Borrowers are cautioned that there should not be a direct substitution of wood poles with steel poles or concrete poles if the original line design was based on Grade C construction. Most manufacturers standardize on steel or concrete poles such that the poles have a wood pole equivalency based on Grade B requirements in the NESC. Table 1 below summarizes the loads used to design poles:

Table 1

An example will be used to demonstrate the confusion when using standard class steel or concrete poles in distribution lines.

A cooperative wishes to build a distribution line meeting Grade C construction. The design calls for 40-4 wood poles but decides in one section to use steel distribution poles. The transverse working load (load without overload factors, [OLF]) is 1200 lbs. two feet (.6 m) from the top of the wood pole.² The common practice is to substitute a standard class steel pole, designated as a 40-4, for the wood pole. From Table 1, the ultimate load for the equivalent steel pole would be 1500 lbs.

But, this is where the problem occurs. The ‘ultimate’ load, to which the steel pole needs to be designed, should be: 1200 lbs. (working load) x 2.2 (OLF³) = 2640 lbs.

In the table above, the 2640 lbs. ‘ultimate’ load corresponds to a ‘class 1’ steel pole.

If a 40-4 steel pole with a tip load of 1500 lbs. were selected, it would be under the required NESC strength by approximately 75 percent. This is where the potential confusion comes into play.

If you would like more information or have any questions, please call H. Robert Lash, Chief, Transmission Branch, at (202) 720-0486 or Harvey Bowles, Chief, Distribution Branch, at (202) 720-1979.

³ Overload factor for steel poles not at a crossing, Grade C construction.