The thermal resistivity of the soil has a significant impact on cable ampacity of buried cables, especially when directly buried without backfill. The lower the thermal resistivity, the higher the cable ampacity for the same conductor temperature $theta_c$.

If the thermal resistivity of soil is unknown, the values from the reference operating conditions in IEC 60287-3-1 are recommended. Or else, the IEEE Standard 442 provides values for some materials and a paper by O.E. Gouda from 2010 lists values for some tested type of soils.

Symbol
$\rho_{\mathrm{4}}$
Unit
K.m/W
Used in
$d_{\mathrm{im}}$
$\delta _{\mathrm{soil}}$
$\Delta T_{\mathrm{4}}$
$\Delta \theta_{\mathrm{e_{\mathrm{t}}}}$
$\Delta \theta_{\mathrm{kp}}$
$H_{\mathrm{tc}}$
$T_{\mathrm{4db}}$
$T_{\mathrm{4et}}$
$T_{\mathrm{tr}}$