This article demonstrates the risks of doing DC wiring aboard a boat without understanding how the whole DC and AC electrical system work together. In particular, let us examine the chassis ground connection. Knowing about these situations is important so you can maintain your boat properly and also for your personal safety.
Refresher #1: Most AC devices have a safety ground, sometimes called a chassis ground. This chassis ground is a backup of sorts in a situation where the primary return path isn’t good or available. This chassis ground is not supposed to carry current.
Refresher #2: On boats, all grounds including DC, AC, RF, Bonding, etc. are tied one common point.
Situation: We recently came across this setup while working on a client’s boat. Someone had taken a dual battery setup (port / starboard) and added a third battery bank for an inverter. They moved the port alternator output to the inverter battery and added an ACR combiner between the inverter and port engine battery. None of the house loads got moved over (they were left on the engine batteries), and the chassis ground is too small for current standards. However, there is a bigger problem in this scenario.
Problem: The new inverter batter bank was incorrectly wired as a floating ground. The negative from the inverter bank was NOT tied back to the negative from the port or starboard engine batteries. Even though the alternator positive was connected to the inverter battery bank, there was NO negative return from the inverter battery bank to the engine.
At first glance, it appears the inverter bank batteries would never charge under engine power because there is no return path for the alternator. After a few minutes of thought and an understanding of the entire system, we found the return path. The 10AWG AC green ground wire joined the inverter to the ground bus near the AC panel. Also, the ground bus was connected to the DC negative as per ABYC requirements. To add to that, the 8AWG chassis ground joined the inverter to the inverter battery negative giving a full return path – one that was over 40 feet of mostly 10AWG – for a 100amp alternator. Not only was it charging slowly, but it is fortunate that the 10AWG wire did not melt and burn.
With full batteries ( i.e. low charge needed from the alternator) and the inverter/charger on float, the inverter chassis ground has 12 amps of current going through it, therefore completing the loop.