When designing electrical power systems, it is often necessary to determine underground cable ampacity. Various methods are in use today, including computer simulation, ampacity tables, and a method that has recently been suggested that includes the effects of moisture migration through soil. Each of these methods can yield substantially different ampacity results for the same installation. Regardless of the method, using the correct value of soil thermal resistivity is critical and using the wrong value can result in cables that are incorrectly sized. This paper examines several commonly used methods and their underlying assumptions. Examples are provided to illustrate the differences in the results obtained from various methods and the consequences of using incorrect assumptions. It is hoped that these examples will provide guidance on the implementation of each method.