I would like to spend a bit of time discussing switches, particularly as they are used in motorhomes, and more specifically the notorious transfer switch. There are several different kinds of transfer switches but I would like to focus on what is normally called an automatic transfer switch. The word transfer refers to the action of the switch when it takes incoming power, either from a shore cable, or from a generator set, and directs that power into the RV, to the breaker panel.

While it sounds simple, this is actually a fairly complex process involving switching a lot of power, making certain that proper grounding and bonding of electrical wiring is taken care of, and doing so in a manner that is convenient and safe.

Consider this scenario: You have been traveling in your generator equipped RV and you arrive at the campground where you pull into your campsite, pull out your shore power cord, plug it into the power pole and then sit back to enjoy your campging adventure.  Since you are lucky enough to have an Onan quiet diesel generator in your RV, in your rush to relax, you have left your generator running, and yet, your RV is now running on the power being supplied through the shore power cable and the power being produced by the generator is being ignored.

This is because you “intelligent” automatic transfer switch has made a decision to select the shore power in preference to the generator power.  This selection is by design, and is logical and appropriate.  In addition, the shore power is isolated from the generator power for safety so that none of the shore power is present at the generator and none of the generator power is present at the shore power cable.  This is obviously for safety reasons.  Another safety feature that happens during this changover is that proper grounding is applied.  When you are plugged into shore power, the grounding for your RV is supplied by the shore cable and comes from the power pole at the campground.  When you are running on your generator, the grounding is supplied by the connection to the generator.

We will talk more about transfer switches in the next post, including contact rating, and how to preserve the life of your transfer switch.

Leave a Reply


  1. larrycad

    John, you are correct, I got distracted and didn’t finish the series. I have noticed that as I get older, I am more easily distracted. 🙂 I will get back on this project and get this subject finished soon. Thanks


  2. John Chehaske

    I just read your July 7, 2009 post, “That’s a switch.” Looked for follow-on posts but could not find any. Did I miss them, or have you not had time to write any more about transfer switches.

  3. Cliff

    Any info on noise from gas generators? I am looking at a 3500 Watt Champion but I am worried about noise problems in campgrounds. Thanks

  4. Harf Overton

    Thank you. I will give it a try.

  5. Harf, actually, the charging of the chassis batteries is not directly dependent on shore power or the state of the transfer switch. The problem you are experiencing is a problem that most RV motorhome manufacturers have not been able to figure out. Generally there is some kind of battery charge controller installed in your coach which senses battery voltage on the chassis batteries and on the coach batteries and directs the output of the motorhome charger to the battery set that needs charged. Of course the house batteries are the most important when you are sitting, camping and thus they get priority from the charger, leaving the poor, neglected chassis batteries to discharge over time. I have an American Coach Tradition and that is what happens with me. Initally I carried a small portable charger which I hooked to the chassis batteries during extended stays and while this worked, I was annoyed with the inconvenience. My investigation led me to the Xantrex Echo Charger which sells for about $125 on the web. I bought one of these things and installed it on my coach along with a digital panel meter to monitor the charge on the chassis batteries and have observed that the chassis battery voltage now doesn’t change by more than about .2 volts DC. If you want to solve your problem, that is my suggestion. Others may have different advice.

  6. Harf Overton

    When we are parked at a spot with shore power for more than a week the chasis battteries will go dead if we don’t start the motor up at least once a week. Don’t the chassis batteries get charged from the shore power? Does this have something to do with the transfer switch?
    Thank you,

  7. Bill is correct on both comments. DC current is difficult to extinguish when opening a contact because DC never stops flowing. AC is easier to stop because it goes to zero each time it changes from positive to negative which happens 60 times per second. However, AC contacts must take into account if there is an inductive or a capacitive load on the system as this can cause arcing problems.

  8. Bill Boedeker

    About switches in general. If you are switching DC, make sure that the switch is rated for the DC current and not only AC current. DC current will arc and the arc will be maintained if the switch is only rated for AC current. You must also be careful with inductive loads.