The 2007 NESC – Part 4

This article is a continuation of what I see as significant changes that are coming in the 2007 revision of the National Electrical Safety Code.

Photo 1. Bonding of cases and enclosures

Grades of Construction

Table 242-1 has been changed by the removal of the delineations rural and urban. The higher grades of construction that used to apply to urban areas, now apply to all areas. This is a significant change for utilities that used to use the lower grades of construction in rural areas. The grade of construction is used to determine the safety factors that must be used in the strength and loading design of the facilities. In general, the higher the grade of construction, the greater the safety factor and the stronger and more expensive the structure.


Construction and Maintenance Loads

Rule 250A2 has been expanded to emphasize the need to consider temporary loads such as lifting of equipment, stringing conductors, or a worker on the structure or component when designing the strength of the structure.

Load Factors

Photo 2. Fiberglass crossarms

Starting in Rule 253, what was called overload factors is now load factors. The Grade C construction section of Table 253-1 has been expanded to delineate between At crossings and Elsewhere. Footnote 5 now addresses fiberglass (fiber-reinforced polymer) portions of structures and crossarms. The table also has load factors for the new extreme ice and concurrent wind loading (Rule 250C).

Alternate Load Factors

The alternate load factors in Table 253-2 and the associated strength factors of Table 261-1B shall not be used after July 31, 2010.

Fiberglass Structures, Crossarms and Braces

Rule 261A3 now addresses the strength requirements of fiberglass structures and Rule 261D3 now addresses the strength requirements of fiberglass crossarms and braces. Table 261-1A now gives the strength factors for fiberglass structures, crossarms and braces.

Strength of Climbing and Working Steps

New Rule 261N addresses the strength of climbing and working steps on structures.

Strength of Insulators

Rule 277 has been changed significantly to better define the strength requirements of insulators consistent with the ANSI® C29 standards for insulators.

Emergency Installation of Cables on the Ground

Photo 3. Insulators on a 138 kv line

New Rule 311C allows for the installation of electric and communications cables laid directly on grade during an emergency under special conditions.

Electric and Communications Cables in the Same Duct

New Rule 252E clearly states that electric and communications cables shall not be installed in the same duct unless all of the cables are operated and maintained by the same utility.

Communications Cables in the Same Duct

New Rule 252F allows communications cables to be installed in the same duct as long as the utilities involved are in agreement.

Bonding of Cases and Enclosures

The NESC requirement that all above ground metallic power and communications pedestals, terminals, apparatus cases, transformers cases, etc., be bonded if they are separated by a distance of 6 feet or less has been moved from Rule 350F to new Rule 384C. Many people were not aware of this rule because it was located in the Direct-buried Cable section of the code. It is now where it should be in the Equipment section.

Arc Protective Clothing

Rule 410A3 now requires employers to make an assessment to determine the potential arc energy exposure for all employees who work on or near energized parts or equipment. If the assessment determines a potential employee exposure greater than 2 cal/cm2 exists, the employer shall require the employee to wear clothing or a clothing system that has an effective arc rating at least equal to the anticipated level of arc energy. This goes into effect as of January 1, 2009.

Work in the Vicinity of Communications Antennas

New rule 420Q limits the radiation level a worker may be exposed to when working in the vicinity of communications antennas.

Existing Utility Location Verification

Rule 423D2 now recommends that the location of existing utilities be verified by exposure prior to the use of guided boring or directional drilling methods.