Embedded Concrete RHO 2

[JGoldberg]

I'm trying to calculate power cable sizing for an underground ductbank in which the RHO of the soil is known to be 120.

The ductbank is currently being designed hence, the RHO of the concrete in unknown.

NEC Annex B.310.15(B)(2) states that concrete = 55 RHO typically. It also says soil RHO = 90 for 90% of the USA.

Annex B, FPN Figures B.310.3, .4, .5, all show RHO Concrete = RHO Soil - 5.
If RHO concrete must be 5 less than RHO soil, then how can B.310.15(B)(2) be correct?

My question is; if I know my soil to be RHO = 120, I should assume the ductbank concrete will be 55 or 115?

Thanks,

Jeff

 

[dpc]

I don't think there is any linkage between the thermal characteristics of concrete, which I suspect will be fairly consistent, and native soil, which can have an extremely wide range of values.

I'd use your local data for soil and the NEC data for concrete (55), in the absence of something better.

I'm not sure what the justification is for assuming the rho of concrete equal rho of the soil minus 5 in the referenced figures. The intent of these figures is to provide adjustment factors for variations in ambient temperature, so maybe this is just a way of reducing the number of variables.

 

[cuky2000]

This example should may not be used to determine the concrete thermal resistivity. The concrete Rho is a property of the material also influenced by the environment conditions.

Although cable ampacity rating for duct bank encased in concrete, is frequently based on Rho= of 55 cm-oC/W, the thermal resistivity of the concrete can vary considerable with the density, porosity, and moisture content of the material and also with its absolute temperature.

Even thought there are wide varieties in the quality of duct bank concrete and different rebar arrangement, the soil thermal resistance is the dominant factor on the overall thermal resistivity of the combined medium for underground duct bank.

Q1- If RHO concrete must be 5 less than RHO soil, then how can B.310.15(B)(2) be correct? Rho can varies significantly with the type of mortar. For instance, structural concrete ( ~145 pcf) or light structural concrete (~110 pcf) can have thermal resistivity at standard tested conditions around 45 cm-oC/W and 170 cm-oC/W respectively.

Q2 - My question is; if I know my soil to be RHO = 120, I should assume the ductbank concrete will be 55 or 115? One visual or practical way to estimate the Rho, consider to determine the concrete characteristic (unit weight, structural type) and the environment conditions. Moisture is an important factor since concrete could retain moisture for long time does to its hygroscopic property.
Rainfall, water table, type of soil, wetland, etc will help to define the expected moisture level. For dry desert areas, high altitude, soil without moisture retention (sand, rock, etc) considers high Rho.