This article is the second part of my electrical system primer. This week I will cover the 120V AC system.

When not plugged into shore power, the batteries will power your DC house system directly. As mentioned last week, these systems include, among other things, coach interior lights, bathroom and vent fans, furnace, slide-out and leveling systems, and direct ignition gas appliances. When the coach is plugged in, the shore power is shared between the 120V AC appliances and the converter or inverter. Most coaches, except after-market installs and high-end diesels, utilize converters.

Converters come in many forms, and combine with the AC breakers and DC fuses to create the coach power distribution center. When the coach is plugged into shore power these components combine to serve three distinct purposes; the converter section takes some of the AC power from the shore line, converts it to 12V DC, and uses it to charge the batteries, while providing the coach with power for the DC appliances through the 12V fuse panel. The distribution section acts much the same as the breaker panel in your house, as it sends 120V AC to the house appliances through the breaker panel.

Inverters work the opposite way, in that they take 12V DC from the batteries and step it up to 120V AC. There are many different types of inverters, from the simple to the more complex that include battery charging functionality. Inverters even in their simplest form are fairly complex devices that require professional servicing should anything go wrong. As such, they will not be discussed in further detail here.

If your coach has a generator, there is a further element to consider. Once again, generator problems are outside the scope of this article, but there are some things to check if you have power problems with the generator that don’t occur with shore power, or vice-versa. If you have AC power while on shore power, but not on generator, the first thing to check is the circuit breakers on the generator. Many users may not be aware that the generator has its own circuit breakers. Locate these breakers, which usually consist of a 20A and 30A breaker, and make sure they are not tripped.

If you don’t have to plug your shore power cord into a separate generator receptacle, you may have a problem with the transfer switch. The transfer switch allows the coach to be powered off shore power or generator, but never both at the same time. As well, it automatically switches between the two. The transfer switch incorporates relays to switch the power, and it is not uncommon for these relays to fail open or closed, which results in the shore power or generator power not functioning properly. The transfer switch is often a black plastic box with three sets of AC wiring entering it. Issues with no AC power while on generator and/or shore power may be related to the transfer switch. Another possible cause of complete lack of AC power may simply be a tripped breaker in the main panel. As mentioned earlier about the generator, it is a good preliminary check to ensure all breakers are in the “on” position before seeking professional help.

Next week I will continue the discussion on AC power and troubleshooting.

About the Author: 

Steve Froese, an avid RV owner, traveler, and Coach-Net member since 2013, is the principal of “A Word to the Wise Technical Communications”, a published RV author, certified RV technician, and licensed Professional Engineer. He frequently collaborates with the “RV Doctor”, Gary Bunzer, and has worked with the RVIA/RVDA as a technical and training writer and consultant. Professionally, he works as a quality engineer and musician. Watch for more of Steve’s work in upcoming Coach-Net publications.