Steps for measuring AC voltage with a digital multimeter
- Turn the dial to ṽ. Some digital multimeters (DMMs) also include m ṽ . If voltage in the circuit is unknown, set the range to the highest voltage setting and set the dial on ṽ.
Note: Most multimeters power up in Autorange mode. This automatically selects a measurement range based on voltage present.
- First insert the black lead into the COM jack.
- Next insert the red lead into the VΩ jack. When finished, remove the leads in reverse order: red first, then black.
- Connect the test leads to the circuit: black lead first, red second.
Note: ac voltage does not have polarity.
Caution: Do not let fingers touch the lead tips. Do not allow the tips to contact one another.
- Read the measurement in the display. When finished, remove the red lead first, black second.
Other useful functions when measuring AC voltage
- Press the RANGE button to select a specific fixed measurement range.
- Press the HOLD button to capture a stable measurement. It can be viewed after the measurement is complete.
- Press the MIN/MAX button to capture the lowest and highest measurement. The DMM beeps each time a new reading is recorded.
- Press the relative (REL) button to set the multimeter to a specific reference value. Measurements above and below the reference value are displayed.
Note: Avoid this common and serious mistake: inserting test leads into incorrect input jacks. Doing so can lead to a dangerous arc flash. If measuring ac voltage, be certain to insert the red lead into the input jack marked V, not A. The display should show the ṽ symbol. Placing test leads in A or MA inputs and then measuring voltage will create a short in the measurement circuit.
Analysis of AC voltage measurements
- In general, all ac voltage sources vary from fluctuation in ac voltage over power distribution systems.
- When different from an expected measurement, voltage is more likely to be lower than normal.
- Generally speaking, voltage measured in ac power systems should be within -10% and +5%.
- Voltage measurements taken at various points in a system vary. Refer to the chart below.
|System Voltage Ranges*|
|Supply||Service Range||Point of Use Range|
|120, 1Φ||114 - 126||110 - 127||110 - 126||106 - 128|
|120/240, 1Φ||114/228 - 126/252||110/220 - 127/254||110/220 - 126/252||106/212 - 127/254|
|120/208, 3Φ||114/197 - 126/||110/191 - 127/220||110/191 - 126/218||106/184 - 127/220|
|120/240, 3Φ||114/228 - 126/252||110/220 - 127/254||110/220 - 126/252||106/212 - 127/254|
|277/480, 3Φ||263/456 - 291/504||254/440 - 293/508||254/440 - 291/504||264/424 - 293/508|
* in volts
Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.