Nautical equivalent of “Old Sparky”

Hi Kevin,

I found your boat wiring website and would be grateful if you might comment on the following for installation of marine electrical systems in an aluminium hull boat. Its a complicated issue I know!

  • AC system includes generator, inverter and marine electrical shore power. The AC system is entirely floating,  i.e., no negative grounding and is interfaced with shore connections via an isolation transformer.
  • DC system includes house battery, engine alternator (isolated negative) and engine battery. All are grounded to one common negative pole on the hull. No other equipment is negative grounded.

I have been told that it would be better to float the DC system entirely as well (same as AC) so that there is no negative ground at all. Is this correct?Shock hazard



Hi Al,

Unless somebody went through and removed the AC/DC ground connection, it should still be intact and must remain. A boat without this connection is an extreme hazard. With an AC fault to the hull, there is no low resistance path back to ground to trip the breaker and the boat will turn into a very large electric chair.

On your boat generator, the AC ground and AC neutral are connected together on the metal engine block of the generator. The generator negative battery cable is also connected to the engine block and your DC negative system. An inventer will have the same AC neutral to AC ground connection. Your AC distribution panel should have a link between the AC ground (Green) and the DC negative on your boat.

Hull ground isolation is nearly impossible on an aluminum boat with an engine and unsafe on an aluminum boat with AC power. The engine and its underwater gear are inevitably electrically connected to the engine block and the DC negative. A positive hull connection to the DC negative gives a low resistance path back to AC ground and DC ground in the event of a fault. A DC fault to an un-grounded hull can create accelerated galvanic corrosion. The area with the positive cable connected to it will corrode to protect your engine and drive.

Hope this helps,