RV Electrical Basics

Red Rover, Red Rover, Send Tige Over


Last Changed 7/14/2006
RV wiring is very much like regular house wiring because that is where is came from.
	Into your house, AC power arrives on two wires, both are what we usually refer to as Hot. If we measure the electrical voltage between these two wires, the reading will be somewhere in the 220-240 volt range. We all know that appliances like ranges and dryers use 220 volt, so where does 120 volts come from? Well the answer is that somewhere along the line from the power company to your house, there is a transformer that has a center tapped winding. The two Hot wires are attached at the end of the winding and the center of the tapped winding is called the Neutral. If we measure from each Hot wire to the Neutral, we will see a reading in the 110-120 volt range. In general, each 120 volt outlet is connected to one of the Hot wires and the Neutral.
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Another basic; we usually refer to the Neutral wire as the Ground wire. In reality, the Neutral and the Ground are different wires. The Neutral is usually at Ground potential meaning that it is at 0 volts compared to Ground. It is important to remember that while the Neutral is supposed to be at 0 volts, it is still a current carrying wire. The current that travel from the power source down the Hot wire has to return via the Neutral. It is just as important not to have the Neutral circuit current travel through you as the Hot circuit current. The Ground wire is properly called the Safety Ground. The Ground should not be carrying current. The Ground it there to intercept current that is not traveling the normal Neutral /Hot routes because of some problem. The Neutral wire should be at Ground potential because somewhere between the power source and you, it is tied to Ground. This should be as close to the power source as possible. In your house, the Neutral is tied to Ground in the Circuit Breaker Panel. In an RV Park, this is also at the Circuit Breaker Panel but it is the Parks Circuit Breaker Panel, not the one in your RV. If you have a generator, the Neutral and Ground are tied together in the generator. This why it is important to have a proper Transfer Switch to move your RV wiring from the Park feed to the generator switching the Hots, Neutral and Ground. When on the generator, Neutral is tied to Ground in the generator. When the transfer switch changes to the park AC power, then the connection of Neutral to Ground is at the park AC Panel. With all this in mind, at the usually RV power post, there are two Hots, a Neutral and a Ground. All four wires are used for a 50 amp outlet. One Hot, Neutral and Ground are used for a 30 amp outlet or for a 15 amp outlet. With a 50 amp RV feed, there is 220 volts coming into the RV but no normal RV appliances use 220 volts. Most importantly, RV Circuit Breaker Panels are not designed for attaching 220 volt appliance outlets. So if buying appliances for your RV, no 220 volt appliances should be considered. Effects of Power The basic thing to remember is that electrical devices use Power. Power, expressed in Watts, is the product of Volts and Amps. Example of Power: 15 amps * 120 volts = 1800 watts When Voltage drops, a common problem especially in older RV parks, an electrical device uses more current. Example: 2000 watts / 120 volts = 16.7 amps 2000 watts /105 volts = 19 amps If the appliance was designed for only 17 amps, the extra two amps being drawn by the lower 105 volts will cause heat and possible breakdown of the appliance. Lets take the case of a 30 amp service RV. At normal voltage, 120 volts * 30 amps = 3600 watts is available. If the Park voltage drops to 105 volts, only 105 volts * 30 amps = 3150 watts is available. You can see that the if the trailer devices were using 3200 watts and the park voltage was 120 volts, we would be fine. But if the park voltage would drop to 105 volts, 3200 watts / 105 volts = 30.5 amps At 30.5 amps, the current draw through the breaker will flip the breaker. We are lucky if the only result of low voltage is the breaker flipping. Burning up an appliance could be a far more disastrous result.
Autoformers for Low Voltage
	One method of low voltage protection is an autoformer. An autoformer is a transformer that senses low voltage and changes it's winding taps so that the output voltage is higher than the input voltage. There are some RV park owners who don't want Autoformers because they feel that the Autoformers will take more power from the park electrical system. Well this is true and false.
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True example; a RV needs 40 amps of current at 120 volts but the park voltage is only 105 volts. 120 volts * 40 amps = 4800 watts = RV power needed 4800 watts / 105 volts = 45.7 amps taken from park power pole. False example; a RV needs 50 amps of current at 120 volts but the park voltage is only 105 volts. 120 volts * 50 amps = 6000 watts = RV power needed 6000 watts / 105 volts = 0 amps taken because the autoformer will try to pull 57 amps from the power pole and flip the breaker. note - in these examples, we assumed 100% efficiency but in reality, autoformer losses would actually result in a little higher park power usage. So yes an autoformer can take more power that the RV might normally use but it cannot take any more power than was offered at the park power pole.

Disclaimer: The information in this site is a collection of data we derived from the vendors and from our personal experiences. This information is meant as a learning guide for you to make your own decisions Best practices and code should always be followed. The recommendations we make are from our personal experiences and we do not receive any compensation for those recommendations.

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