VAC

VAC stands for volts of alternating current, the standard unit for measuring electrical potential in AC circuits. Unlike volts of direct current (VDC), which maintain a steady polarity, VAC voltage alternates direction many times per second, reversing between positive and negative. In North America, household and commercial circuits typically operate at 120 VAC or 240 VAC at 60 hertz; in Europe and most other regions, 230 VAC at 50 hertz is standard.

The choice of AC over DC for power distribution came down to transmission efficiency. Because transformers only work with alternating current, utilities can step voltage up for long-distance transmission (reducing losses in the wires) and then step it back down at the point of use. A small transformer losses far less energy than the copper losses that would occur sending DC power across a continent.

Practical distinction from VDC

On a multimeter or circuit breaker label, you will see VAC and VDC marked separately because they behave differently in equipment. Motors, lighting ballasts, and most plug-in appliances expect VAC. Battery-powered tools, LED drivers, and control circuits often run on VDC. Applying the wrong voltage type will damage sensitive electronics or cause a motor to fail to start. Industrial equipment may use 480 VAC three-phase for heavy machinery, where the three-phase setup provides more stable power and mechanical efficiency than single-phase.

When troubleshooting, always set your multimeter to the correct mode: AC or DC. A reading of 120 VAC at an outlet is normal; 0 VAC means a dead circuit. In industrial settings, VAC is also used to specify the operating voltage of contactors, relays, and solenoids. A 24 VAC control circuit is common because it is low enough to be safe but high enough to carry useful power without excessive current and copper losses.