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Pick any device attached to the circuit you're investigating. Find the wattage (W) listed on the data plate — usually on the back or underside of the device or near where the power cord is connected. This is the maximum power rating of the device, which can be used to calculate amperage. Some devices will list the amperage directly, often labeled FLA, meaning "Full Load Amps". If it does, skip down to the next section to interpret that rating. For household circuits, you can usually assume your house follows the voltage standards of your country. (For example, 120V in North America and Central America, or around 220V to 230V for most other countries.) If you think you are working with an exception, measure the voltage using a multimeter. The answer will be the amperage the device draws on your circuit. For example, a 150-watt device on a 120-volt circuit will draw 150 ÷ 120 = 1.25 amps. Perform the same calculation for each other device on the circuit, or at least the ones with the highest wattage. Write down each answer next to the name of the device. Take the continuously running devices, or the devices that are expected to use the maximum current for 3 hours or more at a time. Add their amperages together. If the result is more than 80% of your circuit breaker's rated amperage, plug one of the devices into an outlet on a different circuit. On top of the continuous amperage, add the amperage of other devices that might be on at the same time. If any combination gets above 100% of the circuit breaker's rating, it will trip the circuit. You can solve this by moving a device to a different circuit, or by remembering not to use high-power devices at the same time. Electrical circuits never operate perfectly. Some of the energy is lost to heat, and devices may draw more current to make up for this. Waste is low in most household circuits (below 10%), but it is still possible to trip the breaker if the total calculated amperage on paper is slightly below the breaker rating. A clamp multimeter (or clamp ammeter) has a pair of "jaws" at the top that close shut to encircle a wire. When set to measure amps, the device will display the number of amps running through that wire. To test a circuit, expose the wire leading to the load side of a circuit breaker. With the clamp multimeter set up as described, have a friend turn on other devices in the house. If the device is on the same circuit, you will see the amperage display increase. Do not attempt this unless you have electrician's gloves and a basic understanding of electrical safety. These wires are live and removing the front panel of a breaker panel will expose you to dangerous voltages, so use extreme caution.

Summary:
Find the wattage of a device. Check the voltage on the circuit. Divide the wattage by the voltage. Repeat for each device on the circuit. Add the amperages of devices that are always running. Add additional amperages. Measure amperage directly with a clamp multimeter (optional).