AC Shore Power Systems, Onboard AC Generation, and Ground Fault Protection
Core concepts
Shore power is the highest-energy system most boaters touch. A 30 A, 120 V inlet provides up to 3,600 W; a 50 A, 125/250 V inlet up to 12,000 W. The shore cable, inlet, main breaker, and panel must match. On board, the AC safety ground (green) and neutral (white) are kept separate; the neutral-ground bond exists only at the source-shore pedestal, generator, or inverter-not at the boat's panel. Reverse-polarity protection, an ELCI (equipment leakage circuit interrupter, typically 30 mA), and GFCI receptacles (about 5 mA) are layered defenses. An isolation transformer or galvanic isolator blocks low-voltage galvanic current on the shore ground. Transfer switches must switch both hot and neutral and prevent shore and generator sources from being connected together.
Traps
The classic trap is bonding neutral to ground on board, which can energize the underwater bonding system and defeat GFCIs. Other traps include using a household extension cord, ignoring reversed polarity, assuming a GFCI replaces an ELCI, and thinking an isolation transformer and a galvanic isolator are the same. Also watch for bootleg grounds and missing ground pins.
Worked example
A 30 A, 120 V shore connection supplies 3,600 VA. If the boat has a 12,000 BTU air conditioner drawing 10 A, a 1,000 W water heater drawing about 8.3 A, and 5 A of miscellaneous loads, the total is 23.3 A-within 30 A, but the 80% continuous rule gives 24 A, so it is marginal. Rule of thumb: keep continuous shore load at or below 80% of the breaker rating, and never parallel two shore cords.