Electricity

Your RV has two electrical systems: a 12-volt DC automotive system and a 120-volt AC household power system. The 12-volt power source is the on-board batteries and the 120-volt source is through the main power cord plugged into an external power source such as a campground pedestal, called “shore power”. The batteries are charged when the RV is plugged into the truck (through the trailer wiring plug) or the 120v shore power.

The 12-volt system powers interior lights, awnings, vent fans, the heater fan, water pump, air conditioner thermostat, and smoke/carbon monoxide detector. The 120-volt system powers the air conditioner(s), microwave, residential refrigerator, TV’s, and electrical outlets.

To understand how this affects your RV, it helps to know the basic principles of electricity.

Current is measured in amps; voltage is measured in volts, resistance is measured in ohms, and power is measured in watts.

Electricity is described through a mathematical equation called Ohm’s Law (I = V / R) and can be thought of as an analogy of water flowing through a pipe:

Rate of flow (current, or I, measured in amps) = pressure (voltage, or V, measured in volts) divided by size of pipe (resistance, or R, measured in ohms).

The amount of usable power (P) is expressed in Watts, and the formula is Watts = Voltage x Current, or P = V x I.

Let’s apply Ohm’s Law to your RV. A campground pedestal normally has three connections for 120-volt AC power: a 20-amp, a 30-amp, and a 50-amp outlet - all with a different style of plug. All three outlets are protected by their own circuit breakers to prevent overload. The 20-amp and 30-amp breakers are single; the 50-amp is two breakers joined together. No matter what circuit you use, if you begin to draw more amps than the circuit is designed for the circuit breaker will pop, shutting off the power at the pole to protect the campground equipment.

It works the same for the 12-volt DC automotive circuit. Hypothetically, let’s say you have a 15-amp circuit and you try to connect a 20-amp device (an amplifier, for example). The 15-amp fuse will blow and is there to protect the wiring and other components in the circuit.

A power management system in the RV will detect if the rig is plugged into a 12-volt source (the truck) or a 120-volt source and automatically switches to the better power source.

Power Protection

I strongly recommend an additional power protection device, like a SurgeGuard product or Progressive Industries Electrical Management System, either portable or hard-wired into the RV.

You can research the devices and determine what you’re comfortable with, but please get something.

I use the Progressive Industries EMS-PT50X portable plug-in. This device tests and verifies the power at the pole before allowing electricity to the RV. It also will shut down automatically any time high or low voltage is detected to prevent damage to the RV.

I use this procedure when plugging into shore power:

Turn the circuit breakers at the pedestal off
Plug the protector into the pedestal
Turn the circuit breakers on
Check for green lights/good indications on the protector
Turn the circuit breakers at the pedestal off
Plug the trailer into the surge protector
Turn the circuit breakers on.

120-Volt AC Power

The electrical circuits within the RV also have circuit breakers to protect the wiring and components. Ever wonder why a 20-amp circuit breaker pops with a hair dryer when something else is also plugged in? Let’s say you have an iron (drawing 12.5 amps) plugged in to the same circuit as the hair dryer drawing 12.5 amps. The circuit is overloaded and the circuit breaker shuts the power off to protect the wiring.

With a 50-amp shore power connection, let’s figure out how much power is available. 120 (volts) x 50 (amps) = 6,000 watts. To give you some perspective, one RV air conditioner uses about 1,500 watts and draws about 12.5 amps - 1,500 watts / 120 volts = 12.5 amps. A residential refrigerator uses about 600 watts and draws about 5 amps, so with two air conditioners and a residential fridge you are drawing about 30 amps of the 50 amps available.

The 30-amp circuit has 3,600 watts available, so you can see that just running two air conditioners is using almost all the power you have.

30-amp power. Power management becomes critical when plugged in to 30-amp service. Virtually all rigs suitable for full-timing need 50-amp service to operate all appliances, but many older campgrounds are only built with 30-amp service. It is very wise to carry a 50-amp female-to-30-amp male adapter so you can plug your 50-amp cord into the 30-amp service panel.

I normally turn off the electric water heater and switch to using the gas water heater to conserve electric power for other things.

Under normal circumstances, you will be able to run one air conditioner, the refrigerator, and other small appliances, but a second air conditioner, the electric water heater, oven and toaster, or hair dryer can require more power than available. It can be a juggling act, but turning off the air conditioner temporarily will usually provide the power needed to run other items.

Generator

A generator is a gas, propane or diesel-powered mini power plant, with the size measured in watts. Most on-board generators are 5500 or 6500 watts, propane powered directly from the RVs propane fuel tanks, and feature an electric, one-button start. A generator of this size will easily power both air conditioners (about 3000 watts), the refrigerator (600 watts), and the microwave (about 1000 watts), with a little power to spare. It is designed to handle the household equipment in your RV.

A transfer switch, usually located in the front compartment where electricity is distributed to the RV, determines if power is coming from the generator or shore power. The relay in the transfer switch makes a loud “clunk” as soon as the RV is powered up, and will prevent both shore power and generator power from conflicting. The transfer switch is designed to prefer generator power, so will always switch to generator as the power source when detected.

Another alternative to an on-board generator is a portable generator carried in the back of the truck. The size of your generator, in watts, determines the capability to run your appliances and air conditioners.

12-VOLT DC POWER

As long as your batteries are fully charged, you won’t have any problem running most of the RV systems on your rig. Your leveling system, slides, awnings, and lights all run off the 12-volt system.

If you have a residential refrigerator, you will also have an inverter, which changes 12-volt DC power into 120-volt AC power. The inverter will be large enough (in watts) to power the refrigerator but nothing else, so your TV, wall outlets and microwave won’t work if only using the 12-volt power source.

The batteries are charged when the trailer is plugged in to the tow vehicle, or when the rig is plugged in to shore power. It is important to periodically check the water level in the batteries.

You have some options on batteries. Two 12-volt Group 24 or 31 RV/Marine batteries wired in parallel are standard in most newer RVs. Here is a great detailed explanation of the different types of RV batteries and their pros and cons.

Solar Power

Solar power is an alternate way of charging the 12-volt batteries. Typically, solar panels mount on the roof of the fifth wheel. Solar energy is converted to electricity and will flow through a charge controller that ensures your batteries do not overcharge. For boondockers, solar panels are a great way to keep the batteries charged without having to use a generator.

Next: Propane

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