The trench shield, in particular, extends 18 inches (0.45 m) over the vertical section of the walls but below the trench's top. As a result, when the shield's top is below grade, it must extend at least 18 inches over the vertical component of the wall. This means that the top of the trench shield must be at least 6 feet above ground level. The minimum depth of a trench required to accommodate a cable or other conduit is determined by the height of the cable or other material you want to install. The depth must be deep enough to avoid damage from water that might enter the trench. Cables and other materials used for underground installation are designed to withstand exposure to moisture and other conditions present under ground. For cables up to 6 inches in diameter, a soil depth of between 3 and 4 feet is recommended. For cables up to 1-foot-in-diameter, a soil depth of between 6 and 9 inches is recommended. These recommendations apply whether or not conduits contain shielding or insulation. The need for depth depends on many factors such as soil type and condition, amount of traffic on the property, etc. For cables larger than 1 foot in diameter, the maximum permitted depth depends on the type of conduit used. For example, armored cable can be installed up to 12 feet deep while metal-sheathed cable cannot be installed deeper than 6 feet.