Neher McGrath Calc UG Ductbanks

[RonShap]

For underground ductbank calculations for ampacities, we typically use a rho of 55 for concrete and 90 for soil.

We concrete encase our ductbanks, then cover with soil up to grade.

We have the contractor send a submittal with lab tested sample / typical material that will be used around the duct banks for installation.

An issue that always comes up is what is the moisture content that should be used for comparison to the calculation. The surrounding natural material has never been exposed to an occupancy as it will after the building will be built, so it is left up to the geotech and us (Electrical) to arm wrestle it out.

After compaction and no vapor barrier, it would seem that the heat emitted off the ductbank itself over time will dry the concrete and soil, so we use 2 or 3%. Since the ductbanks will be typically 24-36" deep, the geotech suggests 10% moisture because he feels only the top layer of soil will vary in those dryer levels and the lower layers will remain at 10% continuously.

What numbers have you used?

Ampcalc software only uses a single homogeneous value for each material.

 

[7anoter4]

IEC 60287-2-1/2006 Table 1 – Thermal resistivities of materials recommends 1 Km/W [100 oC.cm/W] for concrete.
IEEE 835/1994 3.1.4.2 Duct banks Concrete: 60 °C cm/W
3.1.4.1 Earth thermal resistivity: 60,90,120
The Calculation of the Temperature Rise and Load Capability of Cable Systems by J.H.Neher and M.H.Grath [October 1957]
Table VI. Thermal Resistivity of Various Materials : concrete 85 c.cm/W.
NEC Annex B B.310.15(B)(2) Typical Applications Covered by Tables.
Concrete 55 Rho ; Average soil (90 percent of USA) = 90