IEEE 738-2006

The purpose of this standard is to present a method of calculating the current-temperature relationship of bare overhead conductors. Conductor surface temperatures are a function of the following: a) Conductor material properties b) Conductor diameter c) Conductor surface conditions d) Ambient weather conditions e) Conductor electrical current The first two of these properties are specific chemical and physical properties. The third may vary with time and be dependent upon ambient atmospheric conditions other than weather. The fourth, weather, varies greatly with the hour and season. The fifth, conductor electrical current, may be constant or may vary with power system loading, generation dispatch, and other factors. The equations relating electrical current to conductor temperature may be used in either of the following two ways: -- To calculate the conductor temperature when the electrical current is known -- To calculate the current that yields a given maximum allowable conductor temperature For the purposes of this standard, either the electrical current is assumed constant for all time or it is assumed to undergo a step change from an initial current to a final current. The ambient weather conditions are assumed to be constant with time in both the steady-state and transient calculation methods described in this standard. This standard includes mathematical methods and indicates sources of the values to be used in the calculation of conductor temperatures and conductor thermal ratings. However, because there is a great diversity of weather conditions and operating circumstances for which conductor temperatures and/or thermal ratings must be calculated, the standard does not undertake to list actual temperature-current relationships for specific conductors or weather conditions. Each user must make their own assessment of which weather data and conductor characteristics best pertain to their area or particular transmission line. The calculation methods in this standard are also valid for the calculation of conductor temperature under fault conditions.