The availability of accurate thermal conductivity data for all types of materials used inside a reactor for SiC bulk growth is the basis of a realistic numerical heat transfer model as a tool for further process optimizations. Measurements of the thermal conductivity of synthesized SiC powder as a possible source material for SiC bulk growth are reported as a function of pressure and temperature. For the extrapolation of experimental data to high temperature regimes not accessible by measurements, a theoretical model was developed. Based on the structural analysis of the powder and without introducing fitting parameters good agreement between theory and experiment was found.