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 | ||
| Satisfactory | Acceptable | Satisfactory | Acceptable | |
| 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.