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Thermal Resistivity Probe Comparison

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  • Create Date June 28, 2021
  • Last Updated August 4, 2021

Thermal Resistivity Probe Comparison

Underground cable ampacity relies on an accurate measurement of soil thermal resistivity. IEEE Std. 42™ does not specify required dimensions rather it gives example probe designs. Since large probes are difficult to insert into the ground, many testing firms use small probes. Is the measured resistivity the same? The formula for determining resistivity is independent from diameter.


Probe sizes tested (L x Diameter):

  1. Large (standard) 1500 x 19mm
  2. Medium 480 x 6.4mm
  3. Small (lab) 100 x 2.4mm
  4. OEM TR-3 100 x 2.4mm

96 Tests Total: 47 Large, 28 Medium, 6 Small, 14 TR-3

Heat from 0.035 to 0.56 W/cm

Same soil measured at various times during the year.


  • Large and TR-3 probes measure identically (within the same 95% confidence limit)
  • Medium and small (user built) measure identically (within the same 95% confidence limit)
  • Smaller probes are more susceptible to air gaps in the soil (possibly due to rocks), but the results did not indicate this is a problem

Deviations between the two groups may be due to experimental error and soil conditions. Further experimentation is ongoing.

Attached Files

SPI Poster Thermal Resistivity Probe Comparison.pdf