What the COM Jack Is for on a Multimeter

ByKen

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What the COM jack is for on a multimeter is one simple job: it is the common input where the black test lead plugs in. It serves as the reference point for almost every measurement the meter makes, including DC voltage, AC voltage, resistance, and current. The red lead is the one that moves between jacks. This guide explains what the COM jack does, which jack the red lead uses for each common measurement, and where the sources we opened leave details open.

Quick answer

  • The black lead stays in the COM jack for every measurement.
  • COM is the reference, so the reading on the display is the potential of the red probe measured against the black probe.
  • The red lead goes to the V and ohm jack for voltage and resistance, and to a current jack for current.
  • Check the jack labels on your own meter before you connect anything, because the jack names and positions differ between models.

What the COM jack does

COM is short for common. On a digital multimeter, it is the input that the meter treats as the reference for a measurement. The Durham University physics lab guide tells students to connect the black lead to the COM input jack for voltage and current measurements. The Fluke guide on measuring DC voltage gives the same instruction in its setup steps, and it describes the black probe as the connection to the circuit’s negative test point, which is typically circuit ground.

Because the black lead is the reference, the display shows the voltage at the red probe relative to the black probe. The Fluke guide pairs the black probe with the negative test point, and the red probe with the positive test point, which is the pairing the sources give for DC checks.

The sources we opened do not describe COM as a connection to earth. A voltage reading taken with the black lead on COM therefore does not, by itself, show whether a circuit is grounded.

Where the red lead goes

The red lead does not stay in one place. Its jack depends on the measurement. The sources we opened describe three common positions:

  • Voltage and resistance. The Fluke guide says to insert the red probe into the jack marked V and ohm. The same guide adds a caution that the red probe should go into the voltage input, marked V, and not into the A input, which is the current jack.
  • Current up to 200 mA. The Durham guide says to connect the red lead to the mA and microamp port for current measurements up to 200 mA.
  • Current from 200 mA to 10 A. The same guide says to move the red lead to the 10 A port for currents between 200 mA and 10 A.

The Fluke guide singles out the V and A jacks as the place where a mix-up happens, and it says the red probe must go into the jack marked V and not A. The Durham guide also lists separate jacks for the two current ranges. Both sources point to the jack labels as the first check before a reading.

A step-by-step check before each measurement

The following sequence is drawn from the setup steps in the two sources. It is a general routine, not a replacement for your meter’s manual.

  1. Plug the black lead into COM. Keep it there for the whole session.
  2. Select the function you need, such as DC voltage, AC voltage, resistance, or current.
  3. Move the red lead to the jack that matches that function. For voltage and resistance, use the V and ohm jack. For current, use the mA or 10 A jack that matches the expected size of the current.
  4. Set the range on the dial, or confirm that the meter is auto-ranging.
  5. Read the display after you connect the leads to the circuit, and confirm that the function, jack, and range still match what you planned.

Step 3 is the one most people skip. A meter with the red lead still in the current jack will not show a voltage reading that you can trust, and the Fluke guide specifically warns about mixing up V and A. Make the lead change a habit before you touch a live circuit.

Why the reference matters for troubleshooting

Most troubleshooting with a multimeter compares one point with another. A voltage check between a hot wire and the black lead tells you the potential at that point with respect to the chosen reference. A continuity check uses the black and red leads to find a path between two points. In both cases, the black lead is the reference that makes the reading meaningful.

This is why a reading can look wrong when the black lead is on the wrong point. A voltage check taken with the black lead on a neutral wire, when the reference should be the other side of the load, can show a value that does not match what you expect from the circuit. The meter is doing what it was told, and the problem is the choice of reference.

For that reason, we recommend marking the reference point before you start a test sequence. Note where the black lead is connected, and keep it there while you move the red lead to each point. This makes the readings easier to compare and reduces the chance that a reading is taken against a different reference by accident.

Safety notes

Both sources describe the leads and jacks as part of the meter’s measurement setup, and neither describes a shortcut around the rules for live work. Several safety points apply to the COM jack in particular:

  • Check the jack labels and the meter’s category rating before you measure mains voltage. The rating applies to the whole meter and its leads together.
  • Do not change the function dial while the probes are on a live circuit, because the meter can be switched from voltage to current with the leads still connected.
  • Replace leads that show cracked insulation or exposed metal, and do not use a meter with damaged probes.
  • If you work on mains wiring, the circuit must be de-energized and locked out before you change leads or connect the meter to conductors. Do not use the COM jack as a substitute for a proper isolation procedure.

How we chose

We wanted sources that explain the meter’s jacks in plain terms and that are not sales pages. We opened two readable pages. The first is a physics lab guide from Durham University that gives setup steps for a digital multimeter, including the black lead to COM and the red lead to the current and voltage jacks. The second is a Fluke guide on measuring DC voltage with a digital multimeter, which gives a numbered setup routine and a caution about the V and A jacks.

We did not use search snippets for the instructions above. Each jack instruction in this guide comes from the page text we opened. We did not compare prices or ratings for any product, and we did not include any product recommendation.

What we could not verify

  • The two sources describe typical meters. We could not confirm the jack layout of any specific model, so check the labels on your own meter.
  • The Durham guide gives current limits of 200 mA and 10 A for the two current jacks. Those limits may differ on other meters, and we did not confirm them against a manufacturer’s specification.
  • Neither source states the fuse rating behind the current jacks. Do not assume a fuse protects the meter from a mistaken connection.
  • The sources do not explain how the COM jack behaves on meters with a separate ground terminal or with isolated inputs, so we do not describe that case.

Frequently asked questions

Can the black lead go in a different jack?

For the measurements covered by the two sources, the black lead is described as going into COM for every function. If your meter has a second black jack for a specific test, follow that meter’s manual for that test.

Why does my voltage reading show a minus sign?

On a DC measurement, the sign shows the polarity of the red probe relative to the black probe. A negative reading usually means the leads are reversed on the circuit. The sources do not describe every meter’s display behavior, so check your manual if the sign appears on an AC reading.

Which jack do I use for resistance?

The Fluke guide places the red lead in the V and ohm jack for voltage and resistance measurements. Resistance is measured with the circuit de-energized, so no live voltage should be present.

Does the COM jack connect to ground?

The sources describe COM as the reference for the meter’s measurement. They do not describe COM as a connection to earth. Do not treat a reading from COM as proof that a circuit is grounded.

What happens if the red lead is in the wrong jack?

The Fluke guide warns against mixing up the V and A jacks. The sources do not state what happens to the meter in that case, so move the lead to the correct jack before you take the reading.

Bottom line

The black lead goes in COM and stays there. Move the red lead to the jack that matches the measurement, and check the jack and range before you touch the circuit.

Internal reading

For a step-by-step routine on setting up a meter for a specific task, see how do voltage testers work and how do i use a continuity tester.

Sources

  • Durham University, Physics Skills Lab, Multimeters: https://dur.ac.uk/departments/academic/physics/labs/skills/multimeters/ (opened 2026-10-30)
  • Fluke, How to Measure DC Voltage with a Digital Multimeter: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-dc-voltage-with-a-digital-multimeter (opened 2026-10-30)

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