The following information was taken directly out of the 1999 National Electrical Code Book (an updated version of the code book is released every 3 years)
There are 5 different ways to run underground cable for your application
1)Direct Burial Cables or Conductors - 18" deep
2)Rigid Metal Conduit or Intermediate Metal Conduit - 18" deep
3)Nonmetallic Raceways Listed for Direct Burial Without Concrete Encasement or Other Approved Raceways - 18" deep
4)Residential Branch Circuits Rated 120 Volts or Less with GFCI Protection and Maximum Overcurrent Protection of 20 Amperes - 12" deep
5)ircuits for Control of Irrigation and Landscape Lighting Limited to Not More than 30 Volts and Installed with Type UF or in Other Identified Cable or Raceway - 18" deep
article 350(d) Protection from Damage. Direct-buried conductors and cables emerging from the ground shall be protected by enclosures or raceways extending from the minimum cover distance required by Section 300-5(a) below grade to a point at least 8 ft (2.44 m) above finished grade. In no case shall the protection be required to exceed 18 in. (457 mm) below finished grade.
(f) Backfill. Backfill that contains large rocks, paving materials, cinders, large or sharply angular substances, or corrosive material shall not be placed in an excavation where materials may damage raceways, cables, or other substructures or prevent adequate compaction of fill or contribute to corrosion of raceways, cables, or other substructures.
[This message has been edited by Joe Tedesco (edited September 24, 2002).]