Structures supporting electric lines must be designed to support conductors, insulators, and shield wires in a manner that provides adequate electrical clearances and ample mechanical strength as specified in the National Electrical Safety Code (ANSI C2), published by the Institute of Electrical and Electronics Engineers, Inc. Safe clearances must be maintained when the conductor temperature is elevated as a result of large currents flowing in a circuit and when the conductors are ice-coated and strong winds are blowing. Common types of structures include wood poles, reinforced-concrete poles, steel poles, steel towers, and aluminum towers. The total cost of the installed facilities normally determines the type of structure used. Lines built with wood-pole structures have generally proved to be the most economical where adequate physical and electrical requirements could be satisfied. The American National Standards Institute's publication entitled Specifications and Dimensions for Wood Poles (ANSI 05.1) provides additional detailed information for wood-pole structures.

Wood-Pole Types. Southern yellow pine, western red cedar, and Douglas fir are the most commonly used species of trees used for wood poles in the United States. Western larch trees are used for some poles in the western states. Jack pine poles are frequently used in Canada. The type of poles used is determined by the physical requirements of the poles needed to construct the line and the cost of the poles as a result of the shipping expense from the location where the species is grown. The suppliers of the poles usually shave and...