The size of battery-charging equipment has grown to keep pace with larger and more sophisticated batteries. As charger output current has risen, cable sizes have increased in order to limit voltage drops or overheated conductors. This has led to direct-current cabling. It carries the charging current, which is much larger than the alternating-current conductors that operate the charger. This mismatch could pose a fire hazard if the AC grounding wire on the charger case — the green one — had to carry DC fault current home.
Fault current could result if wire chafe or an internal DC connection occurred and caused a short circuit to the case. DC current in the charger’s case would want to “go home” any way it could. The pathway for the fault current is possible because American Boat & Yacht Council standards require a safety connection at the electrical panel between AC grounding and DC negative. This connection must be maintained for shock hazard safety reasons, in case of potentially lethal AC voltages appearing on the DC wiring.
Because the much smaller green grounding wire that runs with the hot and neutral supply wiring, potentially carrying large DC current, could easily overheat, we have a defined standard for installation. The installer must provide a DC grounding conductor from the charger’s case — this applies to inverters, as well — that directly runs to the DC negative bus. It has to be able to carry fault current from the largest supply conductor (DC cable to battery), so it is sized to safely match ampacity.
The positive, or hot, 12-volt conductors in a multiple-output charger may carry their rated currents, but remember that the ampacity of the negative conductor should be the sum of the combined output (charging) current. This negative conductor terminates internally at the charger and is isolated from the case.
Roger Hellyar-Brook runs a marine consulting business, repairing and upgrading boats of all types. He has spent more than 40 years in the marine industry and is the former manager of the systems program at The Landing School in Arundel, Maine.
Paul Mirto is a digital illustrator, longtime boater and former Coast Guardsman. mirtoart.com
This article originally appeared in the June 2018 issue.