Paulp,
1st,look at the top of Table 300.5 It says "Type of wiring method or circuit". The only one that fits a 240v direct bury circuit is column 1 "Direct burial cable or conductors". Then look on the left side titled "Location of wiring method or circuit". The only one that fits is the one titled "all locations not specifically listed below. It says 24".
What column and row are you trying to use that is giving you 18"?
