How is Ampacity calculations as per IEEE 738?

What is IEEE-738?

IEEE-738 is a widely used and accepted method of calculating the ampacity (thermal rating) of transmission conductors under specified weather conditions. Conductor parameters and major weather conditions are input into the method. Major weather conditions affecting rating include air temperature, wind speed/direction, and solar input, although there are some other minor effects. Both steady-state and short-duration ratings can be calculated.

IEEE-738 is based on both experimental data and theoretical equations. The full name of the standard is, "IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors". The standard may be purchased from IEEE (Institute of Electrical and Electronics Engineers, Inc.) at

How to Calculate a Thermal Rating

An equation to calculate conductor rating is developed by first recognizing that the total input heat (per unit length) to a conductor must equal the total output heat in the steady state. The conductor is heated by ohmic losses (I2R) and solar input, and it is cooled by convection and radiation.

The following equation and figure shows this main "heat balance":

Qsolar + I2R = Qconvection + Qradiation

where

• Qsolar is the heat input due to solar radiation, W/ft
• I2R is the heat input due to line current, W/ft (R is a function of conductor temperature)
• Qconvection is the heat output due to convection (a function of wind, air temp, conductor temp), W/ft
• Qradiation is the heat output due to radiation (a function of air temp and conductor temp), W/ft

Heating and Cooling of an Overhead Conductor

Now, the equation can be reworked to solve for current as a function of the weather conditions. The Qconvection, Qradiation, Qsolar, and R terms are all functions of weather conditions and of conductor temperature. If the weather conditions are measured and the conductor temperature is set to the maximum allowable conductor temperature,the calculated current is the rating current as shown in the following equation.