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08 / Thermal
Soil thermal resistivity testing
How quickly the ground carries heat away decides how much current a buried cable can carry. Assume it, and the feeder is either undersized or paid for twice.
Why anybody measures it
A buried cable loses heat into the soil around it. If that heat cannot escape fast enough the conductor runs hot, insulation life falls, and in the worst case the soil around the cable dries out — which raises its thermal resistivity further, which makes the cable hotter still. That runaway is the failure mode ampacity design exists to prevent.
The governing number is thermal resistivity, in °C·cm/W. Design standards allow a default to be assumed, and that default is deliberately pessimistic. On a large feeder or a long duct bank, measuring the real value routinely changes the conductor size — in either direction.
How it is measured
To IEEE 442, using a thermal needle probe: a line heat source is inserted into the soil, a known power applied, and the rate at which temperature rises is recorded. The slope of that rise against the logarithm of time gives thermal resistivity directly.
We test in situ at cable and duct-bank depth rather than only on recovered samples, because a disturbed sample no longer has the density and structure that governs the answer.
The dry-out curve is the real deliverable
Thermal resistivity is dominated by moisture content, and moisture content is not constant. A single wet-season value is close to useless for design.
What the designer needs is a dry-out curve — resistivity measured across a range of moisture contents down to the driest credible state — so the cable can be sized against the worst case rather than the day we happened to visit. Where engineered thermal backfill is proposed, we test that too, since a specified backfill that is not achieving its design value in the trench is a common and expensive discovery.
Where it fits
Data centers, substations, utility-scale solar and wind collection systems, and any long feeder route. On large-footprint projects it usually runs at the same visit as seismic site classification, since both are required early and both are non-intrusive.
Equipment and capability
- Thermal needle probe to IEEE 442
- In situ at cable and duct-bank depth
- Dry-out curves across moisture content
- Native soil and engineered thermal backfill
- Feeder routes, duct banks, substations
- Reported in °C·cm/W and K·m/W
What you receive
Measured resistivity at each location and depth, dry-out curves, moisture and density at test, and a recommended design value stated in the units your electrical engineer's ampacity calculation actually takes.
Ask before you commission
Tell us the target, the ground and the constraint and we will say whether this method answers it, whether it needs a second method alongside, or whether something else entirely is the better route. That conversation is free and it is usually short.