The Revision Cycle
The National Electrical Safety Code® (NESC®) is presently being revised on a five-year cycle. The 2007 edition will be published and available for purchase on August 1, 2006. The NESC is published by the Institute of Electrical and Electronics Engineers (IEEE). The 2007 edition shall become effective no later than 180 days following the publication date. For the 2007 cycle, 297 change proposals were submitted by the July 17, 2003, deadline. In October 2003, the seven NESC subcommittees met to consider the change proposals. The change proposals and the subcommittee’s initial vote and comments on each proposal were published in the “Preprint” on September 1, 2004. The “Preprint” was available for purchase by the general public. Interested parties submitted hundreds of written comments on the change proposals and the initial actions of the subcommittees by the May 1, 2005, deadline. The subcommittees met a second time in September and October 2005 to consider the comments. The final actions of the subcommittees on some change proposals were significantly affected by the comments. As a result of the comments, many actions were reversed or greatly modified.
Strength and Loading Change
As you will shortly see, much more engineering will have to be performed starting in 2007 to design aerial communications and electric supply facilities and, in general, aerial facilities are becoming stronger. In light of hurricane Katrina, maybe that’s not a bad idea. The significant change in the strength and loading requirements of the NESC is the addition of extreme ice with concurrent wind loading (new Rule 250D) for aerial communications and electric supply facilities where the structures are taller than 60 ft in height. To understand the impact of this new rule, we have to look at the loading requirements presently in the NESC.
Ice and Wind Loading
Presently in Rule 250B, the NESC requires all aerial communications and electric power supply facilities with structures to be designed to withstand specific combinations of ice and wind depending upon the location of the facilities relative to a Loading District map in Figure 250-1, page 162 of the 2002 NESC. For example, in the Heavy Loading District, in cities like Boston, Massachusetts; Washington, D.C.; Chicago, Illinois; Kansas City, Missouri; and Bismarck, South Dakota; aerial communications and electric supply facilities have to be designed to withstand a 40 mph wind with the conductors covered with a half-inch radial thickness of ice. In the Medium Loading District, in cities like Raleigh, North Carolina; Nashville, Tennessee; Santa Fe, New Mexico; and Seattle, Washington; aerial communications and electric supply facilities have to be designed to withstand a 40 mph wind with the conductors covered with one-quarter inch radial thickness of ice. In the Light Loading District, in cities like Tampa, Florida; New Orleans, Louisiana; Houston, Texas; and Los Angeles, California; aerial electric supply facilities have to be designed to withstand a 60 mph wind.
Extreme Wind Loading
Presently in Rule 250C, aerial communications and electric supply facilities with structures taller than 60 ft in height shall also be designed to withstand the extreme wind conditions detailed on maps in Figures 250-2(a) and (b), pages 166 and 167 of the 2002 NESC. The wind speeds indicated on the maps are generally 90 mph for the interior of the country and higher speeds along the coasts. In some areas, the wind speed is as high as 150 mph. For example, in Ocean City, Maryland, aerial communications and electric supply facilities with structures taller than 60 ft in height shall be designed to withstand 118 mph wind in addition to the ice and wind loading of Rule 250B.
Extreme Ice and
Concurrent Wind Loading
Numerous change proposals for the 2007 edition proposed addition of a third loading requirement, extreme ice and concurrent wind. The proposal that was finally accepted by Subcommittee 5 calls for designing aerial communications and electric supply facilities with structures taller than 60 ft in height to withstand a combination of extreme ice and wind. How much ice and wind are detailed in maps? Ice thickness goes as high as one and one quarter inch. The greatest impact on design appears to be in non-coastal areas of the country where the extreme wind speed is only 90 mph. For example, in cities like Philadelphia, Pennsylvania; Fort Wayne, Indiana; Springfield, Illinois; and St. Louis, Missouri; where the present NESC calls for all aerial communications and electric supply facilities to be designed for a half inch radial thickness of ice, the 2007 NESC will require facilities with structures taller than 60 ft in height to be designed for one inch of ice under the same wind conditions.
The Future
The present NESC extreme wind loading and the 2007 NESC extreme ice and concurrent wind loading only apply to facilities with structures taller than 60 ft in height. The electric supply facilities with structures taller than 60 ft in height are usually transmission facilities, where the operating voltage exceeds 34 kV. With the extensive damage to aerial communications and electric supply distribution facilities by hurricanes in the past few years, I think there will be considerable pressure to change the NESC in the 2012 edition to apply the extreme wind and extreme ice and concurrent wind conditions to all structures regardless of height. If that happens, I predict that the cost of aerial communications and electric power distribution facilities construction will greatly increase and the cost difference between aerial and underground construction will diminish.
Please send me your comments on this series. If you have general questions about electric power distribution or the National Electrical Safety Code (NESC®), please e-mail me at dave@daveyoungengineering.com or call me at 302-633-1044. NESC® and IEEE® are registered trademarks of the Institute of Electrical and Electronics Engineers.
Dave is a consulting engineer and president of Young Engineering, Inc. of Wilmington, Delaware. Dave has been working with and teaching all aspects of the NESC® and electric power distribution for over 34 years. He is a member of the NESC® Interpretations Subcommittee and represents the Edison Electric Institute® on the NESC® Overhead Line Clearances Subcommittee 4. Dave is also an inspector member of the IAEI®.
|
