Current Issue | Family Motor Coaching

Power Protection

A surge protector helps safeguard your motorhome against a variety of electrical problems.

By Mark Quasius, F333630
July 2017

Many motorhome electrical components and electronic devices — everything from air conditioners and battery chargers to residential refrigerators and

entertainment systems — require 120-volt-AC power, which is available via pedestals at most RV parks. Yet, campground power often is not reliable. Surges, spikes, low or high voltage, and incorrectly wired pedestals can seriously damage an RV’s electrical system.

Most motorhomes do not come equipped with power protection, so an aftermarket device that provides it should be the first accessory purchased for an RV. These devices commonly are referred to as surge protectors, but they are more than a safeguard against surges. Low or high voltage can cause just as much damage. Actually, low voltage is one of the most common maladies affecting the electrical system.A good surge protector guards against low and high incoming voltage as well as surges and spikes.

Campground pedestals take a lot of abuse from the many guests who use them. Frequent plugging in and unplugging takes its toll on the receptacles; the contacts in those receptacles can wear out, especially if a motorhome is plugged into a live outlet while a number of circuits — in particular, large loads such as air conditioners — are left on. If the neutral connector fails, the result is an open neutral, which will put up to 240 volts across both hot poles on a 50-amp supply. That will cause a number of electrical components to fail and will result in an expensive repair bill. So, any device labeled as a “surge protector” should have low- and high-voltage protection as well as open neutral protection.

Surges And Spikes

The electrical components in a motorhome are designed to work within a specific voltage range. I like to compare voltage in a circuit to water pressure. If a motorhome’s water pressure is too high, burst water lines and hoses are the result. Likewise, voltage that exceeds the maximum specified level will damage or destroy electrical components.

Surges and spikes are a function of high voltage. A spike lasts only for one or two nanoseconds (one or two billionths of a second), whereas a surge lasts three nanoseconds or longer. In either case, damage can occur.

Surges and spikes are fairly easy to stop. A metal oxide varistor, commonly known as an MOV, serves as a shunt to divert electricity to ground when incoming voltage is excessive. An MOV looks like a small disc and consists of metal oxide sandwiched between two semiconductor layers. Each layer has a wire lead, with one wire connected to the incoming power and the other to ground. The MOV acts as a variable resistor. When incoming voltage is within specifications, the MOV has a high resistance and the incoming power passes it by without affecting the MOV. However, when the voltage exceeds the MOV’s maximum tolerance level, the MOV goes to low resistance and the current flows through the MOV to ground, dampening the surge or spike and protecting the coach from a damaging increase in voltage. Think of it as a safety valve on an air compressor that bleeds off excess air pressure to prevent damage.

Surges and spikes are rated in joules. One joule is the amount of energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. In other words, the bigger the surge, the higher the rating in joules. Not all MOVs are the same, and ratings can vary, so when searching for a surge protector, look for one with the highest possible rating in joules. The higher the rating, the better the protection. A large enough surge can exceed the capacity of a surge protector. In that case, the MOVs destroy themselves while attempting to save the electrical system. The self-sacrificing MOVs then must be replaced in order to restore surge protection. Some units may need to be returned to the factory for repair, while others can be serviced in the field with a simple circuit board swap.

Open Neutral Danger

Fifteen-amp, 20-amp, and 30-amp pedestals have only three conductors — hot, neutral, and ground. A break in the neutral conductor creates an open circuit, which means no power to the coach. A 50-amp receptacle has four conductors — two hots, one neutral, and one ground. Alternating current (AC) follows the path of least resistance. Ohm’s law says voltage equals current (amperage) multiplied by resistance. So, doubling the resistance allows for double the voltage with the same current flow.

If 30 amps is on one hot pole of a 50-amp supply, and 20 amps is on the other pole, the two phases are not in balance; the excess, 10 amps, flows down the neutral conductor. If the neutral conductor does not exist, the condition is called open neutral. It causes up to 240 volts to pass between the two hot phases, because the resistance in each phase is not equal. Those 240 volts will destroy any electrical component in their path. All this math aside, it’s important to remember that an open neutral can be disastrous to an electrical system because of the presence of up to 240 volts.

Low Or High Pedestal Voltage

We’ve already discussed how high voltage from spikes and surges can destroy equipment. Another problem can occur when campgrounds continually provide voltage that exceeds the maximum allowed. This may stem from a utility transformer that is set too high to allow for long runs to a campground that has power pedestals spread over a large area and an inadequately designed electrical grid. Such a scenario isn’t as common as low voltage, but it does exist.

As noted earlier, generally the most common danger to a motorhome’s electrical system is low pedestal voltage.Many campgrounds have added sites over the years or upgraded from 30-amp pedestals to 50-amp pedestals. But, a campground’s electrical grid may never have been designed properly for the increased load. Once guests arrive in the middle of an afternoon and start switching on their air conditioners, the voltage drops as the demand for power increases.

Electrical components are rated in wattage. Wattage is a true power measurement and equals voltage multiplied by amperage. Amperage is a measurement of current flow. Each conductor is rated for a maximum amperage; excess amperage causes heat to build up from resistance in the circuit, which is why circuits utilize fuses or circuit breakers for over-current protection. Wattage is always constant, so when the voltage drops, amperage increases.

For example, if a 120-volt air conditioner is designed to pull 20 amps as a starting surge, it will pull 24 amps if the voltage drops to 100 volts. The excessive amperage caused by low voltage will increase heat in the wires, and eventually will damage the compressor motor. Other electrical components can be damaged in the same manner.

Choosing A Surge Protector

The key in choosing a surge protector is to ensure that it provides adequate protection. Surge protector ratings differ, so to guard against surges and spikes, look for units that exceed 3,000 joules. Suppliers such as Southwire Company LLC (formerly Technology Research LLC, or TRC) and Progressive Industries offer a variety of suitable devices.

Some portable units are designed to hang on a power pedestal. The advantage is that they can be moved easily from one RV to another. However, they are exposed to the elements and potential theft.

Hardwired units mount inside the coach basement where they are secure and protected from weather. Many have remote displays that can be viewed from inside the coach and provide information such as voltage, amperage, and error messages. Such units must be installed, but typically that’s just a matter of splicing into the shore power cord prior to the transfer switch and maybe running a small phone-type cable to a remote display, if you choose to add one.

You’ll want a surge protector that handles more than surges; it also must monitor voltage levels. Standard tolerances on 120-volt components are plus-or-minus 10 percent, which means a range of between 108 and 132 volts. RV surge protectors with voltage-monitoring capabilities typically lower the minimum voltage a bit to allow for temporary voltage dips when heavy loads are applied to a 50-amp power supply. Therefore, it’s not uncommon to see units with low-voltage cutout values as low as 102 volts. If the voltage exceeds the maximum or drops below the minimum, the device will disconnect voltage from the coach to prevent damage.

Reverse polarity, miswired pedestals, and open neutrals are real dangers as well, so a surge protector should be capable of disrupting power to the coach should any of those conditions exist. Some sort of notification is also necessary so that it’s clear why the motorhome’s power has shut off. This can be as simple as a few illuminated LEDs on the unit itself; or, it can be a digital display, either on the surge protector or mounted inside the coach living area.

Most surge protectors can delay applying power to a motorhome when the vehicle is first plugged into the pedestal. The delay, which may be as long as two minutes, is designed to allow air conditioners time to bleed head pressure from the compressor to reduce the load when the compressor starts. Many modern air conditioners now have an automatic delay built in; in that case, some surge protectors can be set to eliminate the startup delay.

Voltage Boosters

As mentioned, a good surge protector halts incoming power to the motorhome if the voltage drops below parameters. Without power, it may be possible to run the generator, but that consumes fuel, makes noise, and may not be allowed in your RV park. Another option is a voltage booster that also monitors incoming voltage, such as a Hughes Autoformer.

If incoming voltage is 115 volts or higher, the Autoformer passes the current through. If the voltage drops to 113 volts or lower, the Autoformer boosts the voltage by 10 percent. This makes it possible to operate with pedestal voltages as low as 95 volts by boosting it to 104 volts or better. If the voltage drops below 95, the Autoformer won’t provide enough voltage to safely power the coach. But the vast majority of the time, the Autoformer will provide safe and ample voltage when park power is sagging.  

The Autoformer offers replaceable surge protection and has indicators that tell of open neutrals and other dangerous electrical conditions. However, the Autoformer does not shut down the RV when an event occurs, so it should be used in conjunction with a good surge protector. In that case, the Autoformer should be placed in front of the surge protector; that is, the Autoformer is connected to the pedestal, with the surge protector next, and then the RV. Otherwise, the surge protector will cut off incoming low voltage, preventing the Autoformer from boosting it.

Automatic Transfer Switches

Southwire Company makes a line of automatic transfer switches. Some units, such as the 50-amp model 41260, offer some protection against surges, open neutral, and reverse polarity conditions, but not low or high voltage. Model 41260 comes as original equipment in a number of motorhomes, but be sure to purchase a surge protector that provides better surge safeguards as well as low- and high-voltage protection.

Southwire also sells model 40350-RVC, a high-end transfer switch that features better surge protection as well as low- and high-voltage protection. It has RV-C network compatibility, so you may find it as original equipment on the latest high-end coaches with Total Coach or VegaTouch touch-screen multiplex wiring systems. Transfer switches such as model 40350-RVC protect generator power as well as shore power, although generator power issues are extremely rare. If a new transfer switch is installed, the original one is removed, so there’s no need to find additional room for the replacement switch, nor is it necessary to cut into the shore power cord. A coach that has a 40350-RVC transfer switch does not need additional protection, although you may still choose to carry an Autoformer for those times when the voltage needs a bit of a boost.

If a motorhome’s electrical systems are unprotected, it’s not a matter of whether damage from power issues will happen, but when. The cost of repairing that damage far outweighs the cost of purchasing a good surge protector. In other words, a surge protector is inexpensive insurance. It’s a good investment that will pay off in the long run.


Following are some power protection devices, commonly referred to as surge protectors, available from three well-known suppliers.

Southwire Company LLC
Southwire’s Surge Guard brand of surge protectors are available in hardwired and portable models for 30-amp and 50-amp power supplies. A line of automatic transfer switches and a voltage booster also are available.

Model 35550 is a 50-amp hardwired unit that provides 3,850 joules of surge protection. It protects against low voltage (less than 102 volts) and high voltage (more than 132 volts), open neutral, reverse polarity, open ground, and miswired pedestals. The unit mounts in the motorhome basement; an optional remote LCD monitor displays voltage, amperage, and status or error messages.

Model 34850 is a 50-amp portable unit that plugs into a pedestal and accepts a coach’s power cord. The unit provides 3,850 joules of surge protection. It also protects against low voltage (less than 102 volts) and high voltage (more than 132 volts), open neutral, open ground, reverse polarity, and miswired pedestals. LED lights and an LCD display show voltage, amperage, and status or error messages. A new feature of the unit: Power shuts off if the plug or receptacle overheats or if an open ground or open neutral condition is detected.

Model 40350-RVC is a 50-amp automatic transfer switch that provides 3,350 joules of surge protection. It guards against low voltage (less than 102 volts) and high voltage (more than 132 volts), open neutral, open ground, reverse polarity, miswired pedestals, and high and low frequency. A 65-amp contactor is mounted on vibration isolators to ensure quiet operation. The unit is equipped with LED status lights. An optional two-line plain English display indicates voltage, amperage, and power system status messages.


Progressive Industries
Surge protectors are available for 30-amp and 50-amp power supplies in hardwired and portable models. The popular Electrical Management Systems (EMS) series offers a full load of features. Some models are equipped with embedded displays, while others have remote display capabilities. EMS products have a lifetime warranty; the hardwired units can be serviced in the field rather than sent back to the factory.

Model EMS-HW50C is a hardwired 50-amp unit that provides 3,580 joules of surge protection at 88,000 amps. It protects against low v0ltage (less than 104 volts) and high voltage (more than 132 volts), open neutral, open ground, reverse polarity, miswired pedestals, and high and low frequency. A digital display scrolls through various voltage, amperage, and frequency readings, as well as status or error codes.

Model EMS-PT50X is a portable 50-amp unit that offers 3,580 joules of surge protection at 88,000 amps. It protects against low voltage (less than 104 volts) and high voltage (more than 132 volts), open neutral, open ground, reverse polarity, miswired pedestals, and high and low frequency. The unit’s LED display scrolls through various voltage, amperage, and frequency readings, as well as status or error codes.


Hughes Autoformers
While not a complete power protection system, Autoformers can be used in conjunction with a surge protector to boost incoming voltage rather than just shutting down power to the coach.

Model RV220-50SP, Hughes’ latest product, features 4,800 joules of surge protection in addition to a 10 percent voltage boost when incoming voltage drops below 113 volts. An array of LED lights provides park power diagnostics. The Autoformer can be mounted inside a vehicle with an optional connection kit. A 30-amp model is also available.


More Info

Hughes Autoformers
1523 N. Harmony Circle
Anaheim, CA 92807
(888) 540-1504

Progressive Industries, C9298*
1020 Goodworth Drive
Apex, NC 27539
(919) 267-6948

Southwire Company LLC
4525 140th Ave. N.
Suite 900
Clearwater, FL 33762
(800) 780-4324


* FMCA commercial member

Electrical Testing Devices

To ensure a safe electrical connection to a shore power receptacle, Gary Bunzer, “The RV Doctor,” recommends performing three simple tests before plugging a motorhome into any source of electricity. The checks require relatively inexpensive testers.

  • A digital multimeter (DMM) measures the actual voltage coming from the campground. The voltage should be 120 volts, plus or minus 5 percent.
  • A noncontact voltage tester verifies polarity by indicating which conductor or conductors in the receptacle are hot. It also can check for a “hot skin” condition. If an RV is plugged into an improperly wired electric plug or cord, a potentially lethal situation can result, such as electrifying the metallic components or the chassis.
  • A three-light circuit analyzer is used to check receptacles in the interior and on the exterior of a motorhome after plugging in, but only after first measuring the voltage and verifying the polarity of the receptacle at the voltage source.

For detailed instructions on performing these tests, watch Gary’s FMCA Motorhome House Calls video at

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