30" is the prescribed depth for water lines by City Building Department. However, with all things engineering, this is not an absolute answer. This depth is most likely based on a 50 year probability of reoccurance. So frost may go lower but it does not happen often and it probably would not be much lower than 30".
The frost line in Wichita Kansas is 36". That is what the guidelines are in KS that must be followed.
36"
The missouri river averages 75 ft deep, and its deepest point is only 100 ft deep.35 ft deep
Deep River - 2009 was released on: USA: 1 May 2009 (Kansas City, Missouri)
32"
The last question on the attached link. http://extension.missouri.edu/xplor/qa/agriculturalengineering0001.htm
no Frost
In western Pennsylvania the frost line falls at 36", when installing underground waterlines 42" is perfered.
The frost line in the city of Chicago is thought to be 36 inches. The average Chicago winter does not exceed the 36 inch frost line, but extremely cold winters can.
no Frost
The frost line in Salt lake city is 30 in. I would assume that that is average for most of the state.
3ft
i dont know whho cares Most concrete foundations are buried 4' to get below the frost line which is around 3'.
The frost depth refers to how deeply frost can penetrate into the ground. In Martinsville, Virginia, the frost depth is around 18 inches deep.