The mechanical design of the distribution system, and its several parts, must not only be adequate to sustain the normal stresses and strains, but must safely sustain them during abnormal conditions brought about by the vagaries of nature and people. While design criteria for overhead systems are substantially different from those for underground systems, in both instances prudent design takes into account economic and other nontechnical considerations.
For overhead systems, the supports for the conductors and equipment must withstand the forces imposed on them, while the conductors themselves must be sufficiently strong to support their own weight and the forces imposed on them.
National Electric Safety Code (NESC)
Minimum design criteria are suggested in the National Electric Safety Code (NESC) issued by the Institute of Electrical and Electronics Engineers (IEEE). The NESC has received wide acceptance by utilities and other industries in this country and elsewhere.
In general, the code specifies:
1. Clearances between conductors and surrounding structures for different operating voltages and under different local conditions
2. Strength of materials and safety factors used in proposed structures
3. Perhaps the most basic, the probable loading imposed on the conductors and structures based on climatic conditions, approximately defined by geographic areas of offsets and bends in the lines, and of the pressure of wind blowing against them. Both vertical and horizontal loadings include the effects of ice collecting radially about the conductors.
The vertical force on the pole is the dead weight of the conductors with their coatings of ice, cross arms, insulators, and associated hardware. This vertical force exerts a compressive stress that may be considered uniformly distributed over the cross section of the pole.
This loading, however, is almost always overshadowed by the requirements of the horizontal loadings, and is usually not given further attention. Even a very light pole can safely carry the dead weight of a multicircuit, large-conductor line.
The NESC divides the country into heavy, medium, and light load areas. The heavy loading area comprises roughly the northeast quarter of the “lower 48” states, and Alaska; the medium loading area comprises the northwest quarter plus a strip across the middle of the country; the light loading area comprises California and all the southern part of the country to a depth of some 300 to 400 mi, and Hawaii.
The degrees of loading are indicated in Table 5-1, and the geographic areas for the continental United States in the map shown in Figure 5-1. The values and areas of demarcation are approximate and should be subject to other practical considerations, including probable deviations based on actual experience, local codes and regulations, and other environmental requirements.