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Old 01-03-2002
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Correct Voltage

I have a generator that is putting out 133 volts. I am concerned that this is too high and may damage items on my boat over the long term. Is there a range that is considered acceptable for being over and under voltage?

Tom Wood responds:
Small diesel and gasoline-powered motor generators have a bad reputation for putting out "dirty" power—alternating current that has an uneven cycle, undulating voltage, or irregular amperage. In fact, some of them are so bad that a standard analog voltage meter is incapable of reading the line voltage correctly. So you should probably begin by confirming the generator’s 133-volt reading with an RMS voltmeter.

An RMS (Root Mean Square) meter is the only correct way to measure voltage accurately (and the only way to measure the square-wave output of most inverters at all). Since they are expensive, most boat owners don’t want to invest in one just to take an occasional measurement. But every professional electrician has one—the company best known for making RMS meters is Fluke, and many electricians call them Fluke meters. I suggest you beg, borrow, or barter the use of a RMS meter and double-check your voltage output before going any further.

The standard for AC appliances running on over or under-voltage is 10 percent. So a 120-volt appliance would have a running range of between 108 and 132 volts—one volt shy of your measured 133 volts. But the problem is more complex than that. If you start picking up your appliances and reading the tags on them, you will soon see that some were designed to run on 110 volts, a few on 115 volts, and the rest on 120 volts. A motor designed and built to run on 110-volts would operate comfortably on voltages from 99 to 121, but 133 volts would be far too great. You can see that any appliance, whether it was built for 110, 115, or 120 volts would run well on any of the three line voltages and still be within the 10 percent parameters.

If a measurement with an RMS meter confirms that your generator is indeed putting out 133 volts, it’s time to either adjust or replace the voltage regulator. The indication is that the generator itself is fine, but the setting on the regulator is allowing the voltage to drift too high. Best of luck to you.

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