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Tuesday, January 12, 2010

Sky High Electric

I have a Flair motorhome and my electric bill at the RV parks are about three times higher than my neighbors who seem to be using more electrical items. I’m not using the air conditioner and I have a new TV. My refrigerator and converter should be the only things drawing 120-volts AC. All my lights are 12-volt DC. My electric bill runs about $130 a month while my neighbors have bigger rigs plus a wife, which I don’t have. Their usage runs about $35 or $40 per month. My batteries are only two years old. How can I test amp draw on the refrigerator and convertor? John, (Parker, AZ)

John, you’ll need a clamp-on ammeter in order to measure the exact current draw from any electrical appliance while the coach is plugged into 120-volt AC. There is a way to use an in-line meter with an “amperage” scale, but that method means breaking the circuit to the appliance. I wouldn’t recommend that method. There are also aftermarket products such as P3 International's Kill-a-Watt, but devices like theirs are only applicable to 120-volt AC loads equipped with an external cord. It will not work on "hard-wired" appliances, etc.


Another way to calculate current usage is using Ohm’s Law. Ohm’s Law is defined as the way to express the relationship between voltage, current and resistance in any given circuit. They are mathematically relative to one another in the same unique way in each type of circuit. If any two of the three values are known in a circuit, the third can be determined by applying Ohm’s Law.

There are three ways in which to express Ohm’s Law. In text format, Ohm’s Law states that the current equals the voltage divided by the resistance. Also, the voltage equals the current multiplied by the resistance. Or that the resistance equals the voltage divided by the current. Another portion of Ohm’s Law, sometimes referred to as the Power Law, brings wattage into the equation whereby wattage equals the voltage multiplied by the current. You know the voltage is probably close to 120-volts AC so that becomes one of the “knowns.” Most AC devices today will have either the current or wattage rating stamped or printed somewhere on the device or in the user’s guide. I’d begin by calculating how much current (or wattage) your AC devices should be drawing and start there. Factor in how long each appliance is operating and drawing current. Do you get the same result at different campgrounds? If not, there is the possibility of a wiring fault or a metering fault within the campground itself. Sorry, can’t help you with the wife issue.

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