How to Measure an Amp Draw

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Electricity is electrons moving through a conductor. Amps are the unit of measure used to express how many electrons are flowing through the conductor. To measure amperage -- the flow of electrical current -- the meter must be placed directly in the circuit so the current flows through it. Newer clamp-on meters have clamps to place around a wire or conductor instead of requiring you to make the meter a part of the circuit, making measuring the current both safer and easier.

Things You'll Need

  • Multimeter
  • Alligator clip meter leads
  • Clamp-on ammeter

Ammeter Series Connection

  • Disconnect the circuit from any electrical source including power supplies, wall receptacles or frequency generators. Never work on a live circuit.

  • Disconnect the positive supply wire from the power supply terminal. Clip the black meter lead's alligator clip onto the end of the positive supply wire. Plug the other end of the black lead into the meter's common socket.

  • Plug the red meter lead into the AMPS socket on the multimeter. Clip the alligator end of the red lead onto the power supply terminal.

  • Set the multimeter to read either DC amps or AC amps as appropriate for the current being measured. Set the range one value higher than the expected measurement range, or if the range is unknown, choose the highest range possible and adjust it down if necessary.

  • Verify that none of the bare lead ends touch each other or any metal surface. Turn the power supply on and then turn on the circuit. The meter reads the current flow in amps.

Clamp On Ammeter

  • Set the ammeter to the expected current range. The circuit breaker rating is a good way to choose the current range -- a 20-amp circuit breaker indicates a maximum current of 20 amps.

  • Open the ammeter jaws and place them around a single conductor. For example, a lamp cord has two conductors. To measure the current to the lamp, you must first separate the two conductors and then place the clamp around just one of them.

  • Turn the device or appliance on if it is not already on. The ammeter reads the amount of current flowing through the conductor.

Tips & Warnings

  • Clamp-on ammeters are mainly used on circuits to measure AC current greater than one amp. In series meter connections, they can measure much smaller currents and DC currents.
  • Some clamp-on ammeters are capable of measure currents down to one milliampere, but are typically very expensive.
  • Don't work on live circuits with ammeters that require you to make a series connection. Clamp-on ammeters don't require you to touch any bare conductors and are safe to place around live wires.
  • If you place the clamp-on ammeter around two wires, it adds the amps together. This means that placing it around the two lamp wires mentioned previously will result in a reading of zero volts since the electricity is always moving in opposite directions on the two wires.

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References

  • "Introductory Circuit Analysis"; Robert L. Boylestad; 1981
  • Photo Credit Hemera Technologies/PhotoObjects.net/Getty Images
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