How to Calculate Total Amperage in a Light Fixture

Modern homes use electricity to power light-emitting-diode lights, whole-house air conditioners and just about everything in between.
Light fixtures are just one of the many electrical devices in a modern home.Light fixtures are just one of the many electrical devices in a modern home.
As a safety measure, electricians place a breaker box (fuse box in older homes) at the point where power enters the house. If the power drawn by all electrical devices on any circuit exceeds the circuit breaker's rated amperage, that breaker will "trip" and open the circuit. All the devices on the circuit will immediately turn off. Calculating the amperage drawn by any of those devices is a matter of elementary arithmetic.

Step 1

Electricity is delivered to your home with a standard voltage.

Determine the voltage of your household current. In the United States and Canada, standard household current is 120 V. If you live in another country, ask your power company or consult an international table.

Step 2

The breakers in your home's breaker box "trip" when the circuit overloads.

Determine your appliance's power consumption in watts. Appliances and other plug-in electrical devices bear a label printed with the device's power consumption, measured in watts (W). For example, the label on a tabletop radio might say, "Power Consumption: 55W." To determine the power consumption of an electric light fixture, sum the wattage of all bulbs in the fixture. A fixture with four 60-watt bulbs consumes 240 watts of electricity.

Step 3

The wattage of a light with multiple bulbs is the sum of the wattage of all the bulbs.

Calculate the device's power draw in amps. The amperage drawn by any electrical device is related to the voltage in the circuit and the device's power consumption. The relationship is simple: Amperage = Watts / Volts. So to calculate the amperage drawn by a light fixture or other electrical device, divide its power consumption by the voltage of your household current. For example, an electric light with three 60-watt light bulbs on a 120-V circuit draws 180W / 120V = 1.5 amps. On a 240-V circuit such as you might find in Australia, the same light fixture draws 180W / 240V = 0.75 amps.

Things You Will Need

  • Calculator

Tip

  • Light bulb wattage is printed on the glass or stamped into the metal base of small bulbs.

Warning

  • Always exercise caution when dealing with electricity.

About the Author

Kelvin O'Donahue has been writing since 1979, with work published in the "Arizona Geological Society Digest" and "Bulletin of the American Association of Petroleum Geologists," as well as online. O'Donahue holds a Master of Science in geology from the University of Arizona, and has worked in the oil industry since 1982.