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Old 09-04-2010, 06:10   #1
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Isolate Shore Power Loads from Inverter Loads

Our boat has a water heater and battery charger connected to a 240vac shore power socket.

I want to add a device that will disconnect these two loads from the AV bus unless shore power is present. This is to prevent these loads from being driven by the inverter.

I am thinking of doing this with a simple dpst 240VAC 30 amp relay whose coil is wired across the shore power input. one side of the contacts would be connected to the shore power input (via whatever protection devices may be present like a GFI, breakers, etc) and the other side to a terminal strip feeding the loads that I want to disable.

Is there any need to do something more complex?

A primitive schematic of what I propose can be viewed at The diagram ignores grounding and just illustrates the hot feed.

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Old 09-04-2010, 19:35   #2
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Automatic transfer switch

Your requirement is met either by an internal transfer switch, which is a feature that some inverter/chargers have, or an external transfer switch. It is preferable to purchase a transfer switch existant on the market to solve your problem unless you use agency approved products and methods to implement your own design in order to be safe.

One problem with some do-it-yourself implementations of an external transfer switch is the failure to successfully identify and isolate both hot and neutral wires of the loads in order to fully islolate those from inverter-only loads.

"I don't think there'll be a return journey Mr. Frodo". Samwise Gamgee
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Old 18-04-2010, 22:27   #3
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Assuming your inverter also supplies your 240v outlets, then the circuit you have shown will also activate the relay when using the inverter, and unless you disconnect from shorepower manually, it will also connect inverter to shorepower, with obvious consequences.
Ive also used a DP relay, but this has a 12volt coil activated from the 12 volt inverter on/off switch. Disconnect the boat 240v outlet circuit (or whichever devices are to be powered by the inverter) from power source, and connect this only to the relay moving contacts.Ensure there is no other connection to shorepower for this circuit. NC contacts are connected to shorepower live/neutral and NO to inverter 240v live/neutral. Inverter output is not connected to anything else
Existing shorepower connections to other circuits can remain as existing.
Both live and neutral are isolated between shorepower and inverter.
Battery charger, water heaters etc are not powered by the inverter.
If shorepower is switched on with the inverter running, shorepower will supply the above only, however both power supplies remain isolated. (This is for a GRP boat only-not steel)
Care point is to ensure the relay is break before make
hope this helps
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Old 19-04-2010, 01:25   #4
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If you haven't already bought your inverter, look for one which has this function built into it. The Victron inverters do this. Better yet are the Victron inverter/chargers which will fully manage your AC loads, including boosting shorepower with inverter power for inrush loads, shedding loads like water heaters, and regulating the charger current to avoid overloading your shore power connection.

Downside is that they are $$$. I am saving my pennies.
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Old 19-04-2010, 05:02   #5
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Welcome to the Forum!

We will need more info from you before we can provide a good answer.
1. Is the unit an inverter or an inverter/charger?
2. Does it have a transfer switch that passes through qualified shore (or genset) power and automatically shifts to inverting mode on loss of external power?
3. It sounds like you have a 120/240 VAC service with the 120 VAC loads served by the two legs of the incoming service and the 240 VAC loads served by the two hot legs of the incoming service. Is this correct
4. Is the inverter or inverter/charger a marine rated unit?

At any rate, here is a simple scheme for isolating inverter fed from non-inverter fed loads Wiring Inverter Load Group (sub Panels) - Resources - Blue Sea Systems. When you provide the additional information, a better answer can be formulated.

Best regards,
Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
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inverter, shore power

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