How to Calculate Amperage Draw

By Andrea Becker
If you know the wattage of your light bulb, you can figure out how many amps it is drawing.
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The electricity flowing through the wires in your house is often compared to water running through a hose. You can observe the size of the hose, the amount of water flowing through it, the water pressure and the result of the water spraying out. For electricity, the flow of the current is limited by the resistance to flow, measured in Ohms. The amount of current flowing through the wire is measured in amperes, or amps. The equivalent of water pressure is voltage, or volts. Finally, the power produced by the electricity is measured in watts. All of these measures are interrelated. You can calculate amperage if you know the voltage of your power source and the resistance or wattage rating for your device.

Calculating From Watts and Volts

Step 1

Find the wattage of your load. Any device that draws energy is called a load. Examples of loads include a light bulb and a microwave. The wattage is often printed on the device itself, but if you can’t locate the number, you might need to check the owner’s manual.

Step 2

Find the voltage of your power source. In the United States, most household outlets run at 120 volts, although some, such as those for electric stoves or dryers, often run at 220 volts. If your power source is a battery, you will need to look up the voltage. Larger batteries are often 9 or 12 volts, while smaller closed cell batteries, such as C, AA or AAA, run between 1 and 3 volts, depending on size and composition.

Step 3

To calculate the amperage draw from your device, divide the wattage rating by the voltage from your power source. For example, if you have a 100-watt light bulb in a lamp that is plugged into a 120-volt outlet, it will draw 0.83 amps.

Calculating From Ohms and Volts

Step 1

Use Ohm’s law to calculate amps using resistance. Many appliances have a listed resistance. The wire connecting the circuit also has a variable resistance; in the same sense, you can fit less water through a garden hose than a fire hose. You don’t need to include this resistance unless you have a lot of wire or need to be very accurate.

Step 2

Find the voltage of your power source as you would when calculating from watts and volts.

Step 3

Ohm’s law states that the voltage equals the amperage times the resistance, so if you divide the voltage of your power source by the resistance of the load, you will find the amps. For example, if you plug a 40-Ohm dryer into a 220-volt outlet, the appliance will draw 5.5 amps.

About the Author

Based in Wenatchee, Wash., Andrea Becker specializes in biology, ecology and environmental sciences. She has written peer-reviewed articles in the "Journal of Wildlife Management," policy documents,and educational materials. She holds a Master of Science in wildlife management from Iowa State University. She was once charged by a grizzly bear while on the job.