Any time that a wire will be installed through any thing you should use chafe protection. That is the only function of the grommet. This would apply to passing the wire / cable through either your pedestal, or the pedestal guard.
If you are concerned about waterproofing, get one of these;
BTW - these are called Deck Glands
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USCG Licensed OUPV Captain, ASA 101/103/104/105 Certified Instructor - Also certified in Recreational Marine Electrical Systems
Last edited by eherlihy; 05-18-2010 at 04:15 PM.