Electrical question.

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Nomadb1

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Helping a friend wire an antique trailer from scratch. He wants to run a 240v cooktop so I need to set up a 50 amp 240 feed. He also wants an inverter as he plans to run a dorm fridge.

Assuming I use one leg of the 240 to run the 110v system when plugged into shore power how do I get the 110v system powered from the inverter when he unplugs the shore? Also once the system is running on the inverter what keeps it from putting one of the male leads of the shore power plug from being hot (a potential shock/short)? Another related question - if he plugs into the shore and still has the inverter online how do I keep the smoke in the inverter?

Thanks,
Brian
 
Where does he think he is going to get 240VAC?  Certainally not in a camp ground as they only have 120VAC available.
 
Nomadb1 said:
Helping a friend wire an antique trailer from scratch. He wants to run a 240v cooktop so I need to set up a 50 amp 240 feed. He also wants an inverter as he plans to run a dorm fridge.

Assuming I use one leg of the 240 to run the 110v system when plugged into shore power how do I get the 110v system powered from the inverter when he unplugs the shore? Also once the system is running on the inverter what keeps it from putting one of the male leads of the shore power plug from being hot (a potential shock/short)? Another related question - if he plugs into the shore and still has the inverter online how do I keep the smoke in the inverter?

The answer is you want one and only one source of AC power connected at a time - shore power or inverter, but not both.  There are a couple of ways to do this.

The first is to get an inverter with a built-in transfer switch, one that will disconnect itself and pass through shore power when the inverter is off.  Connect it to a single 120 volt circuit after the electrical panel, then use that circuit to power the refrigerator and whatever else he wants to run on the inverter (TV, a couple of wall plugs, etc.)

When the inverter is off it disconnects itself and passes shore power to the circuit, when you turn on the inverter it disconnects the shore power and feeds inverter power instead.

Here's one example:

https://www.amazon.com/Power-Inverter-Automatic-Transfer-Switch/dp/B00TI1D5JK

The second option is to put a 50 amp socket on the output of a conventional inverter (one without a transfer switch).  Connect the inverter only to the side of the socket that powers the trailer's 120 volt circuits.  Leave the other hot leg disconnected.

When you want inverter power, plug the trailer's power cord into that inverter socket.  When you want shore power, plug the trailer cord into the shore power socket.

This way there's no possibility of cross connecting the two power sources.

The third choice is to get a 50 amp automatic transfer switch to do the above without all of the plugging and unplugging.  The key point is the transfer switch connects either the inverter or shore power to the trailer while leaving the other source disconnected so power can't backfeed to it.

As far as campgrounds only being 120 volts, that's true for the 20 and 30 amp outlets, so your friend won't be able to use his 240 volt cooktop if he uses an adapter to plug into one of these.

However, 50 amp outlets are 120/240 volts so the cooktop will work when he's plugged into one of them.

 
First they may 240 volt invertes. Cheaper

But how to do one leg..
The proper way to wire an inverter is as follows, NOTE: This applies to any inverter with POWER PASS THROUGH, I'm going to use an an example of a 2000 to 3000 watt inverter

Shore---Main breaker box (This is where the magic happens) Branch  curcuits to A/C, Water heater, Fridge, Converter (if not part of the inverter) and all loads NOT on what I call the "E" panel properly called the Inverter Sub Panel (Televisions Bathroom Kitchen and microwave)

30 amp branch from main panel TO inverter's AC-in (10 ga wire at least)
more 10GA (NOTE use a sharpie to write IN and OUT on the wires)
Back to a SUB panel this is a 2nd breaker box but the "Main" breaker is optional on this panel (If you specify it is an inverter sub panel when you buy it it should not need a main breaker)

Branch breakers in this box to TV's, Microwave and GFCI chain.

The main box splits the 240 volts into two 120 volt legs. The inverter passes power to the sub panel (120 volt) if shore is present and feeds it the hard way (inverter feed) if no shore power.



Shore---Main--Inverter--Subpanel--Inverter powered stuff
 
donn said:
Where does he think he is going to get 240VAC?  Certainally not in a camp ground as they only have 120VAC available.

50 amp 4 wire plug is 120/240v with a neutral and ground. Two 120v circuits to the neutral and 240v between the 2 hot legs. They have to make 240 or else the neutral could carry 100 amps.
30 amp plug is 120v only.
 
Assuming I use one leg of the 240 to run the 110v system when plugged into shore power
You can use both legs - just wire on the 120v circuits between one of the hot legs and neutral. Just a house is wired.

You can use an inverter with its own built-in transfer switch, so that the branch circuits get powered through the inverter at all times and the auto=transfer selects shore or inverter power as needed.  That inverter can also be the converter/charger, so every thing in one unit.

Alternatively, you can add your own transfer relay for the 120v branch circuits.  Here as article I co-authored showing 4 methods of converting an existing trailer to use an inverter.
RV Inverter Install: DIYRV

There are several topics on this site dealing with wiring an inverter - try the SEARCH button on the menu bar.
 
Thanks all for your great answers.  I think I can make some educated suggestions for my friend now.

Thanks,
Brian
 
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