Final answer:
The National Electric Code typically requires direct buried cables with GFCI protection at 120v, 20 amperes to be at least 12 inches below grade in a residential setting.
Step-by-step explanation:
For a residential location installing direct buried cables that are GFCI protected at 120 volts and 20 amperes, the National Electric Code (NEC) requires the cables to be buried at a minimum depth depending on a few factors including whether the wiring is residential and the type of protection applied to the cables. However, as a general guide, a typical residential branch circuit requires burying the cable at least 12 inches below grade if it is not under a concrete slab and it is GFCI protected. It is important to consult the latest version of the NEC or a professional electrician to ensure compliance with all local codes and regulations, as they may vary by location.