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Old 08-14-2015, 09:57 PM   #1
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Inverter and Water Heater

This spring I installed a Magnum 2,000 watt inverter so that we could operate the refrigerator, hair dryers, toasters etc. I soon learned it would also operate the water heater. If you disconnect the shore power, the inverter picks up the AC load including the water heater if it's breaker is on. This draws a 120 amp load from the batteries and even though I have an 880 ah battery pack, this will not work for very long. I have a 120 A alternator so this will also max out the alternator. I have been trying to remember to open the breaker when I disconnect the shore power but this is a sure way to disaster.

The solution is a relay. I installed an AC relay in the circuit to the water heater and connected the wires to the normally open side of the relay. The relay is pulled closed when the shore power is connected and drops out when it is disconnected. Now it is idiot proof which is just what I need.

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Old 08-14-2015, 10:03 PM   #2
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Nice.
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Old 08-14-2015, 10:23 PM   #3
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Sounds like you solved your problem. The other approach is to have two "legs" of 120 vac, one powered only by shore power, the other by shore power if available or inverted power if not.
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Old 08-15-2015, 01:10 AM   #4
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Similar system we use. Our hot water tank is on a single circuit and only 110. We schedule showers around anchoring times when the tank is hot using circulating water from run time. Other than on shore power the gen set is employed..
We isolated a single circuit for the inverter. On this we use the fridge, microwave, and other items. While running the 1500 inverter handles the fridge well, when we anchor we shut the fridge down as a part of the anchoring check list.
Actually, we (hence the "Frdge Off" step in anchoring) switch off the inverter when we anchor and rely on house batteries. Being a small craft, conservative steps are in order.
We have found that if opening the fridge is restricted the contents remain cold enough to last till the gen set is fired up or we start running again.

We installed an additional small 25 watt inverter for charging cell phones and small items used during running that require 110. All works well within our confined system.
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Old 08-15-2015, 03:38 AM   #5
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Nice solution. An alternative, wire your breaker panel so that certain items such as the water heater and ac units are powered only by shore power and generator.

This is easy to do as modern inverters have pass through functions which allows shore generator power to pass through the inverter, thus allowing those items that will be powered by the inverter to be powered by all three sources.
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Old 08-15-2015, 07:31 AM   #6
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The solution of "splitting" the breaker panel into inverter powered loads and non inverter powered loads such as the water heater, air conditioner, etc is a better one. A relay adds complexity and is another component to fail.


One of my previous boats was wired this way and it worked well. My current boat powers all circuits from the inverter and I am too lazy to change it.


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Old 08-15-2015, 08:16 AM   #7
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Just buy a spare relay now.
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Old 08-15-2015, 12:59 PM   #8
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A relay is a bad idea. Post #5 is the standard, proven solution. Water heaters don't belong on inverters of that size/type.
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Old 08-15-2015, 05:02 PM   #9
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A relay is a bad idea.
How come relays are a bad idea?
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Old 08-15-2015, 06:03 PM   #10
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Well, for many boats, it will take a double pole relay to switch the feed to the water heater and the air conditioner, two appliances that you probably don't want to power with an inverter.

Splitting the main panel buss isn't that hard. Hopefully the two big power users that you don't want to run from the inverter are on one end of the heavy brass DC buss that supplies the breakers. You cut that buss with a hacksaw to separate it from the other breakers. Make sure that the gap is significant, maybe a 1/4" because if that gap closes it will fry your inverter.

If it is on the end that feeds the buss, fine. Otherwise you will have to separate the buss from the main breaker feed and run heavy 10 gauge wire to feed those two breakers and leave the others disconnected from the main breaker.

Now feed the AC input to the inverter's transfer switch from the main breaker. Run the inverter's AC output back to the main panel and connect it to the buss that feeds the remaining breakers.

If the two heavy load breakers aren't next to each other at the end of the buss you can try moving them or worse case jumper wire them to the output of the main DC breaker.

This is all difficult to explain in words. Xantrex and Freedom before them used to have a diagram of how to do it in the Freedom 10/20/30 installation instructions, but I haven't looked in a long time to see if it is still on line.

This way you have a mechanically sure wired way to separate the big loads from the inverter that doesn't depend on any relays.

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Old 08-15-2015, 07:30 PM   #11
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Well, for many boats, it will take a double pole relay to switch the feed to the water heater and the air conditioner, ...

snip snip snip

This way you have a mechanically sure wired way to separate the big loads from the inverter that doesn't depend on any relays.

David
Sure sounds like a lot of work when a pair of matchbox sized solid state relays can be installed in minutes.
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Old 08-15-2015, 10:07 PM   #12
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While I have a split panel, I have no issue with the relay solution.

It works, it's easy, inexpensive, etc...

One thing... Install a three way inverter bypass switch. If the inverter fails throwing the switch will allow use from shore or gen power
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Old 08-16-2015, 05:40 AM   #13
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Sure sounds like a lot of work when a pair of matchbox sized solid state relays can be installed in minutes.
While electrical panels differ, I have worked on a couple of different brands and they were all easy to work with as to splitting the water heater and AC off from the inverter. Bay Pelican's for example has a separate breaker for the inverter with the inverter supplied outlets / appliances being fed from the inverter breaker.

Last time I worked on my panel the toughest part was getting my wife off the boat so I could have an electrically dead boat with which to work. In that job I removed the refrigerator and freezer from the inverter feed. My new units were AC/DC and the AC was to be used only when on shore or generator power.
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Old 08-16-2015, 08:48 AM   #14
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The OP's solution is intriguing. It's interesting that I have never seen it recommended by any inverter manufacturer or used by any boat OEM. One question I would have is where is the relay in the circuit relative to the circuit breaker for the water heater? How is shore power brought to the relay, separate from the inverter power? To me it just seems to add complication and another component to fail, but I'm interested in learning more.

I like having the inverter circuits on their own panel; admittedly I was dealing with a pretty complex boat, but as others have noted it's a very easy wiring job.
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Old 08-16-2015, 09:11 AM   #15
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A 2 pole relay any place in the normally wired circuit would do the trick....it would just be the coil feed that goes to any place on the shore power feed.

Unless I am missing something...of course the feed wire should be protected depending on how wired.
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Old 08-16-2015, 06:25 PM   #16
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I keep it seperate

My water heater only works on shore power or generator power and is not run through the inverter. It is kept totally seperate.
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Old 08-16-2015, 08:39 PM   #17
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My water heater only works on shore power or generator power and is not run through the inverter. It is kept totally seperate.
+1, also my A/C does not run through the inverter.
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Old 08-17-2015, 03:56 AM   #18
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We installed an additional small 25 watt inverter for charging cell phones and small items used during running that require 110.
I know a 25 watt inverter can't be expensive, but I believe that nowadays all cellphones, iPods, iPads and laptops can be charged from an USB charger. 12 volt only, thus simplifying even more.
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Old 08-17-2015, 06:20 AM   #19
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I know a 25 watt inverter can't be expensive, but I believe that nowadays all cellphones, iPods, iPads and laptops can be charged from an USB charger. 12 volt only, thus simplifying even more.
Bell and a couple of other manufacturers make multiple port 12v USB charging stations. I have installed two of these for a total of eight ports and have run a separate cable and fuse so that I avoid the inverter.
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Old 08-17-2015, 06:49 AM   #20
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A 2 pole relay any place in the normally wired circuit would do the trick....it would just be the coil feed that goes to any place on the shore power feed.

Unless I am missing something...of course the feed wire should be protected depending on how wired.

You aren't missing anything. The relay should be "downstream" of the heater breaker and a single pole on the hot lead is all that is needed.

Here you go ... $6 on ebay:
1pcs Soild State Relay SSR 25 AA AC AC 25A 250V 80 250VAC 24 380VAC | eBay
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