Pole classes are numbered from 1 to 10, with 1 being the strongest. Wood poles are used mostly in distribution systems and light-duty transmission lines. The class of pole used depends on what the pole is used for. The length of pole used is determined, in part, by the clearances required for the voltage of the circuits on the poles, the number of circuits, and the location of the pole in relation to streets, railroads, buildings, and so forth. Clearances are also required to provide safe working conditions for linemen working on the lines.
Engineers also consider local conditions when determining the length of poles.
For example, poles located in densely popular high-traffic areas need to be higher than those located in sparsely populated rural areas. Concrete poles are preferred where the life of wood poles is shortened by local conditions. Concrete poles may be solid or hollow. Solid concrete poles are made in a trough form with steel reinforcing rods running lengthwise. The hollow type of pole is made by placing the concrete and reinforcing rods into a cylinder of the desired length and taper and then revolving the cylinder in a lathelike machine.