I worked for a city in WI, west central WI. The city requires water mains to be buried minimum 7' deep to keep the lines from freezing. One year, they must have had a tiny leak in a mainline. The ground got saturated and that frost went deep! The watermain was 9' down and it froze and burst - in mid-February. What a mess to fix a busted watermain in negative degree temps. I felt bad for the utility guys.
As a kid we never had a water line that was less than 3 ft deep and most were 4 ft down. That was in Ct. Freezing and breaking a water line in the "sub zero" cold north is a real headache as @wyoDreamer mentioned. Here in the mountainous area west of the Blue Ridge mtns, I think they require 30 " but I would never want a line to be less than 3 ft down. Better deep than not deep enough.
When I moved to Cheyenne, Wyoming I was amazed to find out they only buried their waterlines 2.5 feet. After living there for 6 years, I realized they barely got any frost in the ground at all. It would snowstorm for 3 days and we would 6' snow drifts covering the roads and driveways - and then 2 days later we would have 50 degree temps for 3 days.