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VAC usually means Volts AC or Volts Alternating Current.
In electrical or electronics contexts, it describes an AC voltage. For example:
VAC is a way of specifying voltage when the voltage is alternating, meaning it changes polarity periodically.
| Term | Meaning | Typical example |
|---|---|---|
| VAC | Volts Alternating Current | Wall outlet power, transformer output |
| VDC | Volts Direct Current | Battery, USB supply, DC power adapter output |
With DC, the polarity is steady: one terminal remains positive and the other remains negative.
With AC, the voltage repeatedly swings positive and negative. In most power systems this happens as a sine wave at a fixed frequency, commonly:
So a label such as:
Input: 100–240 VAC, 50/60 Hz
means the device can be powered from an AC mains supply between 100 and 240 volts RMS, at either 50 or 60 Hz.
When you see a value like 120 VAC, it usually refers to the RMS voltage, not the peak voltage.
For a sine wave:
\[ V{peak} = V{RMS} \times \sqrt{2} \]
So:
\[ 120 \text{ VAC RMS} \approx 170 \text{ V peak} \]
and the peak-to-peak voltage is about:
\[ 340 \text{ V peak-to-peak} \]
That is why mains AC can be dangerous even when the stated number seems modest.
If you are measuring VAC with a multimeter:
Outside electronics, VAC can also mean other things depending on context:
But if you saw it on a device label, power supply, schematic, breaker, relay, transformer, or multimeter, it almost certainly means Volts AC.
VAC = Volts Alternating Current. It means the voltage is AC rather than DC. A rating like 120 VAC or 230 VAC tells you the effective AC voltage a device uses or can tolerate.
