Article 300 Wiring Methods
mikeholt.com: The intent of this editorial change to the NEC was to clarify that a warning ribbon is required for service conductors (exposed or in a raceway) buried more than 18 inches. Author’s Comment: Often service lateral conductors are installed in nonmetallic conduit (which must have 18 in. of cover [Table 300.5]) by the electrical contractor; therefore, a warning ribbon must be placed at least 12 in. above the underground raceway. 
