The thermal resistance of the earth surrounding a ductbank depends heavily on soil resistivity (ρ). IEEE Std 835 provides typical values that are often used when detailed geotechnical data is unavailable.
Typical resistivity values:
- 60 °C·cm/W – damp soil or clay
- 90 °C·cm/W – average native soil
- 120 °C·cm/W – dry sand
- 150 °C·cm/W – dry sand and gravel
Values outside this range may be encountered in unusual conditions. The ductbank route calculator accepts custom inputs but will warn when they fall outside the typical IEEE range.