Alternator Ground Connection


I have recently purchased my first boat, a 59' retired commercial steel trawler, that I am preparing to completely rewire. I just came across your videos about boat wiring and have been eagerly watching them. In the last video, Jeff was talking about the potential for the battery connection at the alternator going to ground on the engine block. I thought the alternator ground terminal and all other grounds were isolated from the boat structure - hence the need for ground wires to each load. That lead me to believe that an accidental connection of a battery positive to the engine or hull structure would not be a short to battery ground. The starter also seems to have an isolated ground post connected directly to battery negative. Am I missing something?  - Jeff


Hi Jeff, you bring up lots of good questions. On fiberglass boats, most alternators have a non-isolated ground, meaning there is no negative cable on the alternator, only a positive, and the alternator uses the negative cable on the engine block as a return to the starting battery. Some alternators, albeit rare but gaining popularity, have an isolated ground (i.e. negative) post on the alternator that requires a negative cable. An isolated ground post on a alternator is preferable.  - Jeff

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