on Researching Interpretations
Photo 2. U-guard should have a back-plate
Referring to the National Electrical Safety Code Interpretations Collection 1943 to 1997 Inclusive, the interpretations appear in the book grouped together as they were originally published, i.e., 1961 to 1977, 1978 to 1980, etc. Within each group, the interpretations are presented in order by rule number. If you are looking for interpretations associated with a particular rule, this is where to look. There are also listings in order by interpretation number. Note that some of the rule numbers have changed. For example, the NESC horizontal clearance requirements between line conductors are specified in Rule 235B in the 1997 NESC. In the sixth and prior editions, it was in Rule 235A2. This type of information can be found in the NESC Handbook. The contents of the NESC Handbook can be very helpful in understanding the NESC but “should not be considered to be an official requirement or an interpretation of the NESC.” People occasionally request an interpretation of a rule in a prior edition of the NESC. In that case, the interpretation will reference that edition and only applies to the rule as it appeared in that edition. If the wording of the rule was changed in subsequent editions, the interpretation does not apply to the new rule.
An Example Interpretation
In April 1997, an engineer requested an interpretation of Rule 239D “Mechanical Protection Near Ground” of the 1993 NESC relative to the use of U-guard without a back-plate on cable terminal poles. The 1993 edition Rule 239D states, “Where within 8 ft (2.45m) of the ground, all vertical conductors, cables, and grounding wires shall be protected by a covering that gives suitable mechanical protection.”
Photo 3. U-guard not having a tight fit to the pole
The interpretations subcommittee decided that conduit or U-guard (with or without back-plate) may be used to satisfy Rule 239D, however, where “U-guard is used without a back-plate, the U-guard must have a tight fit to the pole surface, without gaps between the U-guard and the pole. Otherwise, the U-guard should have a back-plate.” It is difficult to comply with this requirement on wood poles because of the very uneven surface. (U-guard not having a tight fit to the pole)
The interpretations subcommittee also pointed out that “Only certain types of conductors, cables and grounding wires can be attached directly to a pole, see Rule 239A1 and the exceptions in Rule 239D (1993 edition). All other types of conductors and cables must be fully enclosed, either by a conduit or a U-guard with back-plate.”
If you have any general questions about the NESC, please call me at 302-454-4910 or e-mail me firstname.lastname@example.org
National Electrical Safety Code and NESC are registered trademarks of the Institute of Electrical and Electronic Engineers. The National Electrical Safety Code, NESC Handbook, and the National Electrical Safety Code Interpretation Collection 1943-1997 are publications of the Institute of Electrical and Electronics Engineers, Inc.