There will be heat transfer out of the duct bank radially, in a plane perpendicular to the direction of the ducts. For an infinite length duct bank, this is the only direction of heat loss. Conduction along the wires is generally ignored. Since the heat produced is the same per unit of duct length at all points, this means that the temperature of the wire will not depend on the length of the bank.
For a finite length duct bank, the vector of heat loss near the ends will have a longitudinal (parallel to ducts) component and some heat will transfer into the soil beyond the ends of the duct bank. This will allow for slightly higher ampacity, but only for lengths which are comparable to the cross section dimension of the bank. In practice this is generally neglegible. Instead of using the simple two dimensional Neher-McGrath formulation, a three dimensional differential equation needs to be solved or approximated numerically.
The SKM modelling program may be asking for the length just to determine the voltage drop rather than the ampacity. Or it may be trying to include the very short bank case.
