Thermal · Cable design
The assumed 90, and the two ways it is wrong
Almost every buried cable in the country is sized against a thermal resistivity nobody measured. Sometimes that costs money. Sometimes it costs the cable.
Ask an electrical designer what soil thermal resistivity they used and the answer is usually 90 °C·cm/W. Ask where the 90 came from and the answer is that it is what everybody uses. That is true, and it is standard practice, and on a great many sites it is perfectly defensible. It is also an assumption carrying a conductor size, and assumptions that carry conductor sizes are worth examining.
Where the number comes from
Ampacity tables have to assume something about the ground when no site data exists, and the figures in general circulation cluster tightly: roughly 60 for damp soil, 90 for average soil, 120 for dry. Ninety sits in the middle and has become the default of record. Published guidance describes it as conservative for most moist soils in the United States, which is a fair description of what it is for — a reasonable number for ground nobody has looked at.
The difficulty is that soil thermal resistivity across real sites spans something closer to 30 to 500. A single mid-range default sitting inside a range that wide is not conservative in any general sense. It is conservative against some ground and optimistic against other ground, and which one you have is a site question, not a national one.
Wrong in the direction that costs money
On a consistently moist, fine-grained site — much of the glaciated Midwest, a good deal of the East — the real figure may sit nearer 60 than 90. Designing to 90 on ground that performs at 60 means the conductor is carrying less current than the earth around it would allow.
On one feeder that is a rounding error. On a long duct bank run, a collection system across a utility-scale solar site, or a data center campus where the same detail repeats hundreds of times, it is a different quantity entirely. Published estimates put the ampacity available from properly characterising and engineering the thermal environment at something like 10 to 15 per cent, and occasionally around 30. That is conductor copper, trench width and installation labour, bought to cover a number nobody checked.
Wrong in the direction that costs the cable
The other failure is the serious one, and it is not a failure of the soil. It is a failure of the assumption that soil holds still.
A loaded cable heats the ground around it. Heat drives moisture away from the cable. Drier soil conducts heat worse, so the cable runs hotter, which drives off more moisture. That is thermal runaway, and it is the mechanism ampacity design exists to prevent. A sand that measures comfortably when damp can pass 150 as it dries and, in the driest state, go higher again — figures above 300 appear in the design literature. Against ground doing that, a design value of 90 is not conservative. It is the optimistic end of the range, chosen before the cable was energised.
Homogeneous sands are the usual offenders, along with loosely placed or poorly compacted backfill, which both holds less moisture and gives it up more readily. Arid sites never had the moisture the default assumes in the first place.
Why the boring log does not answer it
It is tempting to read the soil description off the geotechnical report and pick a value from a table. Thermal resistivity does not work that way, because the variable that dominates it is not what the soil is but how much water is in it and how firmly it is packed.
The same sand can sit near 70 saturated and well past 200 bone dry. Density matters nearly as much: the same material placed loose and placed compacted gives materially different answers, which is why a sample recovered, bagged and recompacted in a laboratory has lost the structure that governed the result. Testing in situ at the depth the cable will occupy is the only way to measure the material in the condition it will actually be in.
The curve, not the number
What a designer actually needs is not a value but a relationship: resistivity measured across a range of moisture contents, down to the driest state the site can credibly reach. That is the dry-out curve, and it is the deliverable that answers both failure modes at once. It shows the wet end, where ampacity is being left unused, and the dry end, where the runaway risk lives.
A single measurement taken in April tells you what the ground was doing in April.
Backfill as placed, not as specified
Where engineered thermal backfill is specified, the specification is a statement of intent. Guidance suggests a properly designed and installed thermal backfill should hold below about 100 dry, and good ones do considerably better. Whether the material in the trench is achieving that depends on what was delivered, how it was placed, and how well it was compacted on the day.
That is worth testing rather than assuming, for the same reason the native soil is. A backfill underperforming its specification is a common finding, and an expensive one to discover after energisation rather than before backfill closes.
What to ask for
Ask for in-situ testing at the design depth rather than laboratory values on recovered samples. Ask for a dry-out curve rather than a single figure. Ask that any engineered backfill be tested as placed. And ask what the measured dry end does to the ampacity calculation — because if the answer is that nothing changes, the measurement has confirmed the design, which is worth knowing too.