Modified values for geometric factor of external thermal resistance of cables in duct banks

The authors previously described a technique for calculating the geometric factor G/sub b/ for extended ranges of the height/width ratio of the duct bank or backfill that enabled the Neher-McGrath method to be applied to a wider range of cable configurations. However, the technique was based on the assumption that the duct bank surface represents an isothermal boundary, which can result in some errors in the derived G/sub b/-values. A finite-element-based technique is described for calculating modified values of G/sub b/ on the basis of the actual heat-transfer mechanism (constant total heat flux) around the duct bank surface. The technique was applied to several cable configurations, yielding more accurate values.>