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Old 05-20-2018, 04:56 PM   #1
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120 volt versus 240 volt from my generator

I have an 8K westerbeke diesel generator. It has two 120 volt legs. One feeds the house loads and the other side feeds the air conditioners. My question is can I run a leg off each side to power a 240 volt hot water heater without affecting the existing circuits?
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Old 05-20-2018, 05:00 PM   #2
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Yes, if both legs together give 240vac. Use a meter and check that. IT is going to use power from both legs, so the other 120vac loads will have less available amps to run those things. Your total watts gen output is constant, no matter what configuration is used.
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Old 05-20-2018, 05:07 PM   #3
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do you have the genset manual?

there are some basic pictures to show how the wiring gives you all the different combinations of voltage.

if not, I can post the pics from mine.
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Old 05-20-2018, 05:18 PM   #4
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Yes, as noted above it will work. You need to install an appropriately sized double pole breaker and feed it from each leg of your current AC panel. Then wire it to your heater.



But most water heaters can have a 240V or 120V heater element installed. It may be easier to change out the heater element and power it with 120V, probably from the house side. Or you could just hook up 120V power to the 240V element. It will heat at 1/4 the normal rate, but that also might work for you.


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Old 05-20-2018, 09:15 PM   #5
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My only concern is the power would not backfeed into the 120 appliances and burn them up. I'm contemplating using a tankless water heater information you're standing the 120 tankless heaters do not work so well. And yes according to westerbeke this unit will supply 220 volts
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Old 05-20-2018, 09:21 PM   #6
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I just don't get the concept of a tankless heater on a boat. You gonna start the genset every time you need hot water?
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Old 05-20-2018, 10:05 PM   #7
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I just don't get the concept of a tankless heater on a boat. You gonna start the genset every time you need hot water?
Sure why not. Running the generator for 5 minutes beats the hell out of taking a cold shower
But honestly I am still exploring the options of solar hot water. It is little more money up front but I won't use any energy to heat the water
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Old 05-21-2018, 05:43 AM   #8
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IF you have the volume a 30G or 50G Rheem HW heater only looses about 5 drg F in 24 hours .
Rheem Marathon 50 Gal. Tall 4500/4500-Watt Elements Non Metallic ...

https://www.homedepot.com/p/Rheem......r.../205466186


You can heat the water when the noisemaker is on and use the water a day later with only the DC FW pump operating.
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Old 05-21-2018, 07:19 AM   #9
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Well send the boat so I think my heater is any 12 gallon. And yes I do have water for two sometimes three days but it takes a half hourof generator time to bring it up to temperature.
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Old 05-21-2018, 11:19 AM   #10
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I have my water heater in the engine room, and in the summer, after running a while, engines are hot, then all the water is hot, even the cold water lines which run next to the hot water lines. I dont even need to turn on the water heater.

I dont really like that, cause if you take a shower it is all hot water and maybe too warm until it draws from the main tank, BUT the tank also sits in front of the generator which also heats that tank. I was thinking to put that insulation foam on the cold water pipes. Seeing it is a boat, you don't want to waste water running it to get to an even temp. I think putting some insulation on the pipes is going to make it better. I have copper pipe, and they conduct heat very well from the hot engine room air.
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Old 05-21-2018, 01:27 PM   #11
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My only concern is the power would not backfeed into the 120 appliances and burn them up. I'm contemplating using a tankless water heater information you're standing the 120 tankless heaters do not work so well. And yes according to westerbeke this unit will supply 220 volts
It doesn't work that way. Your 120 vac appliances will have one 120v leg and one neutral leg, plus a ground. Your 240 vac appliances will have two 120v legs and a ground.

Just a suggestion, but given your experience level you might want want to have an electrician either do it, or have them check your work before powering up. I have been wiring up stuff, including multi-phase, for years on my own and am still perfectly capable of making bone headed mistakes. On boats, those mistakes can be much more dangerous to you, the boat and anyone in the water around you.
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Old 05-21-2018, 01:46 PM   #12
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I realize how 120 and 240 or wired my only concern was that when the 240 is wired to one heating element. I just wanted to be sure it could not back for you so is single 120 hook up
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Old 05-21-2018, 01:51 PM   #13
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I realize how 120 and 240 or wired my only concern was that when the 240 is wired to one heating element. I just wanted to be sure it could not back for you so is single 120 hook up
As another posted, if it is a 240 vac appliance you'll wire it up through a two pole breaker, so the legs are separated and cannot "back up" on each other. should that happen, you'll know it from the sparks and the fact that the boat goes dark. If you are still conscious.
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Old 05-21-2018, 01:54 PM   #14
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I have my water heater in the engine room, and in the summer, after running a while, engines are hot, then all the water is hot, even the cold water lines which run next to the hot water lines. I dont even need to turn on the water heater.

I dont really like that, cause if you take a shower it is all hot water and maybe too warm until it draws from the main tank, BUT the tank also sits in front of the generator which also heats that tank. I was thinking to put that insulation foam on the cold water pipes. Seeing it is a boat, you don't want to waste water running it to get to an even temp. I think putting some insulation on the pipes is going to make it better. I have copper pipe, and they conduct heat very well from the hot engine room air.
I think some pipe insulation when your cold water line would probably help you. I agree in South Florida in the summer I do not even need hot water and that's without running the motor
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Old 05-21-2018, 01:57 PM   #15
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I realize a standard home type hot water heater has to 110 volt elements each feeding from one leg on the 220 v. I am still looking for a sketch on how 220 volt instant hot water heater is wired
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Old 05-21-2018, 03:55 PM   #16
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Your Westy has two 120v windings. Can be wired to provide 240v and still keep your two 120v legs in the panel the same. But little details in how breakers, transfer switches and shore power are wired can actually turn this quite complicated.

How much current draw is the instant water heater? Most draw LOTS of kW and thus are not popular on smaller boats.

I would suggest just getting a regular tank heater. It might come with two 240v elements, but you can easily swap them out with 120v lower wattage elements and not have to mess with your gennie and panel wiring at all.
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Old 05-21-2018, 04:06 PM   #17
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Look in the manual. We had a 7.5KW Onan and it was wired for two 120 outputs. Had to change some jumpers for it to provide the 240.
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Old 05-22-2018, 05:09 AM   #18
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If your wired for 120 only on a gen which can provide 120 and 240 vac, then when you rewire the gen to provide both, each 120 vac leg loses half its former amperage capacity to deliver power, since they were formerly combined, NOT a problem if the loads for each 120vac leg are properly balanced.

What you dont want to do is under load one leg and overload the other leg which means blowing breakers and or stressing the windings.
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Old 05-22-2018, 09:21 AM   #19
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"It might come with two 240v elements, but you can easily swap them out with 120v lower wattage elements and not have to mess with your gennie and panel wiring at all."

Larger units (40-50+ gallon) will come with ( 2) 240V elements.

240V as noted can be swopped for a 120V , and it would be a simple wiring job to use which ever (or both ) legs of 120 have the amps to operate the unit.

Each element is already connected to a thermostat ,only the top/bottom hand off would need to be disconnected, and an extra rd or black switch added.


Under $10.00 at the box store, unless you need to buy 120v elements too..
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Old 05-22-2018, 09:47 AM   #20
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On a boat typical on this site, I would not see any advantage to going above like a 12gal water heater. 12gal can do a couple showers in a row, wash dishes, run a washing machine and not take that long to heat up. 12gal can be sourced with 120v elements off the shelf, too (I think).
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