How to Power Your RV

Your RV’s 120 volt electrical system is fed through a central distribution panel where all of the circuit breakers are located. In order to power this panel from two different power sources we can choose one of two methods. The first is the more cost effective and least costly and is commonly used in lower cost travel trailers. That method entails hard wiring a power cord to the breaker panel’s inputs. When not plugged into shore power this cord can be plugged into a generator set outlet to provide power to the RV’s electrical systems. While this method is inexpensive, it’s less convenient because you have to physically switch the plug from the shore power receptacle to the generator receptacle. The second method is to install an automatic transfer switch.

Manual vs Automatic Transfer Switches

Manual transfer switches need to be manually switched by moving the switch lever on the box and are uncommon in a motorized RV. Automatic transfer switches commonly used in an RV will do this automatically and are logic controlled to switch under a given set of conditions. Generally automatic transfer switches will default to the generator inputs and are mechanically held. Once shore power is present the magnetic coils will pull the switch’s contact relays over to the shore power side. This is why you typically hear that clunk when the switch engages shortly after connecting to shore power. As soon as the shore power is no longer present the switch will revert back to its generator priority position.

Generators for Your RV

Sometimes the 120 volt devices in your motorhome need to be powered when you are not plugged into a campground pedestal, frequently referred to as shore power. When shore power is unavailable, the on-board generator set can be used to power these devices. Generators can be used in a number of situations, such as camping in remote areas where access to power is not available. A number of systems, such as lighting, water pumps and fans, are powered by your 12 volt battery. However, there will come a time when these batteries need to be recharged. The on-board generator can be used to power the coach’s battery charging system in the absence of shore power. Another use for generators is to power the rooftop air conditioners to make for a more comfortable motorhome interior in hot weather.

Generators for Gasoline Engines

Gasoline powered Class A motorhomes will be equipped with a generator powered by a gasoline engine. The generator shares the same fuel tank as the vehicle engine but uses a separate fuel pickup within that tank. Usually these pickup tubes are cut short so that the generator will not run if the fuel level gets below a ¼ tank but this amount can vary from one manufacturer to the next. The reason for this is so that you can’t totally drain your fuel tank while dry camping. The ¼ tank remainder ensures that you will always be able to start your RV’s engine and drive to a refueling location. These generator sets are designed to slide into a basement compartment that is designed for that purpose so access to the controls and engine service points is done through the removable side cover on the generator, which also serves to contain the cooling airflow and add additional sound deadening capability.

Generators for Diesel Engines

Diesel powered generators can be found on motorhomes that are powered by diesel engines. Some of the entry level class A diesels are really front engine gasoline chassis with a diesel engine in place of a gasoline engine. Also, many smaller B+ and Super C motorhomes are now being made with small diesel engines. In this case the diesel powered generator will be fairly small (in the 4 KW to 7 KW range) and will be mounted in a side compartment in the same fashion as the gasoline powered generators. On a rear engine diesel pusher, the front of the coach is clear of engines and radiators. In this case you’ll find a diesel powered generator that is mounted in the front cap on a set of slide rails. The generator can be slid forward out of the coach to allow better access for servicing the unit. These units generally start at around 7,500 watts and run up to 10,000 or 12,500 watts in size. 

Some older motorhomes used a propane powered generator that ran off the RV’s on-board LP tank. However, these are rare and less desirable because the limited capacity of the propane tank didn’t give the generator a very long running time before the LP tank was empty. Used coaches with propane powered generators are very hard to sell and their resale value reflects this.

In-Phase vs Split-Phase Generators 

Generators that are rated up through 8,000 watts are usually single pole 120 volt-only generators, referred to as “in-phase” generators because both windings are in the same phase. Larger generators, 10,000 watts and up are two pole split-phase 120/240 volt generators with a center tap neutral. Because motorhomes rarely have any 240 volt appliances you may think that this arrangement isn’t needed. But if you refer back to our split phase 120/240 diagram you’ll recall that each phase has a given amount of amps available. That’s what allows you to balance your load and eliminate the heavy wiring required for a 10KW generator, which would output 83.33 amps if it was an in-phase design. Note that a 50 amp electrical service is equivalent to 12,000 watts, which is the size generator required if you needed to supply a full 50 amps to your RV’s breaker panel.

What to Consider When Choosing a Generation System 

Small portable generators tend to be noisy because an engine running at 3,600 RPM isn’t very quiet. They were designed to be light and portable. In a motorhome this isn’t important because the generator doesn’t need to be lifted because it will be mounted into the RV. What is important is the noise level. If we add a second set of field coils to our generator, we will double the frequency to 120 Hz when running at 3,600 RPM, which isn’t good and will burn up our electrical devices. But if we take that same 4 pole generator and slow it down to 1,800 RPM we will still have 60 Hz. It’s just that we are passing twice as many magnets during the same revolution so by reducing the number of revolutions by half we will still maintain 60 Hz.

This is the way many RV generators are set up. The 1,800 RPM speed of the engine allows for quieter operation. Because we don’t have to lift this generator we can also add a nice large muffler to it. The big drawback is that we have to use a larger engine. A small 8 HP engine is capable of creating 3,500 watts at 3,600 RPM but when you slow it down to 1,800 RPM it will only put out around 5 HP, which isn’t enough. By going with a 12-14 HP engine, we will still have 8 HP available at 1,800 RPM to then make our 3,500 watts of power. This means that the engine will be physically larger, weigh more, be quieter and more durable, and cost more than a smaller engine that is running higher revs. But the gains are well worth it in an RV application. Large utility power plants carry this even further and can use as many as 24 field coils in their generation systems so that they only have to turn 300 RPM. Less RPMs means greater life but more weight and size, while less magnets means less weight and size but greater speed and wear.