Thermal Resistivity Probe Comparison: Does Size Matter?
Objective: Properly size underground cables, DC and medium voltage AC
Methods: Sizing cables requires accurate soil thermal resistivity values, but these can vary depending on the measurement method even while complying with the ASTM D5334 standard. The in-situ soil thermal resistivity was measured using 4 thermal resistivity measurement probes at various times during the year of the same soil. Analysis was performed for both the heating and cooling curves. Two probes measured similar resistivities while the other two probes measured higher resistivities. Lab soil thermal resistivity measurements were also performed.
Results: The consequences of using a particular measurement in underground cable ampacity calculations are illustrated with an example ampacity calculation.
Conclusion: It is proposed that cable ampacity calculations should use the upper limit of the 95th confidence interval from the resistivity measurements.