Automatic Transfer Switch

What is an Automatic Transfer Switch and why should you care?  An Automatic Transfer Switch automatically connects your generator to the RV 120-volt system when you start the generator.  Without one you have to plug the shore power cord into the generator receptacle.  Depending on the location and access to the power cord and the receptacle, plugging it in can sometimes be somewhat onerous.  I've seen arrangements where the receptacle is in the back corner of a very small power cord compartment that is only accessible through a 5"x5" door, making it quite difficult (Hah!  Nearly impossible!) to reach, even after pulling all the cord out of the tiny compartment to get to it.  Pulling all that cord out and stuffing back is a nuisance and especially messy and difficult in bad weather. Plugging into the generator receptacle is virtually impossible with 25' of fat 30 or 50 amp cord in the way.  I've seen way too many RVers complain their generator wasn't working when the only problem was they hadn't plugged their power cord into it!  An Automatic Transfer Switch eliminates that problem.  But they do add one more possible point of failure.

An Automatic Transfer Switch is a nice addition to any RV with an on board generator.   Having an automatic transfer switch means you don't have to go outside in bad weather and wrestle with the power cord to connect your generator.  All you have to do is fire it up when you get to camp and the switch does it for you.  The switch has two inputs:  the shore power cord and the generator.  It has one output, connected to the 120 volt breaker panel.  The default position of the switch (when there is no power from the generator) is for the shore power cord.  Once the generator begins to deliver power, a control circuit in the transfer switch will switch to generator power after about 30 seconds. The 30 second delay is designed to give the generator motor time to "settle in" before putting any load on the generator.  If yours doesn't settle down and run smoothly in 30 seconds or less you should have it tuned up.

If you decide to install a transfer switch on your RV, be sure to purchase one rated to match the RV panel capacity and power cord connection -- 30 or 50 amps.   A 50 amp switch could be used on a unit with 30 amp power but you'd just be paying extra for something you don't need.  NEVER use a 30 amp switch on an RV with 50 amp service.  Doing so could result in an overload condition that could burn out the switch and even lead to a fire.  If you add an automatic transfer switch it might be a good idea to retain the original generator receptacle connection so have a fall back if the switch fails.  I had a switch fail and ended up spending almost as much to bypass the switch as I spent to install it -- and lost the use of my generator for several days on the road before I arrived where I could purchase the necessary parts for the bypass.

Automatic Transfer Switches start at about $50.00 for a 30 amp model.  50 amp version will cost more.  You can buy even more expensive models that may have a heavier duty cycle and might last longer but I would carefully compare the features, warranty, and life expectancy before spending a lot more money.  Unless you are using your RV extensively where the switch will get a lot of use, the lower priced models will usually be adequate for most people since our RVs normally only get occasional use.

Check whether you have access to the wiring and a place to mount the switch before you buy.   No sense spending money on a switch you can't use.  It is ideal if the power cord compartment is large enough and you have good access to install the switch there.   It makes the installation fairly easy.  If there is room there you usually have access to all the connections you need to install the switch.   I had to install one near the breaker panel under the bed in one RV because there wasn't enough room or good enough access for the installation in the power cord compartment.  The door was only 5"x5" square and there was barely room to stuff the power cord in the compartment.  Decide where you're going to mount the switch and make sure there is sufficient room for it and some sturdy structural component to mount it to.  It should NOT be mounted to just paneling.  You will need access to both the wiring from the power cord and the wiring from the generator as well as a way to run wiring from the switch to the breaker panel.  If you have to run any extra wire, make sure you use at least 10 gauge wire for all connections for a 30 amp switch.  For a 30 amp feed the black should be hot, white should be neutral, and green or bare copper is ground.  Check with an electrician to confirm wire sizes and colors for a 50 amp installation and note that a 50 amp power cord has 4 conductors where the 30 amp cord has only 3.  A typical 50-amp cord as three 6 gauge wires and one 8 gauge wire.  You'll probably need a little professional help figuring out the correct wiring for a 50 amp switch.  The first one I installed didn't work.  I had my installation checked by an electrician and found that I'd hooked up the wrong "hot" wire from the generator to trigger the switch.  Didn't know there was any difference but there was.

Most switches will come with very good wiring diagrams that anyone who can change a residential switch or outlet should be able to follow to make the connections.  Basically you disconnect the shore power cable from inside the cable compartment and connect it to the power cable terminals on the switch.  Then remove the generator receptacle and connect the generator wiring to the generator terminals on the switch.  Alternatively you could splice into the line between the generator and the receptacle, leaving the receptacle connected.  That way, if the switch fails you can still plug the power cord into the receptacle until you can get the switch repaired or replaced. Then connect the panel terminals to where the power cord was previously connected in the cable compartment and you should be good to go.  A word of caution if you choose to the splice option.  You may have serious problems if you plug in the shore power when the generator is running using the receptacle or start the generator with the shore power plugged in!  Don't know how you could ever have the cord plugged intothe generator receptacle and the shore outlet at the same time.  Everything should be OK as long as there is nothing plugged into the receptacle.   Also, during installation, be sure the power cord is disconnected and the generator off until the switch is completely installed.  If you have to extend any of the wires, be sure to enclose any splices in a proper electrical box, never just twist or solder them together and tape them up.  If you are not comfortable doing 120-volt wiring, have it done by a licensed electrician or a qualified RV technician.  If you have to run any new wiring instead of simply using existing wires, make sure you use wiring of the right gauge for the application (30 or 50 amp).  If you have to splice into existing wires, the splices should be contained in a proper electrical junction box.  The connections inside are usually made with wire nuts and you should use stress relieve fittings where the wires pass in and out of the box.  I did have some trouble getting a 50 amp switch to work and enlisted the aid of a licensed electrician to help me troubleshoot it.  Turns out I had chosen the wrong leg of the 4-wire feed from the generator to connect to the switch.  Hooking up the other leg instead solved the problem. 

Switch on!