All such protection shall extend at least 450 mm (18 in.) into the trench from finished grade.

FPN: See 300.5 and Article 340 for conductors or Type
UF cable used underground or in direct burial in earth.

551.81 Receptacles. A receptacle to supply electric power to a recreational vehicle shall be one of the configurations shown in Figure 551.46(C) in the following ratings:

(1) 50-ampere — 125/250-volt, 50-ampere, 3-pole, 4- wire grounding type for 120/240-volt systems

(2) 30-ampere — 125-volt, 30-ampere, 2-pole, 3- wire grounding type for 120-volt systems

(3) 20-ampere — 125-volt, 20-ampere, 2-pole, 3- wire grounding type for 120-volt systems

FPN: Complete details of these configurations can be found in ANSI/NEMA WD 6-2002, National Electrical Manufacturers Association's Standard for Dimensions of Attachment Plugs and Receptacles, Figures 14-50, TT, and 5-20.