The increased utilization of power grid requires operating power cables close to their thermal limit. Buried power cables in such condition experience thermal instability where thermal resistance increases as the moisture migrates away from the proximity of the cable forming a dry-out zone. While it is common to use a two-zone model to account for the dry-out zone in ampacity calculations, there is a limited number of studies on characterizing backfill materials for their critical temperature rise ( Δθx ), especially for crushed rock sands. In addition, dependency of the Δθx on moisture content is sometimes not acknowledged. This study measures the Δθx for three typical backfill materials and investigates the relationship between Δθx and moisture content. The results show that crushed rock sand has higher Δθx compared to natural sand. Overall, large range of Δθx was measured depending on moisture content and type of soil. Ampacity calculations with the established Δθx show that at low moisture content, the thermal resistivity of the soil has a higher influence on ampacity than Δθx . At relatively high moisture content, the Δθx becomes the predominant factor governing the overall ampacity.