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Old 08-19-2013, 02:05 AM   #11
Money Sponge
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City: Anacortes WA
Vessel Name: Money Sponge
Vessel Model: 55' Ocean Alexander Mk 1
Join Date: Jul 2013
Posts: 6
Wow, thank-you for the detail and the time it took you to respond to my question. You mention several things in your post and I will comment on several of them to leave this thread as complete as possible should others refer to it. It was also clear that even after carefully crafting my original post I see that I left some misunderstandings to everyone trying to help me.

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There are 50A 120V outlets which provide only 120V service. These have a certain shape plug so you only plug in a 50A 120V cord.
Yes, I am familiar with the plug types which is why I ended up wanting a detailed understanding of how they were wired. I bought my 50A 125V cord because that's what my marina has at the pedestal. It is a three wire cord with only one leg of the utility service coming aboard.

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In addition, there are 50A 120/240V outlets which provide BOTH 120V and 240V service. The key here is that they provide BOTH voltages, not just one or the other. These outlets are also unique so you can only plug in a compatible cord.
Here is where my misunderstanding was originating but later in your reply you give me the needed tidbit as did some other posts. The plugs are indeed unique and the 125/250V outlets at the other marinas will not accept my cord and they shouldn't. In the case of the 125/250V rated cords they are four wire cords and bring both sides of utility transformer aboard the boat. This is why I was concerned about accidentally getting 240V on a boat that was wired for only 120V service. What I didn't make clear is why I had this concern in the first place, it wasn't because I thought the plug would fit in a wrong outlet but because I assumed that the outlet would be wired differently by the shore power electrician depending on whether or not the second hot leg was needed for the boat. I fell in this trap because I jumped to a conclusion when my marina told me they only install this outlet when 240V service was required.

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What's interesting about the 120V/240V service is that whatever is at the other end of the cord you plug in can choose to use the 120V, 240V, or both services in any combination. And there are adapter cords that break out one of these services and make it available on a different receptacle type. For example, there are adapter cords that plug into 120V/240V service at a dock and allow you to plug in a 120V cord which would otherwise not fit the dock receptacle.
This single yet so simple paragraph actually answered my question. Knowing now that the plug is always wired for 240 volt service and that the 125V rated cords, and adapters, simply use just one of the available two hot legs made me feel pretty silly.

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This doesn't really make sense. A properly wired 120/240V outlet should have both voltages available. Perhaps what you are seeing is the result of plugging in an adapter cable that only picks up the 120V service. Such adapter cords are made to allow 120V boats to plug into and use these more common outlets.
By this time I had already found my missing link in your reply but this made it crystal clear.


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Now I'm confused. I thought you said that your Marina only allows 120V boats to use the 120V outlets, and 240V boats to use the 240V outlets? I'm missing something here.
I think it was my wording of my original post based on my then incorrect assumptions of the wiring that confused everyone. It was my translation of what the Harbormaster told me that was incorrect. You are right on. My marina uses almost all 125V outlets but when a boat needs the other they change it. But now I know they never need to change them back! Just get an adapter that pics up only one of the two now available legs and your good to go. I see now why the newer marinas use the 125/250V outlet.

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Does this make more sense to you now?
Absolutely! And thanks again for your time. I'm embarrassed that with my understanding of this subject that I missed something that was clear to everyone else!

As an aside, it may be worth mentioning that my new 50A cord is a replacement for 2-30A cords run in parallel using a reverse wye 50A adapter. This is fine when using the adapter connected to either the 125V or the 125/250V outlets but I was very concerned that when plugging into two different 30A outlets, when no 50A power outlet was available, that I could accidentally create a short between the outlets if they were on opposite legs of the utility transformer. Each time I had to do this I had to check voltage to be sure that both 30A outlets were on the same utility leg. I am much better off with this installation. I left the second 30A inlet at the boat connected and wired it to Shore Power B so that when 50A power isn't available I simply use one of my old 30A cords to this inlet and live with 30A service.
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