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 (Rho) 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."
As many engineers who
have used "90" as a safe and typical rho value have
discovered, the NEC is simply wrong. Because there is no "average
soil," wet or dry, these numbers are essentially meaningless.
90: Not the Magic Number
Forty years of soil thermal research shows that:
- Soil and rock rho values
actually vary from 10 to 1000 C 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.
“Many engineers who have used 90 as a safe and typical rho value have discovered that the NEC is simply wrong.”
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