Autotransformer Use: "Stealing Power?"
Dave and Helen Damouth www.damouth.org
26 June 2005
We occasionally see advertisements touting the advantages of using an autotransformer in an RV to correct a low-voltage condition at an RV park, or we hear glowing reports about autotransformer use from other RVers. See, for example: Hughes Autoformer. Yet few RVers understand just what an autotransformer does and how it does it, and some folks have expressed a vague feeling that using an autotransformer is somehow "stealing power".
The most important motivation for using an autotransformer is that certain appliances, most notably an air conditioner, may overheat and shut off or suffer permanent damage if operated for long periods at a voltage substantially lower than normal. Opinions vary as to what is a safe voltage. On my old trailer, I once operated the air conditioner at about 98 volts continuously for several days, with no apparent damage. Others have said that the limit should be placed at around 104 to 106 volts. Every air conditioner has a thermal switch which protects it by shutting it off if gets too hot, turning it on again after it cools down. But milder overheating which doesn't trip the thermal switch may, over a long period, shorten the life of the product, presumably by degrading the electrical insulation in the motor windings.
The following article uses a simplified example to explain the function of an autotransformer and the consequences of using it."Stealing" may not be the right word. But the use of an autotransformer when the campground voltage is low will indeed result in you getting more power at the expense of your neighbors, whose voltage will drop even lower when you turn on your autotransformer.
Consider the following very simplified scenario:
You and one neighbor in a remote part of the RV park share a long power feed from the park's distant main feed. I'll assume that the main feed is always at 120vac, but that the long power wires shared by these two remote sites have a resistance of 0.25 ohms (not unlikely - that's only 200 feet of #8 cable).
Further assume that both of you are using exactly the same appliances which would add up to each of you drawing 25 amps if the voltage remained at 120 volts.
But the voltage won't remain at 120 volts (because of the resistance in the long power feed). Under these conditions, the voltage at each site will be 108.68 volts and each site will be drawing 22.64 amps.
Now, suppose you, but not your neighbor, installs an autotransformer. The Hughes Autoformer product increases voltage by a fixed 10% in a low-voltage situation, so that inside your RV, you now have 118.38v. This raises your current draw inside the RV up to 24.66 amps. (This additional load causes more voltage drop in the shared line, dropping the voltage coming into each RV to 107.62 v. so your inside voltage increased a bit less than the expected 10%).
Your use of the autoformer has reduced your neighbor's voltage from 108.68 v. to 107.62 volts, reduced his current from 22.64 amps to 22.42 amps, and reduced his power usage from 2.46 kilowatts to 2.41 kilowatts. Your voltage inside the RV increased from 108.68 to 118.38, your inside current increased from 22.64 amps to 24.66 amps, and your power from 2.46 kw to 2.92 kw. Your neighbor has a 2% power reduction while you see a 19% increase in your own power usage. This analysis assumes that the loads in your RV are resistive - see footnote.
This seems like a pretty good tradeoff, unless your neighbor has a power management system that shuts off her power when the voltage drops below a certain level. In some situations, your ability to use additional power by using the autotransformer could result in enough additional incoming voltage reduction so that your neighbor loses power completely. If you want to be a nice guy, you can turn off a bit more of your own power usage, returning his voltage and power to where it was before you used your autotransformer while still retaining the advantage of having normal voltage in your own rig.
Note that even though you may have a 30-amp power pedestal, you can not increase your internal current all the way up to 30 amps in a low-voltage situation. This is because when the autotransformer increases the inside voltage by 10%, (which is a 20% increase in power), it also must increase the current drawn from the power pedestal by a bit over 20%. So if you turned on 30 amps of loads inside the RV, you would be drawing considerably more than 30 amps from the pedestal, tripping the 30 amp circuit breaker.
Also note that if you and your neighbor both have autotransformers, you both win, since you both have higher voltage and more available power inside your rig. The campground owner loses a bit - a few hundred watts of additional power loss in the campground wiring.
We won't argue about whether the campground wiring in the above example meets code or not - a major reason for buying an autotransformer is that many rural campgrounds have wiring that is not designed to modern codes.
The ethics of increasing your own share of an inadequate power supply at the expense of your neighbors are debatable. Some might say it's unethical to run the heater (or the air conditioner) at all, in situations where there is inadequate power to supply everyone. Others will say "hey - the campground owners are at fault, I paid for 30 amps, and I'm simply using what I paid for".
A special situation occurs when the regional power company has inadequate generating capacity during a heat wave and deliberately institutes a "brownout", reducing everyone's voltage as a way of "fairly" distributing the discomfort to everyone. (Their only alternative would be rolling "blackouts", completely shutting off the power to some customers in order to maintain full voltage to the rest - a strategy that is sometimes used, usually with advance notice, remuneration, and the cooperation of the customers to be shut off).
In a brownout situation, it indeed would seem to be a bit unethical to use an autotransformer, unless one also reduces total power usage by turning off some loads, so as to keep the full 120 v. available to critical loads while not increasing one's total share of the limited resource.
Footnote: Although most electrical loads in an RV are "resistive" (meaning a positive linear relationship between voltage and current), an air conditioner (or any other inductive motor) is a special case. If a major part of the RV power usage is due to an air conditioner (a common situation), the above analysis changes significantly. These motors actually have an inverse current/voltage relationship. As the voltage drops below normal, the current rises. At very low voltages, the current increases to the point where the motor may overheat. In this situation, using an autotransformer will prevent the overheating, and may not significantly increase your total power use (it could conceivably even slightly reduce total power use!)