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.