Here is the code NEC article 3005 for burial depth of low voltage wiring.
Landscape lighting wire depth. According to the National Electrical Code the wiring necessary for this lighting must be installed at minimum 18 depth. For example 12 gauge wire is thicker than 14 gauge wire. Wire needs to be buried a minimum of 6 inches deep and 24 inches deep under driveways and sidewalks.
Otherwise also use 102. 2-3 directional accent lights are often needed to illuminate the pole and the entire flag properly. Use 10-gauge wire for the main lines from the transformer to where the lights begin then switch to 12-gauge wire between the lights.
Remember to take your time and plan your lighting with care. But you are to install them correctly selecting a suitable wire diameter. -All locations not specified below - 6 inches.
Deep so they wont be damaged if the lawn is aerated. -Under a building - In raceway only. Voltage Drop Equation For Landscape Lighting.
-Under roads alleys public driveways and parking lots - 24 inches. If the wire run consists of more than 100 watts of load you will definitely need to use the 102. One of the biggest remodeling trends in landscaping today is the installation of outdoor living spaces.
Total watts on low voltage wire length of wire run in feet wire size constant listed in chart below VOLTAGE DROP. Unless covered by concrete or installed directly under a building the minimum cover depth in column 5 is six inches. The pole height and flag size are key.

