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How to Measure Currents

Measuring the flow of electric current in a circuit is one of the basic tests performed when troubleshooting a malfunctioning branch circuit or electrical appliance. Knowing the amount of current drawn by an electrical load will expedite isolating the cause of electrical problems.

Digital multimeters and clamp-on digital ammeters are two ways of measuring current.

Things You Will Need

  • Amprobe digital clamp-on multimeter
  • 3 prong line separator

Measuring the flow of electric current in a circuit is one of the basic tests performed when troubleshooting a malfunctioning branch circuit or electrical appliance.  Knowing the amount of current drawn by an electrical load will expedite isolating the cause of electrical problems.

There are two ways of measuring the current flowing in a circuit.  You can physically connect a conventional ammeter in series (in line) with one of the current-carrying conductors, or you can use a clamp-on meter which has jaws that snap around the current-carrying conductor.


General Instructions

  1. Access the electrical wiring that supplies the load for which you are measuring the current draw. A load is anything that consumes electrical energy, such as a light bulb, clothes dryer or washing machine. Depending on the load, you may need to remove a junction box cover, a sheet-metal panel or some other cover. After gaining access to the wiring, carefully separate the current-carrying wires supplying current to the load. The current-carrying wires supplying a 110 volt load will be a black and a white wires. The wires supplying a 240 volt load are a red and a black insulated wires.
  2. Turn the clamp-on multimeter power switch to the on position. Set the clamp-on multimeter function switch to its amperes position. Squeeze the lever, opening the clamp-on multimeter's movable jaw. Slip the open jaw over one of the current-carrying wires and release the lever, closing the jaws around the wire. In order to make sure you get an accurate reading, make sure that you have only one of the current-carrying wires inside the jaws.
  3. Turn on the load being tested and read the amount of current drawn by the load on the liquid crystal display on the meter. The 120/240 volt line voltages found in your home are nominal voltages. Nominal voltages may vary by plus or minus 10 percent and still be acceptable. If the load current drawn falls within 10 percent of that current stamped on the loads name plate, it is OK.

Using the 3-prong Line Splitter

  1. Plug the 3-prong line splitter into any 120-volt receptacle. Plug the cord of any plug-connected load (vacuum cleaner, coffee maker or other small appliance) into the receptacle on the 3-prong line splitter. The line splitter serves to separate the two current-carrying conductors inside the appliance cord. Without the splitter, you would have to slit the cords outer covering to separate the conductors.
  2. Set up the digital clamp-on multimeter as you did in Section 1.
  3. Snap the clamp-on meter's jaws through one of line splitters openings and take the reading. The same percentages as given above apply to these readings.

Things You Will Need

  • Amprobe digital clamp-on multimeter
  • 3 prong line separator

About the Author

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.

Photo Credits

  • Polka Dot Images/Polka Dot/Getty Images
  • Polka Dot Images/Polka Dot/Getty Images