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I have a new boat with an electric stove (240v), it works great on my split phase generator. The dock power is a single phase run thru 2 breakers. Dock original had dual 30 amp plugs, I ran the 2 hots to my 50 amp cord and I understand they are not split phase but I don't run a huge load till I got my new boat.

In my mind I need a 120v -> 240v transformer, then a 240v to split phase transformer to solve my stove problem and have the circuit correct at my current dock.

Is this the solution, or is there a product that can do this?

How much energy am I wasting with the transformers?

The boat is 240v 50amp.

A new stove that can run on propane is $800 so is this worth it?

Thanks for your thoughts. Diagram

enter image description here

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    Something doesn't add up about what you just said. I'm quite sure you've gotten some of your facts wrong. Single-phase would NOT be run through 2 breakers. You need to get very clear with your dock manager's electrician exactly what they are supplying, and get on back with us. You can tinker with residential power but DOCKSIDE POWER KILLS PEOPLE EVERY DAY. DO NOT FOOL AROUND WITH IT AND DO NOT TAKE CHANCES WITH IT. Jury-rigging something so "it works" is not acceptable and could get you a manslaughter charge. If it isn't what you need, pay the dock to make it what you need. – Harper May 26 '17 at 4:46
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    Can you post photos of the dockside configuration? – ThreePhaseEel May 26 '17 at 11:44
  • I have added an image, rather then have 2 30 amp cords running to the boat I used 1 50 amp cord, just hooked the 2nd hot into the second breaker for the other plug. This has been fine for years for my smaller boat I understand that the neutral is undersized but the cable is short and I don't push much current. Can I get some kind of transformer setup to get true split phase service? – SBTech May 26 '17 at 15:04
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    I see your added drawing. Stop Doing That Right Now!!!! This is just the kind of setup that is "fine for years" and then kills someone, as I said. "i don't pull much current" is self-serving logic, if that were true you wouldn't need two hots. – Harper May 26 '17 at 17:41
  • "Split phase" is single phase, split into two legs coming out of the north and south poles of the utility transformer. If you're getting 240v power from connecting things this way, not just two 120v circuits sharing a neutral, then your hot wires are on different buses (different transformer poles) and your neutral is not undersized, presuming it is the same size as the hits, since it carries the imbalance of the load on the two "phase legs." If you were just getting two 120v circuits, you would be overloading the neutral since the current would be additive in that case. – Craig May 30 '17 at 15:01
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The single worst application for electricity is dockside. Thousands of people die every year when a shock that might not kill them stuns them, and they drown. Typically this can't be distinguished from regular drowning, even in an autopsy. The giveaway is that multiple people drown, often rescuers. As such, dockside power is serious business and not to be trifled with.

Keep in mind: dockside power is not "perfect except for you". There are obsolete installations, negligent marina owners, and lots of idiot boaters who are hacking things just like this. Some guy on the other end of the dock doing something else stupid can interact in unexpected ways with what you're doing, and blam, somebody's dead. What will your defense be? Claim he violated the electrical code?

On the other hand, transformers are a "force for good" in dockside electrical safety. Electricity wants to get back to source, not ground, and transformers break "source" into zones.

The best way: Pay your dock to install the power you need.

This is best because of liability. A dockside drowning will bring lawsuits, and if there's any code violations, good chance it will bring criminal charges as well. You want these to be somebody else's problem. You want to be doing nothing more than plugging a boat that was professionally inspected, into a dockside receptacle that was professionally installed and is owned by someone else.

The transformer approach

This is a bad idea because of the liability issue. If you botch this or if something goes wrong, you could face civil and criminal liability. So I would not consider it without a sign-off from both your AHJ and your insurance company; and also having a professional engineer certify it (which shouldn't be too hard).

Even if you do everything perfect and to code, someone could still be electrocuted by the idiot two docks over. And if he can coverup his codevio, and there's your obviously homebrew/hacked together transformer setup, it won't matter if it's code-perfect, their lawyer will spin it around and put it on you. "Preponderance of the evidence".

You are correct that you can merge two power sources using a transformer, but you shouldn't. Because power moves both ways through a transformer, and if one side is energized, it will "light up" the other. That can have huge impacts to safety. For instance if they are sourced from two different panels or services, you have now biased those services against each other, creating current loops in unexpected ways. Doing this dockside is the act of a murderous sociopath, because anything can happen with dockside power!

Then there's the ordinary problem: you would actually be backfeeding from source A to source B or vice versa, and that's bad for exactly the same reason it's bad with generators.

What you can do, however, is use a transformer to take ONE of the 120V/30A circuits and kick it up to 240V for transmission to the boat, then have a second onboard transformer to turn the straight 240V into 120/240 split-phase for the boat proper. A 5 KVA transformer should suffice and those go for about $100 used. You would not be able to use all the burners on the stove at once.

  • I updated the drawing for a clear picture of my goal, I would like to get the properly sized transformer to power the whole boat correctly. I would like to end up with 50amp per phase service. – SBTech May 26 '17 at 20:22
  • Based on the last paragraph about the transformers the cost and size of the transformers makes it cheaper/better investment to remove the 240 stove and replace it with propane. Thanks for helping with that decision. – SBTech May 26 '17 at 20:56
  • For 240V/50A you would want to be looking at a 12KVA (12 K VA, with VA to watts, so 12,000 watts) transformer. However that is not a common size, and not so cheap. – Harper May 26 '17 at 22:00
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As an electrician who's main hobby is sailing including open water regattas in the Gulf Coast. I agree with the above comments. You should not try and modify your boat's electrical system any more than you would modify a car or airplane electrical system. Leave it up to the experts in your area.

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