The 120 volt AC system on modern RVs allows the same conveniences and comforts of a stationary house. Most high power appliances such as air conditioners, coffee makers, electric water heaters, and washer/dryers are only practical in an RV with a 120 volt AC power connection. The campground 120 volt AC connection also keeps the 12 volt DC battery bank charged and operating the 12 volt lighting system. Many RVs even have full time AC power available with an inverter connected to the battery bank, although the AC power is limited by the inverter and the size of the battery bank. Also many RVs include a 120 volt AC generator to power appliances and charge the battery bank when campground power is not available.

The electronics and power required by recreational vehicles has evolved over the decades. When I was a kid in my parent’s travel trailer the electrical system was very simple. The system was powered by a 12 volt DC supply which charged an automotive battery. The rest of the system consisted of several 12 volt DC lamps. All that was needed was to plug in the charger/converter every few days to charge up the storage battery. There were no advanced electronics such as televisions, computers, air conditioners, pumps, water heaters, etc. The charger/converter during this timeframe was usually a ferroresonant transformer design which regulated the the output around 13.7 volts DC to keep the battery on a float charge. This system was simple and inherently robust against most voltage fluctuations and line spikes, as there was really not much to fail.

Moving forward to the 21st century, RVs have almost all the same electronics and appliances as a regular fixed home. The difficulty is that the RV is plugged into different and unknown electrical systems every night when traveling. When an RV is fed by an overloaded system dipping below 100 volts (instead of the nominal 120 VAC) problems can quickly surface such as air conditioner compressors stalling, refrigerators not cooling properly, and fuses blowing. Very low voltage, or excessively high voltage, can strain motors and insulation resulting in premature appliance failures. Another dangerous condition is when the AC supply at the campground is running at the proper voltage but is incorrectly wired allowing high voltage (shock hazard) on normally grounded connections. Surges from campground connections are also common with heavy loads being connected and disconnected at other power pedestals. Summer thunderstorms with lightening always present a large surge potential. This is where a power monitor with surge protection is a good investment to protect the electrical systems in the RV.

Below are some fundamentals of 120 volt AC for the RV owner to understand for the proper functioning of the electrical systems. These are presented in (hopefully) an easy to understand manner. There is often misunderstanding among RV owners with electrical basics.

120V AC Basics
Voltage and current are fundamental units for all electrical systems. Recreational vehicles typically have 2 independent systems: A 120V AC system that is similar to a house, and a 12 V DC system powered by a battery bank. Voltage is electrical potential where current is the flow of electricity. The best analogy is a water pipe connected to a water system. The potential (voltage) is equivalent to the pressure supplied by the water system. The current is equivalent to the flow volume of water in the pipe.

Power is the next unit to consider and is always equal to voltage times current. Power is the rate of doing work. From the water system analogy think of power as the water spraying out of the end of the open pipe. A low pressure water supply with a small pipe will not produce much volume from the water coming out of the open end, it may only trickle out. If the system pressure (voltage) is increased, and/or if the pipe size in increased to allow more flow (current) there will be much more water volume spraying out (power) of the end of the pipe.

Energy is the next concept. Many people confuse power and energy. Power is the rate of doing work where energy is the capacity for doing work. In an electrical system power is measured in watts and energy is power over time, or watt-hours. Think of a 100 watt incandescent light bulb. When the bulb is on it is using 100 watts of power. After the bulb has been on for 2 hours it has used up 100 X 2 = 200 watt-hours of energy. Since electric power and energy consumption can be large, the units of kilowatts or kilowatt-hours are commonly used where 1000 watts are equal to 1 kilowatt. The abbreviations for these units are: Watt – W, kilowatt – kW, watt-hour – Wh (or Whr), and kilowatt-hour kWh (or kWhr). The measure of kWh is the useful energy delivered to you by the power company, this is measured by the power meter on the side of the residence.

A more complex concept of power will be presented next: Reactive Power. Remember from above that power is always voltage times current. However, in reality there are two different forms for AC power, reactive power and real (or active) power. Real power is the actual power used by the load to do work as in the power used to turn the shaft in a fan motor. Reactive power is the non-dissipated power delivered to the load (like the fan motor) which is stored in the load that is returned to the power system on the next cycle. Remember “AC” stands for alternating current, and the polarity of the voltage and current switches back and forth during every cycle. For normal 60 Hz power, the polarity actually reverses back and forth 120 times a second. The presence of reactive power is the result of the type of load connected to the power system. For example, the fan motor operates on the principle that alternating electric current is converted into motion by a rotating magnetic field. This magnetic field is produced by the alternating electric current flow in many coils of wire that make up the motor. In reality not all of the generated magnetic field is turned into motion, and the excess field will be returned to the power system during the next AC cycle. This is the reactive portion of power.

The concept of reactive power is difficult to comprehend so we turn again to the water flow analogy. When a voltage is applied to a coil of wire (in the case of a motor winding) it takes time for the magnetic field to build up in the coil of wire. Therefore the magnitude of voltage applied to the coil and the current in the coil are not matched in time. The voltage builds up before the current starts to increase. Think of a water faucet. As soon as the faucet is turned on water starts to flow out. Now connect a long hose (the equivalent of a coil of wire) to the faucet and turn it on and observe what happens. It takes several seconds for water and the pressure to build up in the hose before water starts to flow at full force out the end. The longer the hose (big coil of wire) the longer it takes to achieve a flow out the end.

Why be concerned about reactive power? Power companies and large power distribution systems such as campgrounds are very concerned with reactive power. To get power to your RV, electric current must travel many miles through utility lines and through the campground distribution system. The main cost of this power distribution besides the real delivered power is power loss in wires and transformers. This real power loss is directly dependent on the apparent power delivered by the system. The apparent power is the sum of the reactive and real powers. Therefore high reactive power present on a distribution system results in high distribution system losses without the benefit of delivering additional real power to loads. Note that utility transformers have markings for their apparent power rating such as “25 kVA” or 25 kilo-Volt-Ampere. This indicates that the transformer is limited by apparent power, not watts of real power. Ideally we want reactive power to be low as possible. The perfect case is when there is zero reactive power, and apparent power then equals real power.

For normal residential connections the utility company charges for real energy (real power times elapsed time) only- even though high reactive loads results in extra system distribution losses. Advanced power monitors such as the OSI report instantaneous real power, average and peak real power, and total real energy used. An additional display field is included which shows the power factor (PF) in percentage. Power factor is the ratio of real power to apparent power used by the load (the RV). For example, a 90% power factor with 1000 watts real load indicates there are 1110 watts of apparent power being drawn from the campground electrical system. Normally power factor is not a parameter of much concern with a recreational vehicle when connected to the campground electrical system. However, power factor can and does affect operation from an inverter or generator as these power sources are rated in total watts delivered at a given power factor. Most small generators assume a power factor of 1.00 or 100%. For example, a popular recreational generator is advertised as 2000 watts, and checking the detailed specification sheet the manufacturer designates this is for a 100% power factor. Assume an RV is running 93% power factor total as a load, this means the maximum the generator can deliver is 1860 watts of real power! Just like the utility company, a high power factor will limit how much a generator or inverter can power loads. The last point is the user should check how a power monitor reports power (real power or apparent power). Less capable monitors will only report apparent power which will give misleading results when sizing an actual load to a generator or inverter.

Hot and Neutral
“Hot” and “Neutral” are terms that refer to the current carrying conductors in the AC system. Normally the “hot” conductor is at the full AC system voltage of 120 volts. “Neutral” is the return conductor for the current and is usually near ground potential. The description of usually is given since the neutral conductor may be at a significantly higher potential than ground and still meet national electric standards. Some inverters use a split voltage scheme where the normally hot conductor is at 60 volts and the neutral is at 60 volts -opposite phase- with respect to ground, thus producing the normal 120 volts AC from hot to neutral. Note that a ground wire should never be used as a current return path for an AC circuit, this is the purpose of the neutral conductor.

What is Ground?
Ground is a conductor used for safety that, by definition, is always at earth ground potential. Proper grounding is important in an RV or in any electrical application to prevent dangerous voltage levels on surfaces of appliances or even on the RV frame itself. Imagine standing outside on the earth at ground potential and reaching up and touching a metal door handle on an RV. Receiving a shock or even a slight tickle indicates a ground problem and a potentially dangerous situation. A current of only 30 milliamperes (0.030 Amps) running through the human body is enough to cause cardiac arrest. Recreational vehicles, like houses, incorporate ground fault interrupt breakers to detect this problems in certain living areas. The OSI power monitor also detects and switches off AC power from the campground if it detects excessive leakage in the grounding system.

Recreational vehicle AC electrical systems are wired in the same was as a stationary residence with a major exception. With a house, the incoming neutral is connected to ground at the main breaker panel. There is a ground rod near the incoming electrical service that is connected to the house ground. In an RV the ground and neutral are not connected at the RV breaker panel. The ground to neutral connection occurs in the campground system at a main distribution panel or at the power pedestal. Since RVs sit on electrical insulating rubber tires there is no practical way of achieving a good earth ground except through the campground electrical system. This configuration normally works well except when the campground has a broken, miswired, corroded, or loose ground connection. As stated above, a faulty earth ground can result in having excessive voltage on the RV frame that result in shocks when touching any metal part on the RV. I have observed this effect at several campgrounds over many years of camping.

15/20 Amp, 30 Amp and 50 Amp RV Service
Recreational vehicles and campgrounds have a variety of electrical outlets and plugs, the standards being 15 or 15/20 amp, 30 amp, and 50 amp. The 15 amp outlet is identical as found in residential homes. The 15/20 amp outlet is wired with heaver gauge conductors to allow a maximum of 20 amperes current. This outlet is identified by a “t” shaped neutral slot. A 15 amp cord can be plugged into a 15/20 outlet. Only older and small travel trailers may have a 15 amp power connection, as these units have limited power needs without high power appliances such as air conditioners or electric heaters. A 15 amp circuit can supply a maximum of 1,800 watts of power. A common connection for many towed trailers and smaller to medium sized motorhomes is the 30 amp outlet. This is a unique outlet and plug for recreational vehicles. A 30 amp circuit can supply 3,600 watts maximum power. This is enough to run higher current appliances such as a single air conditioner, or an electric water heater. Note that many high power appliances cannot be operated simultaneously without exceeding this maximum power level. For example, running an air conditioner, electric water heater, a space heater and a refrigerator at the same time will likely exceed 3,600 total watts. Larger RV towables and motorhomes use 50 amp service. This level of power is necessary for large units running multiple air conditioning units and washer/dryers. The 50 amp plug/receptacle is different than its lower current counterparts in that it has 2 “hot” connections, where each hot lead can handle 50 amps of current. The receptacle is wired with 2-phase across the hot leads so there is 120 volts from each hot to neutral, and 240 volts from hot-to-hot. The 50 amp connection allows for a significant increase in total power since each hot can supply 50 amps of current at 120 volts. This gives a total power capability of 12,000 watts maximum.

Common AC Power Problems
Armed with the understanding of the basics of 120 volt AC power operation we are ready to investigate issues that occur when plugged into a campground or other outlet such as a storage lot.

Voltage is too high or too low: The nominal AC voltage in the US is 120 volts. Variations around this take place due to system loading and electrical appliances are designed to accept these normal fluctuations. However, excessive low voltage can cause compressors to stall in air conditioners and refrigerators, and heaters not to put out sufficient heat. High voltages can stress wiring insulation and cause power supplies in televisions, computers, and microwave ovens to burn out. Excessive voltage can also cause heating elements to get too hot and burn out. The OSI monitor continuously monitors voltage and disconnects from the source if the voltage is under 100 volts or above 136 volts AC.

From a casual survey of RV parks up and down the Eastern US, the most common AC system issue encountered has been low voltage. Ironically this problem is not just found in older parks but has been witnessed in brand new installations which appear to be lacking proper capacity for the number of RV spaces in the park.

A somewhat common problem is mistaking a 30 amp 120 volt RV power outlet as an electric dryer outlet. These outlets are easy to confuse since they look similar (the ground lug shape is slightly different). An inexperienced technician will wire the RV outlet as a dryer outlet with 240 volts instead of 120 volts between hot and neutral. Unfortunate RV owners that have experienced this condition report thousands of dollars of damage to appliances and wiring in their units.

Low voltage can adversely affect absorption refrigerators operating from an electric heating element. Consider a nominal 360 watt heating element at a 120 volt supply. Dropping the voltage down to 105 volts will only produce 276 watts from the same heating element significantly impacting the refrigerator’s cooling capability! How many times have people blamed poor absorption refrigerator operation on an “old” or “bad” model without even checking the line voltage before condemning the appliance?

Improper outlet wiring, hot – neutral reversed: This situation causes full voltage on the neutral conductor where the hot side of the circuit is at or near ground potential. It is caused by improper wiring to the RV outlet. Operation with this configuration may not cause damage and may go unnoticed to the user. However, some power supply models and computer equipment have been known to fail quickly under this condition (high voltage difference between neutral and ground).

Improper outlet wiring, hot – ground reversed: The hot connection switched with ground, or hot connected to an open earth ground, is a dangerous condition. Any surface that is meant to be at ground potential such as the body of an RV will have full 120 volts AC present resulting in a serious shock hazard.

Improper outlet wiring, neutral – ground reversed: This is another situation that could go unnoticed by a user without a power monitor with this detection capability. The neutral carries the circuit current and the ground connection should never carry significant current except in the case of a serious system fault (it is there for protection). Allowing the ground circuit to carry return current can cause the ground connection voltage to rise to an unsafe level. Ground wiring is normally smaller gauge (size) than the current carrying hot and neutral conductors, so running high continuous current on ground conductors can cause overheating and burning.

Improper outlet wiring, open or poor ground: In normal operation the ground circuit carries no or very little (leakage) current. Unfortunately a ground connection that has been disconnected or come loose may not be noticed without a capable power monitor. When a campground ground is not connected to the frame of the RV, the frame will be floating and any leakage currents or faults from appliances will cause voltage to be present on a normally grounded structure. This will create shock potentials.

Over current / drawing too much power: As discussed earlier, having too many high power demand appliances operating at the same time can cause excessive current and power draw on the electrical system. Power pedestals have current breakers against this condition but these have slow reaction times to allow large loads (compressors) to start. Drawing excessive current, but not enough to trip the main breakers, can cause the RV power cord to overheat and power plugs to melt.

Incorrect line frequency: The line frequency standard for North America is 60 Hz or 60 cycles per second. The national electrical grid controls this frequency to a very high precision which is needed to share power among utilities along with other reasons. Operating from an incorrectly adjusted generator or faulty inverter can produce large deviations from 60 Hz. Certain types of power supplies will not function properly and may shut down under these conditions. Motors in air conditioners and compressors will run at the wrong speed affecting their operation.

Normally the line frequency is not of concern when operating from the campground supply which is connected to a major utility. There are some rare cases in remote locations where the campground may not be connected to a major utility but powered from a municipal generator where frequency variations can be experienced.

Surge Protection
Surge protection capability is another important parameter that needs to be discussed in detail. Surges on the electrical system are very short, but damaging, spikes in voltage. A typical surge can peak at tens of volts up to thousands of volts above (or below) the normal line voltage. Power supplies for sensitive electronics such as computers, televisions, and cell phone chargers can accept low level surges but their power capacity to safely dissipate large surges is very limited. The components used internally on computer based electronics to absorb surges are sacrificial, meaning they will attempt to dissipate the surge energy but will only withstand a certain number of surge “hits” before they are no longer effective. The best place to capture and dissipate line surges is before they enter the RV electrical system.

Surges are typically produced by two main sources: Man-made and natural. Most people are familiar with lightning surges and the damage they can produce. A nearby lightning strike can produce a spike of thousands of volts on the line voltage. A heavy-duty high-energy surge protector is needed at the electrical entrance to the RV to protect against this type of spike. Other surges are produced by man-made actions. Some examples are switching on-or-off high power reactive (e.g. motors) loads. A neighboring RV site that “pulls the plug” with the air conditioner running can produce a spike in your electrical connection. While this spike may be unnoticed at the time, it can cause latent damage to a computer power supply causing an ultimate failure in the future.

A high energy surge protector is located at the power entrance of the RV or at the incoming breaker panel of a house. Most units are marketed with the peak energy they can dissipate in the unit of Joules. The Joule is a measure of energy equal to one ampere of current in a resistance of one ohm for one second. This can also be expressed as one watt for one second. Surge devices are optimized for very short duration lightening strikes or switching arcs. An example is clamping the spike to less than 400 volts peak while conducting up to 10,000 amperes for 20 microseconds (20 microseconds is 20 millionths of one second). Note the high current capability but for a very short period of time! The Joule rating can be misleading since a key parameter is the peak clamping voltage of the surge protection device. A model of surge protector can have a high Joule rating but a poor voltage clamping rating. Most appliances can accept short voltage spikes less than 400 volts peak but even some RV units only will clamp voltages to 1,000 volts or more which can still damage these sensitive electronics. It is good advice to find out the maximum clamping voltage when comparing and purchasing a high power surge device. Many manufacturers of surge protectors conveniently leave off the voltage clamping parameter and only report a Joule rating.

Another aspect of surge protectors is that they meant to be sacrificial devices. This means that the surge protector is meant to dissipate the energy from the line surge instead of the surge traveling through the RV electrical system into expensive appliances. Depending on the peak energy and/or the frequency of surges the surge protector unit itself will fail (it is meant to sacrifice itself!) and will need replacing. Any quality surge protector should include an indicator to show when the unit has been damaged to the point that it needs replacing. Inexpensive units do not always have indicators with the consequence that they are no longer protecting against line surges when the user thinks he is still protected. Some units also have the option to replace the actual surge assembly at a nominal cost instead of the entire unit. Also having a high power rated unit will minimize replacements since these will accept more low energy surge “hits” before failing. A common question is how long a surge unit should last, there is no standard answer to this since a freak lightening strike could take out a unit on the first camping trip or it could last for decades of trips.

Why Do I Need a Power Monitor / Surge Protector?
The quick answer is all of the above — and then some more!

When considering a power monitor it is important to compare and understand the features of different units. Some units only measure voltage. Does the power monitor disconnect the line when a serious fault is detected? Can the monitor detect incorrect wiring scenarios described above? The OSI power monitor has been designed as a comprehensive unit to detect all major electrical faults while also displaying power and energy usage.

There are key differences in a power monitor and surge protector. Not all power monitors have surge protection, and not all surge protectors have power monitoring capabilities. Some advanced units do have both functions- such as the OSI monitor describe on this web site. There is confusion between functions in discussions with RV owners where a $50 surge only protector is being compared to a $400 full feature power monitor. This is comparing apples to oranges.

A Power Monitor has the following capabilities depending on the model: Voltage, current, frequency, power, energy, improper source wiring, and the ability to automatically disconnect the load (RV) when a serious electrical fault is detected. Surge protection may be included. All power monitors do not include all of these monitoring functions. Manufacturer’s detailed specifications and operator’s manuals need to be studied before choosing the right unit for the application.
A surge protector has only line surge protection capability. Some surge protectors on the market may include a line voltage readout or indicator lights for improper wiring but may or may not have automatic disconnect capability for under or over voltage conditions. These units are good for information but will not protect the RV from serious electrical problems.

From informal surveys on RV forums and campground discussions, RV owners are split on their opinion whether they need a power monitor and surge protection or not. Most of the believers in protection have first hand experienced problems at campgrounds where the voltage has dipped so low that air conditioners and other appliances do not run properly. Some have experienced a “hot skin” shock on their RV from an improperly grounded system. Many have also experienced first hand the devastating effects of a close lightening strike or sudden line surge taking out a television or some other sensitive electronic device. On the other hand there is a false sense of security from people that have been camping for x number (x can be 5, 10, 20, etc.) of years and “never had a problem”. Many campgrounds are actually well maintained and have few electrical system problems. But consider this in closing, even the best campgrounds have hundreds of cycles of plugging and unplugging into pedestals, weather and mechanical stress on pedestals, and electrical surge events such as summer thunderstorms which can overtax the electrical system.

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