Currently Wandering » Outdoor Family, Adventure Travel, Simple Living - Family of 5 Traveling the United States in an Airstream.

Power Upgrade – The Bliss of Automatic Transfer Switches

Our Airstream didn’t come with an inverter installed. There is one AC panel inside the Parallax power distribution center which distributes the 30 Amps from the street connection to all internal plugs, the refrigerator, the DC power converter, and the air conditioner. Adding an inverter complicates this setup considerably, for reasons I will now attempt to explain.

We wanted the inverter to run the aforementioned blender and provide power to the AC outlets throughout the Airstream. There were a few things that we NEVER wanted the inverter to power. The air conditioner was off-limits as it draws more than the 2,000 watts our inverter can produce. The refrigerator also has a heavy draw, and I’d rather just burn propane while boondocking. But the MOST IMPORTANT THING to NEVER RUN OFF THE INVERTER is the DC Converter.

See, the DC converter uses AC power to charge the batteries and provide power to the DC systems in the Airstream such as lights and fans. The inverter uses DC power to produce AC power. Powering the DC converter (AC->DC) with the inverter (DC->AC) works these two components against each other, causing them to both lose excess power in the process and drain your battery down quickly with no benefit.

It is common to make sure the DC converter has a switch that can turn it off when the inverter is running to prevent this problem. If your AC panel has the converter isolated on a breaker, you can just switch the breaker off and no harm is done.

At this point, I’ll remind you of the primary reason for our Solar Install and Power Upgrade: Laziness.

I didn’t want to have to remember to switch the converter off. Luckily, somebody smart invented something called an Automatic Transfer Switch. This wonderful device can switch between two power sources. It is often used to wire in a backup generator, but in my case I’m going to use it to automatically switch between the inverter and shore power.

With the Automatic Transfer Switch and some reworking of the AC wiring, I’ll explain how I created a system that automagically does the right thing at the right time. Brace yourselves: there will be diagrams. First, here is the ‘before’ diagram. Notice that I’ve already included solar power, though it isn’t technically required for the inverter install.

Old Wiring Diagram

The A/C side of this situation is pretty simple here. Power comes into the Main panel, and is distributed as needed. Now, observe the ‘after’ diagram, as it exists at the end of my install.

New Wiring Diagram

There are a few changes here, so let me guide you through what I did.

The main concept is that I split the Main Panel into two separate panels. The Main panel has the main appliances that I never want to power from the Inverter. The new sub panel only powers the A/C plugs and receives power from the Main panel (when plugged into shore power) or the inverter (when inverting from the batteries). The miracle here is the Automatic Transfer Switch, which automatically selects the input that currently has power. This switch was semi-expensive ($75), and you can just install a battery isolator switch like I used in other places in my install. You have to mount the switch somewhere, and remember to turn it to the right power source depending on where you are drawing your power from.

Remember that my primary reason for this power upgrade was laziness. I didn’t want to see a switch mounted somewhere, and I didn’t want to have to remember to switch it. The Automatic Transfer Switch solved my problem nicely. With it in place, I can just use the power button on my inverter remote to turn on the inverter, and the power is automatically connected where I need it.

The benefit of having two power panels is also laziness. I never have to worry about switching off the converter, or having the refrigerator drain my batteries if accidentally left in A/C mode.

Splitting the Panel

The diagram above looks nice and clean, but the reality is a little more complicated. Because of the location of the AC panel in the Parallax Power Center, it made a prime location for the new Sub Panel. I added a new Main Panel on the other side of the Airstream underneath the wardrobe. Moving the main panel meant re-routing the wiring for the air conditioner, refrigerator, and the converter to the other side of the Airstream.

New Main Panel

During my solar install, I routed the solar cable through the wire chases that run underneath the floor. The chase in our Airstream (2008 International 27FB) runs from under the wardrobe to behind the power center. There are two of those wire chases right next to each other, and I used them to run additional romex from side to side. The Air Conditioner and Converter both needed to be run to the wardrobe side, and the Refrigerator needed to be modified to run to the new Main Panel. The new main panel is shown in the picture above, part way through my wiring process.

I also ran a new wire to connect the Main panel to the Automatic Transfer Switch, which mounted right to the back of the Parallax Power Center. Below is a picture taken after wiring it up, but before attaching the back cover.

Wiring the Automatic Transfer Switch

If that whole process sounds like a mess, it was. Honestly, I’m surprised it all worked out, and I didn’t make any critical mistakes. In the middle of the process, the hallway looked something like this:


Maximum GFCI

In the factory configuration, the power plugs in the kitchen and bathroom were protected by a GFCI breaker. In my rewire, I added a few more so that every plug (including the refrigerator in the new main panel) was on a GFCI protected circuit. It might be overkill, but I thought a few more protections couldn’t hurt.

The Fateful Moment

I wired up everything, had all the circuit breakers in place, and all loose wires connected to the right places. To give things a test, I ran a power cord into my Brother’s garage and took a deep breath. Then, I flipped my main breaker on….. and nothing happened. After a few very confused minutes, I found out that I had tripped the GFCI outlet in my brother’s garage. A few resets and immediate faults convinced me that I had a wire fault somewhere. After isolating the main wiring components (because I have two panels now…) I discovered that I had over-tightened one of the wire stays, which pinched through the wire insulation and improperly grounded  a wire. After a quick fix, I was back up for another test. This time, everything worked. A quick celebration, and I re-mounted and closed everything up.

This brings us to the general close of Power Upgrade posts. I’ll post a cost breakdown and final thoughts, but I’ve described all the parts of our upgrade already. It was fairly hard to consolidate our experience down into just a few posts, and I’m sure I missed critical steps. Please reach out if you have questions about something I’ve missed.

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  • Doug Hewins - Thanks for posting this information — might be very helpful down the road. Quick question: on your diagrams, you show the line from your solar controller going to the DC bus, and then to the battery. Wouldn’t that line go to the battery first, then to the DC bus? Otherwise, you be powering your lights and fans directly from the solar panels. Thanks again.ReplyCancel

    • Sam - This surprised me as well during the install: The existing converter (battery charger) was also just wired to the DC Bus. But really, it does make sense. The battery doesn’t have different charging terminals than usage terminals, so connecting at the bus is nearly the same as connecting at the battery terminals themselves. Running only one set of wires from battery to Bus is actually much simpler as well, with only one set of connections to deal with at the battery.

      That DOES mean that incoming solar power powers the lights and fans before charging the battery, but that is actually the desired behavior anyway. There is a loss involved charging the battery vs using the power immediately, causing immediate use to be better. The presence of the battery in the circuit means that the fluctuating voltage of the solar panels is smoothed into normal 12V ranges.

      The battery will charge if the voltage on the DC Bus is higher than its internal voltage, and discharge if the voltage on the DC Bus is lower than it’s internal voltage. The Solar Panels (and also the Converter) charge the battery by raising the voltage on the Bus.

      Does that make sense?ReplyCancel

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