How to Size a Home Generator
Back-up power systems in homes ensure both safety and comfort during power outages from storms or other emergencies. Home generators form the basis of these back-up systems. Incorrectly-sized generators, however, can cause system failures, unreliable performance, maintenance problems and safety issues. It is vital that homeowners understand and follow the actions required to size their generators correctly. Does this Spark an idea?
Things You'll Need
- Hand-held clamp-on amp meter or power analyzer with Peak function
- Line-worker's rubber gloves
- Assistant
Instructions
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1
Make a list of each piece of electrical equipment the generator will power, such as appliances, furnaces, lights and computers.
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2
Measure continuous power rating for the generator or the amount of power consumed by all items running simultaneously. Locate the main disconnect box, and remove the front cover. Wearing gloves, clamp the meter to the line wire. Take separate current readings for each hot---usually black---wire. Add volt-amp readings from each wire together, and multiply by 120 for wattage figure. To ensure power consumption is measured accurately, take several readings approximately 10 minutes apart, and select the highest reading. Add an additional 20 percent safety margin to the total wattage.
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3
Determine surge current requirements. Many appliances such as refrigerators, furnace fans and air conditioners are driven by motors, which have higher current requirements when initially starting than when running. This factor must be incorporated into the total wattage requirements for the generator. Remove the cover from the distribution box. Set the meter to Peak or surge reading. Wearing gloves, clamp the meter to the wire feeding appliance. Ask an assistant to turn on an appliance; record the surge current reading. Repeat for each appliance to be powered by the generator. Assuming only one appliance will start at a time, determine which item has the greatest difference between running and surge current. Add the difference to the total current for all items.
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4
Determine if more than one appliance will need to start simultaneously. If so, the generator selected should provide enough starting current for multiple loads to prevent overheating, burning out or tripping the generator's circuit breaker. Refer to earlier surge current readings for appliances. Add the difference between running and surge currents for each selected item to get the total current requirement.
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Make a list of appliances and equipment with solid state circuitry, such as radios, televisions, microwave ovens and computers. These items are highly voltage sensitive and can be damaged by harmonic distortions in electrical power. If they are to be powered by the generator, it should stabilize its output and prevent distortions.
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6
Select a generator with a continuous rating equal or larger to the total wattage requirement calculated and which can produce a wave-form with less than 10 percent harmonic distortion.
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Tips & Warnings
Generators must be wired into homes by licensed electricians to ensure proper safety precautions and eliminate power feeding back to municipal power supplies.
Additional appliances to be powered increase generator size and cost. Limit items to be run by the generator to only the most essential, such as refrigerators, lights, medical equipment, security systems and septic systems.
If the generator purchased is unable to power the entire house, rotate the use of essential appliances during the emergency. Run each appliance for only an hour, and switch to another.
If in doubt or fearful about measuring the electrical current, consult a licensed electrician.
Generator fuels may be flammable. Keep in approved containers and out of reach of pets and children.
