This is a nice drawing of the RV power systems and what they run. It leaves out solar which has its own controller/charger that delivers solar power to the batteries. But it shows the function and position of an inverter. In the Class C RV the DC fuse panel and AC breaker box are in the same location but the functions are separate as in the drawing.
There can actually be two transfer switches. There is a power cord in an exterior cabinet that is either using shore power, i.e.plugged into a grid power outlet, or the generator to supply 120 volt power. In most Class C RV’s this transfer is manual. The power cord is either used at an outlet or plugged into the internal outlet powered from the generator. But it could be an automatic switch that senses which device is providing the power and automatically switches between them.
The second transfer switch senses whether power is coming from shore/generator or from the inverter. It is wired to provide power to the charger if it’s grid or generator sourced but not if it’s inverter sourced. That prevents a loop from battery to inverter to charger back to battery; the attempt to charge the batteries with their own power. In this transfer box both options are wired with AC type wiring, in this case #12/3 wire including a positive, negative, and ground.
This drawing shows how the wires are connected. Incoming power is either at 5/6 or 7/8. The charger/converter (from AC to DC) gets power if 5/6 are hot. The switch also provides power directly to the AC breaker box. If AC power is coming from the inverter at 7/8, the power still goes to the AC breaker box but not the charger. DC is provided directly from the batteries.
The 120 AC output from the inverter is wired to this transfer switch. AC wire does not lose current like DC wire does so regular household wiring is used. In this case #12 gauge. The inverter’s top output is 25 amps but the circuits it feeds are no more than 20 amps.
DC wire does lose current over distance so very thick wires are used to connect the batteries to the inverter. And distances should be as short as possible.
For distances from 3 to 10 ft the minimum wire size is #4/0. That is 0000 wire. The inverter is about 18”x 10”.
The closest storage area to the battery compartment in the RV that is large enough for the inverter was the dinette bench. There was already a wiring hole in the floor here because several DC wires feed the switches on the other side of the dinette bench.
I was not sure how much wire I needed to reach the battery compartment so I bought 10 ft of black and red. This was expensive wire, about $100. I opened the hole in the floor which was patched with spray foam and ran the wires to the battery compartment. The distance was only about 4’ so I needed far less wire.
I mounted the inverter in the dinette seat. There was not much room for the wires on each side so I removed it to wire the DC posts and the AC box and replaced it on the outside wall. It does not sit on the floor but has about an inch of clearance beneath it for cooling.
The ventilation for the inverter might need extra air and if I get temperature errors I will cut vents in the seat walls.
The battery box is getting very crowded at this point and the shunt post is just about full.
I brought the negative wire in from the opposite side from the positive and all the other wires to have more space for the connections.
The inverter has very little room in the dinette seat compartment so the separate remote was an excellent addition.
This remote turns the inverter on and off. It also has a display for the battery volts etc. It is wired from the inverter. I ran the wire through an existing hole under the refrigerator and up the stairwell side so it is conveniently near the door. Sometimes it is needed for the outside outlets. We only turn on the inverter if we need it.
I patched the floor holes with stainless steel wool that I used to close spaces in the siding to keep mice out. Then covered the steel wool with spray foam both above and under the RV. These RV power systems have been completed.