If the electrical system to be installed operates at a potential higher than 600 volts, the nonmetallic conduit must be
A. installed in not less than 36 inches (900 mm) of earth.
B. installed at a depth of at least 18 inches (460 mm) of concrete.
C. installed at a depth of 24 inches (60 mm) deep and cinder filled.
D. encased in not less than 2 inches (50 mm) of concrete.
Answer: D
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