AC Shore Power Adaptors

BEOWULF spends very little time whilst cruising tied to docks – almost none, in fact. However, now that we are in Southern California, and still looking for a permanent home, we have been doing some dock-hopping.

So far we’ve been on docks with 30 amp 110V service, 50 amp 220V power, and now 20 amp 110V power. Each has a different style plug at the dock end.


We went to West Coast Marine Electric in Marina Del Rey for a refresher course in plug adaptors. This sketch, scribbled on a piece of scrap paper, was the result – along with a bag full of plug parts and some heavy duty wire. This shows adapting our 30 amp 10v single phase service to the 50 amp 220V dual phase on the dock. Note how only one of the two hot leads is connected from the shore to our plug.

BEOWULF does not have very high power requirements and has just a single 30 amp 110v circuit. Which has us thinking about adaptors.

Over the years we’ve just opened up the end of our shore power cord or the dock box itself, and made up some form of Mickey Mouse adaptor on the spot. But as we mature, and become more aware of our own mortality, we’ve become more cautious.

As a result, we’ve now got a “system” of plug adaptors – the same approach might be of benefit to you.

The first element is a plug which adapts to the dock end of our shore power cord. In our case, the shore power cord has a male plug on its shore end, so our adaptor is a female receptacle.

In the U.S. 110V single phase plugs are wired more or less as follows: green wire or terminal is for the ground wire (this is usually the plug prong with a right angled blade); white wire or terminal is for neutral; and black or red wire/terminal is for hot.

We’re tied up right now behind the house of our friend Dave Wyman, in the Ventura Cays. Dave has a 20 amp circuit on his dock. His plug has three prongs, set in a circle. It looks a lot like our 30 amp plugs, only it is a little smaller in diameter. At a local hardware store we picked up a 20 amp male plug to match, and then wired it to our female adaptor which ties to BEOWULF’s shore power cord. The wiring is easy – green to green, white to white, black to black – all of the terminals, on both ends, are color coded.

When we go back to Marina Del Rey next week the dock where we’ll be staying has 220V 50 amp service. In the U.S. 220V service plugs have four wires. The ground wire (still green) goes to the case of the plug. Then there is the white wire for neutral. What’s different is there are two black – hot – leads. If you put a voltmeter between these two hot leads you will get 220 volts. Check the voltage between either of the hot leads (black) and the single neutral (white) and you will find 110 volts.

To adapt this to BEOWULF’s shore power system we need to connect three of the four wires from the dock plug to BEOWULF’s shore power cord. We start with a male 220V 50 amp plug which will eventually be connected into the dock power plug. Ground (green) has a terminal on our male 50 amp/220V plug which connects to the side of the plug, to which our green ground wire from our adaptor plug goes. Then we connect the neutral (white) prong. Finally we connect one of the two black (hot) leads to our black terminal. Now we’re working off one “leg” of the shore power system.

Sounds easy, right? It is, but obviously you want to know for sure what you are doing. Making a mistake with this can kill you, or someone in the water nearby, so make sure you do it right – or have expert advice. In fact, you just might want to have two or three adaptors made up in advance, by someone who is familiar with AC wiring.

Posted by Steve Dashew  (July 11, 2002)

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