How to test a 240-volt circuit

Written by raul avenir
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How to test a 240-volt circuit
Your air conditioner runs on 240 volts. (Klimaanlage image by Imagemaker from

Electricity flowing through your circuit breaker is rated at 120 volts, but it is commonly referred to as 110 volts, just as electricity at 240 volts is typically called 220 volts. It takes two 120-volt circuits and a ground wire to form a 240-volt circuit. A 240-volt circuit is typically installed to provide power for heavy-duty appliances, such as air conditioners, clothes dryers, stoves, ovens and water heaters. Learn how to test your 240-volt circuit to help you diagnose problems with your electrical circuit.

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Things you need

  • Multimeter

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  1. 1

    Observe your 240-volt wall outlet and learn its configuration. A 240-volt wall outlet commonly has three openings: two identical slots slanted at 45 degrees (sometimes vertical), and one middle slot placed lower than the two vertical slots. Each of the two vertical slots carries 120 volts, while the third or middle slot connects to the ground.

  2. 2

    Turn the dial of your multimeter to a range of 240 volts AC, and attach the black and red test probes to your multimeter. Remember never to touch the metal tip of the probes when they are in use to avoid electric shock.

  3. 3

    Slip the metal end of one test probe into any of the slanted 120-volt slots, and slip the metal end of the other test probe into the next identical slot. Look at the reading displayed by of your multimeter. The two 120-volt circuits connected to your multimeter are producing a total current of 240 volts. Your multimeter should display a reading of approximately 240 volts; if it doesn't, one or both circuits might have a problem.

  4. 4

    Turn the dial of your multimeter and set it to120 volts. Insert the metal tip of the red probe into any of the slanted 120-volt slots, and slip the metal tip of the black probe into the middle (ground) slot. Your multimeter should read approximately 120 volts AC. If it does not, then that circuit is defective. Perform the same procedure on the other 120-volt slot to find out if that circuit is working.

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