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Basic Electricity - Page 7 - AC Circuits

Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Intro to AC Circuits. Marine Cords, Polarity.

I suppose we all remember our mother telling us not to stick things in the wall socket, and not to put our fingers in the light socket. Hopefully we all learned from this. Because Alternating Current (AC) electricity is very common. Like gasoline, we use it all the time, and although it is dangerous, we become complacent. What we have in our homes is installed in a way to keep us safe from harm.  Alternating current systems are very dangerous. If you have the slightest doubt about your knowledge and ability to understand them and install them correctly, don't. Hire a professional.

In the following pages about onboard AC systems I am only going to deal with 120V Single Phase AC.   There are other systems, up to and including 120/240 Volt Three Phase. If you don't know what this means then see the next paragraph. 120V AC is often referred to as 110V or 115V.  I will always use 120V. 

I strongly recommend you hire a marine engineer, naval architect, or certified marine electrician to help you design your AC system for your boat, and have a certified marine electrician install it. By certified I mean, they have passed the ABYC certification course for marine electrical systems. I cannot emphasize this enough. An electrician may be very competent at his job and may have installed wiring in thousands of homes. But AC circuits on boats are NOT the same as AC on shore. There are significant differences. So make sure you hire someone who has passed the ABYC certification exam.

Direct Current will give you a tingle but AC will fry you in an instant. Do not fool around with it. It can kill people in the water near your boat and it can also burn your boat to the waterline, as can DC, so be careful and make sure it is installed correctly, to the latest ABYC or ISO Standards. 

Please forgive my attempts to scare you. I am a bit of a purist and frankly I would prefer to have a strictly DC system. But most boats over 25 feet today have AC systems on board. This is mainly for convenience because they can then use AC appliances and air conditioning. If installed and maintained properly AC can be safe and reliable. The key is installing it correctly and maintaining it properly.

Things that are most important in installing an AC System:

Picking the right service level, that is, 20 amp, 30 amp or 50 amp.

Picking the right shore power connection and cord for the boat.

Keeping the Black, White and Green wires separate and of correct polarity.

Sizing overcurrent protection. Picking the right amperage for Circuit Breakers.

Using Ground Fault Circuit Interruption (GFCI) to protect people.

Isolating the system to prevent galvanic corrosion.
See Corrosion On Boats. 

Preventing current leakage into the water or current impressed on a metal hull. This results in Stray Current Corrosion and can cause Electric shock deaths to anyone in the water. See the following:
Corrosion On Boats.

Electric shock Drowning

Creating an effective path back to ground.

Marine Cords

I covered where AC comes from in the beginning of this series. The power lines from the substation extend out onto the dock, where there is a power outlet. A cord is used that has plugs on both ends and plugs into both the outlet and the boat. This is not just an extension cord.  These cords are designed specifically for marine use. They should be Marine Rated or UL - Marine Listed. Do not use any thing other than marine cords.

Here is a link showing a cord, shore side receptacle and boat inlet receptacle. This is not an endorsement of their product although Hubbell is widely used. 
Hubbell Cords and Receptacles 

Marine cords are designed to fit in a specific type of socket depending on the amperage rating of the cord.  The blades are a different configuration for 20 amp, 30 amp and 50 amp. This is so you cannot plug in the wrong cord. They also have a mechanism that locks them to the socket and prevents water from getting inside and the plug from easily pulling out. Male ends have molded on boots at the connector, and female ends have a locking ring.  Here is typical shore power inlet.

The socket, or shore power inlet, on your boat has to match the plug on the cord, for the same reasons as given above.  This inlet will have a cap that covers the socket and keeps water out when it is not being used. Here is a link to a Coast Guard Boating Safety Circular81 that has an informative article on AC Electricity on boats, page 1-4. It also shows diagrams of the various socket and plug connections. This is public information. You need Adobe Reader to read and print it because it is a pdf file. There is a section about AC power cords and why they must be the correct cord.

Since I first wrote this in 2007 a new shore power plug has come on the market. It complies with both ABYC Standards and USCG regulations.  Here is a link to the Smart Plug.  The inventors of this plug claim it solves many of the problems of conventional shore power plugs.  It is being widely used on boats and on RVs. It is up to you to decide if you wish to use this type of plug.  It does the same thing as the other plugs; transferring power safely onto your boat. However, keep in mind if you buy a cord with a Smart Plug on it, you must also buy the socket it goes into.  This will not work with a conventional shore power socket.


One of the primary reasons for the types of sockets is to maintain polarity. In DC systems we learned that one side is positive and the other side of the circuit is negative. This is polarity. In DC the polarity must be maintained. Otherwise the equipment simply won't work.

AC polarity is important for two reasons, the proper operation of the equipment, and for your safety on the boat. In AC circuits you have three wires. The black is the hot, the white the neutral, and the green the grounding or safety wire. On the shore side the black is never, ever, connected to ground. The white wire is connected to ground. If at some point you accidentally switch the wires, now the black wire is connected to ground and the white is not.  Now both the black and green are hot (meaning current is flowing in the wire) and you have a serious potential for shock because the metal case of everything on the boat becomes hot, and if it is a metal hull, it is hot.

So throughout the AC System, from the power plant to onboard your boat the polarity must be maintained.

This is why appliances either have three prongs, or one prong is bigger than the other. This is to maintain polarity. This is also why you should never cut the third prong off. The third prong is the green wire. If you cut it off and a short occurs, now the current has no way to get to ground, except through you.

All AC electrical equipment has color coded connectors, just like the wires. The hot side is black, the neutral side is white or silver, and the grounding side is green. In 220 and 440 systems there is also a red wire and red connector. Fortunately there are polarity indicators you can use to tell you if it is right. These range from small handheld devices you plug into a socket, to indicators that are built into the electrical panels and shore power inlets. These will tell you if something is hooked up wrong. Do not use the circuit until the polarity problem is corrected!

I cannot emphasize this strongly enough. Reverse polarity is dangerous to you, and some AC equipment can be damaged. Other equipment simply won't run.  I recently spoke to a person who upon plugging into shore power got a reverse polarity indication. He also noticed his batteries were not charging.  The charger simply would not work with reverse polarity.  The problem was on the dock. The white and black were switched in the shore power socket.  

Reverse polarity most often occurs when either the shore side is wired wrong, or someone has added an appliance on the boat and wired it in reverse. Have the system checked by a certified marine electrician.

Links to Offsite References:
Wiring Your Boat

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Basic Electricity
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