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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-04-2008
I33 I33 is offline
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240 volt shore power

I need to add 240v capability to my boat as our new air conditioner requires this voltage.
I would appreciate any qualified persons taking a look at this drawing and critique it please.


I know this would work in a land application, but I do not know how well it fits into a marine environment.

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Old 09-04-2008
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Gil,

I am not a qualified marine electrician.

I see one thing that makes me nervous. There is a potential in this system to disconnect one of the "Shore Power Cable's" (A or B). If A is disconnected you have N and L1 connected to the 240 box but no ground. If your AC cycles you will smoke your compressor motor in a hurry. If B is disconnected you have G and L2 but no neutral. The results would be similar and not pretty.

I think that you need to find a way to make sure that both G and N are always connected to the 240 box if either shore power cable is connected. Also, you need to ensure that the AC can't cycle if you only have one hot leg connected.

I have learned the hard way how expensive it can b if a 240 motor only gets 120...
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Old 09-05-2008
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You absolutely have the right idea. The only fly in the ointment is running two thirty amp, 120 lines into the boat. You can do it that way, but you'd be much better off running a single 50amp, 240VAC Hubble cable from the shore to the boat.

The reason I say this is that you'll be sure you're getting 240 from the power post. I can't even begin to tell you how many dual 120 receptacles I've seen wired incorrectly in marinas. To make sure you've really got 240 available, you really have no choice but to measure from one 'hot' lead to the other. If it's wired correctly, you'll see 230-240 volts. If you get a 'zero' volt reading, the receptacles are just a single 120 source, and it will do Bad Things to your 240 a/c system.

The second reason I'd suggest a single 240 cable is that more and more marinas are putting in the 240 receptacle and a single 30 amp 120 receptacle, so the option of running two cables just isn't there. My last reason is just a casual observation with no science to back it up: for some reason, 30 amp cables just seem to go away easier. I'm not sure if it's the contacts, the way the cables are produced or whatever, but they seem to fry themselves with little or no provocation and without warning. My guess is that it's because the receptacles' contacts get worn out and loosen up, causing bad connection.

Something else I noticed in your drawing is that you don't have galvanic isolators shown. If you're doing a major overhaul to your electrical system, bite the bullet, purchase them and install them. They'll protect your running gear and through-hulls in marinas that don't have good grounds. (And a lot of them don't!) They'll also help protect you from the idiot power-boaters who leave their power cords in the water, turning the whole area into one huge battery.

The other thing I notice is that you don't show a pair of 30 amp circuit breakers on the incoming, split 120 lines. The 120 also needs breakers just 'south' of the incoming lines, or otherwise your 120 systems won't be protected.

Sorry that my modified drawing isn't as neat as yours, but I used a paint program, not a cad program to modify it.
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Old 09-05-2008
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Breakers are on hand

Gary,
Thanks for your comments. The breakers for the 120 circuits are on hand, just not installed yet. They are going to be placed just behind the entry connectors, not at the electrical panel, as I have a long run between entry and panel.

I also plan on adding the galvanic isolators, I just didn't put them on the drawing for simplicity's sake.

I want to stay with the twin 30-amp shore power cables as I will have at least one 30a most places. I have one 120v air conditioner and will therefore always have at least some air available. When we have 50a, 240v available we will be able to run both ACs.

I agree that many marinas have both L1 & L2 tied (incorrectly) to the same leg on a 50a service, but don't see that as a danger to the 240v air conditioner. In that instance, there would be zero volts between L1 and L2, and the motor could not be damaged by that. Sure there would be 120v from either L1 or L2 to ground, but the motor winding is not tied to ground. The same situation with Neutral as there is no Neutral connection on the air conditioner, just L1, L2 and safety Ground.
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