Rho Values
Rho Values
Thermal resistivity, often denoted by the symbol , is a measure of a material’s ability to resist the flow of heat. The thermal resistivity of a material is typically expressed in units of K.cm/W (Kelvin centimeter per watt) or °C.cm/W (degrees Celsius centimeter per watt), depending on the unit system used.
Thermal resistivity affects the ability of the duct bank to dissipate heat generated by the electrical cables within. When cables carry current, they generate heat due to resistance. If this heat is not adequately dissipated, it can cause the cables to operate at higher temperatures. Higher cable temperatures can reduce the overall ampacity of the cables, as excessive heat can degrade insulation and other components, ultimately limiting the current-carrying capacity.
In the context of direct buried ducts and duct banks, materials with lower Rho values dissipate heat better and thus enables higher current capacity for a given cable size.
Typical Rho Values:
| Material | Rho Values |
|---|---|
| Engineered Backfill | <55 |
| Concrete | 55 |
| Damp Soil | 60 |
| Uniform Crushed Limestone | 70-80 |
| Average Soil | 90 |
| Dry Soil | 120 |
| Water | 165 |
| Polyethylene (PE) | 450 |
| Rubber | 500 |
| Paper Insulation | 550 |
| PVC | 650 |
| Air | 4500 |
Notes:
- “Average Soil”: While a Rho value is 90 is often quoted by code books as the “average soil” for majority of the US, it is highly recommended to do site specific testing for a more accurate value of Rho as independent tests have shown great variation in Rho values from region to region. This testing can be done with other Geotech tests to minimize costs.
- Moisture Content: Thermal resistivity decreases with increase in moisture content of the soil. Water provides a thermal path for heat to escape via a mechanism known as thermal bridging. It should also be noted that heat from a cable can dry up moisture thus increasing the thermal resistivity of the adjacent soil. This should be taken into consideration when designing such underground raceways.
- Compaction: Poor or improper soil compaction leads to increased air pockets between the soil particles. These air pockets or gaps interrupt thermal bridges, resulting in higher overall heat resistivity.