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Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Simple AC Circuits, schematics.

Below is a very simple basic AC circuit diagram of a simple electrical circuit you would find on a boat. These types of diagrams are called schematics.  The power comes in from the shore,  through the power cord, on the left side of the diagram.  In this case I have shown a 220 VAC 50 amp cord.  We know this because it is a four wire 220 VAC circuit. If this were only for 120 volt AC then it would only have three wires, Black, White and Green.

It goes from the cord through the shore power plug to the main breaker which should be in a panel box.  It then goes out to the boat and splits into branch circuits each which has its' own circuit breaker. This splitting would probably be actually done in the main panel box by using buss bars for each branch circuit and then running wire from the buss bars out to the equipment. That is about as simple as it gets.

 AC Circuits

Some things have been left out.  If we want to protect the boat from galvanic corrosion, there would be a  galvanic isolator in the green line where it enters the boat, probably next to the main breaker panel box. (ABYC E-11. 7 and A-28 Galvanic Isolators)

AC Circuits with Galvanic Isolator

To protect people from shock hazards such as ground faults or reverse polarity there would be a GFCI included in the outlet.  To protect against reverse polarity there would be a polarity indicator at the main breaker panel wired between the black and the white wire. 

ABYC recommends that the main breaker include either an ELCI (Equipment Leakage Circuit Interrupter) or an RDI (Residual Current Device). ABYC AC and DC Electrical Systems On Boats E-11.11.1.  These devices prevent AC current from leaking into the waters around the boat. See

Also I have stopped the red wire, that brings in 220 VAC, at the main breaker.  However, it would be available in case of electrical equipment that needs 220 VAC, such as a washer and drier. Many boats don't have a 50 amp system.  Most have only a 30 amp 120 VAC system.  This will run almost all AC appliances, even such items that draw a lot of current such as microwaves and air conditioners.

AC Circuits

The next  schematic adds a polarization transformer. This effectively isolates the onboard system  from the shore system and at the same time protects against reverse polarity.

 You could also use an isolation transformer.  The wiring to the transformer would be slightly different. Here the input to the transformer is 220 VAC so a 50 amp cord is necessary. The output is 120VAC.  However you could very easily tap off a 220 VAC circuit as well. Beyond the transformer itself the circuits are the same.

ABYC E-11.7 Isolation of Galvanic Currents.

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Wiring Your Boat 2007 All rights reserved. revised 04/27/2021