The article refers to the problem of calculating the effective thermal conductivity k_ef of a steel porous charge. In proposed approach for each heat transfer mechanism, which occurs in the considered medium, the corresponding thermal resistance is assigned. The model values of the k_ef coefficient were determined twice for the same input data (geometric dimensions, emissivity, temperature, thermal conductivity of steel and gas). The difference between successive calculations depended on the use of two different methods to determine the thermal radiation resistance R_rd. In the first approach the resistance R_rd was calculated using the exact method and in the second approach the simplified, approximate method was used. The discrepancy between the obtained results of k_ef in both approaches provides the evidence to use the approximate method to determine the resistance R_rd. A bundle of square steel sections were used to demonstrate the challenge. In the exact method, data regarding the temperature distribution within a single profile was used. These data were obtained based on experimental research using a guarded hot plate apparatus. The calculations were performed using a temperature range between 200ºC and 800ºC for two sections: 60×60mm and 80×80mm and three emissivities: 0.5, 0.7 and 0.9.