Thermal resistivity: real rho values for the professional power engineer

Thermal stability

Heat transfer in a porous medium like soil can be a complex process.  Heat is conducted through soil solids and water but is also transported as latent heat in the soil pores.  This makes the modeling of heat flow in soil both interesting and complex since it involves thermal and hydraulic processes.

Vapor movement across pores can carry substantial amounts of latent heat, but if the soil around the heat source isn’t wet enough for the water to move back and evaporate again, the soil at the heat source will dry out.  Soil drying around a heat source like a power cable can create the potential for thermal runaway which can lead to cable failure. Understanding a soil’s thermal stability can help power engineers more accurately design power distribution systems to prevent thermal runaway.

Accurate ampacity calculations depend on measured rho values

Appendix B of the National Electrical Code (B.310.15(B)(2)) states, “Typical values of thermal resistivity (rRho) are as follows:

  • Average soil (90 percent of USA) = 90
  • Damp soil (coastal areas, high water table) = 60
  • Very dry soil (rocky or sandy) = 120.”

However, as many engineers who have used “90” as a safe and typical rho value have discovered, the NEC is simply wrong. These numbers are essentially meaningless because there is no “average soil”, wet or dry.

90 is not the magic number

Forty years of soil thermal research shows that:

  • Soil and rock rho values actually vary from 10  to 1000 ℃ cm/W.
  • There is no “typical” value for 90% of soil types.
  • Thermal resistivity of porous materials like soil, rock, and concrete are not constants.
  • Resistivity changes with density, water content, and temperature of the soil or concrete.

Measure, don’t guess

Even in a well-designed underground cable system, the soil may account for half or more of the total thermal resistance. Soil and backfill thermal properties should not be assumed. These properties are relatively easy to measure in the field and in the laboratory. A safe, professional installation requires actual measurement and evaluation of thermal rho.

A rho value reported in a void can be misleading

If a soil thermal resistivity report only reads “Soil X has a thermal resistivity of XXX °C-cm/W”, seek clarification. What was the moisture content? How densely was it packed? Are there organics in the soil? Soil moisture, density, and soil makeup are critical factors in determining a soil’s thermal resistivity. Any reporting of thermal resistivity for the purpose of design should include moisture content and density data (see example). A physical description of the soil should also be included. The thermal dryout curve is the most comprehensive way to report soil thermal resistivity.

METER’s KD2 Pro complies with the ASTM D5334-14 standard

D5334-14 is a significantly updated version of the Standard Test Method for Determination of Thermal Conductivity of Soils and Rock by Thermal Needle Probe Procedure. It represents the best practices in accordance with current research in heat and mass transfer. For accurate measurements, it is important to specify and use the most current version of this standard. The specific elements of KD2 Pro compliance to ASTM D5334-14 are:

  • All needles have sufficient length-to-diameter ratio to simulate conditions for an infinitely long, infinitely thin heating source
  • Device includes linear heat source and temperature measuring element
  • Device produces constant current
  • Device reads voltage and current to nearest 0.01 V and ampere
  • Device measures time to the nearest 0.1 second
  • Accessories included are capable of drilling pilot hole with diameter and depth equal to the dimensions of the needle
  • Temperature decay with time is included in analysis to minimize effects of temperature drift
  • Microprocessor-based analytical methods comply with all specifications of ASTM D5334-14
  • Device is calibrated to ensure accurate measurements. Accuracy verification standard material is included

IEEE 442-1981 standard

The 442 standard includes a statement from the IEEE Standards Board. “Every IEEE Standard is subjected to review at least once every five years for revision or reaffirmation. When a document is more than five years old and has not been reaffirmed, it is reasonable to conclude that its contents, although still of some value, do not wholly reflect the present state of the art” (page ii, IEEE Std 442-1981).

The IEEE 442 standard was last affirmed in 2003. While the standards committee is considering updating this standard (which was last subject to thorough revision in 1981), it may be advisable to specify and follow ASTM 5334-14 which, due to its recent revision, better represents current state of the art in heat and mass transfer. Inaccuracies that may occur when explicitly following the field probe dimensions and probe heating times outlined in IEEE 442-1981 are outlined in KD2 Pro Compliance to ASTM and IEEE Standards.

The “Equipment Required for Field Measurements” section in the IEEE 442 standard was obviously written to enable an engineer to construct equipment out of available materials (stainless steel tubing, automotive batteries). The KD2 complies with all theoretical assumptions upon which IEEE 442-1981 is based but makes full use of technologically superior sensors and microprocessor-based analysis over the homemade probes and oversimplified pencil-and-paper analysis methods which were in common use in 1981.

Methods of soil analysis: part 4

Chapter five of the Soil Science Society of America (SSSA) Methods of Soil Analysis Part 4 addresses soil heat. The KD2 Pro probe needle sizes, heating times, accuracy specifications, and internal data analysis meet or exceed recommendations outlined in the SSSA methods.

Thermal properties testing

The KD2 Pro is a fully-portable field and lab thermal properties analyzer. It uses the transient line heat source method to measure thermal conductivity, resistivity, diffusivity, and specific heat.  Sophisticated data analysis is based on 40+ years of research experience on heat and mass transfer in soils and other porous materials.

Equipment rental

Want to do thermal properties testing but not quite ready to make the full investment? Consider renting the KD2 Pro to get the data you need.  Contact METER for pricing, availability, and rent-to-own details.

Lab services

Accurately measuring material thermal properties is easy with the KD2 Pro, but establishing an effective measurement protocol and carefully controlling important factors that affect thermal properties can be challenging and time consuming.

METER scientists have over 40 years of experience making high-quality thermal properties measurements.  We offer convenient thermal properties lab services. If you don’t have time or aren’t completely comfortable making the thermal properties measurements, our services could be perfect for you.

Contact METER for information on lab services.

Rho resources

Accurate readings at top speed

For over four decades, scientists and engineers have relied on our KD2 Pro for measuring thermal properties in almost anything—and we mean anything.