120 Volt Single-Phase AC
The phrase single phase refers to the single sine wave oscillation occurring. With 120 volt single phase, the current actually oscillates from 170 volts to minus-170 volts. One complete oscillation from 0 to 170 volts, down to minus-170 volts and back up to 0 takes 1/60th of a second. Sixty phases per second is standard for all household and industrial electrical AC power sources.
240 Volt Single-Phase AC
With an alternating current of 240 volts, two sine waves occur within a single phase. Even at 240 volts, it is still single phase because both oscillations of voltage are occurring within the same phase, only in opposite charges. The second sine wave oscillates exactly opposite of the first wave.
Single-Phase AC Vs. Three-Phase AC
Single-phase current differs from three-phase electrical current because the latter is actually three single-phase oscillations offset by 120 degrees. Three-phase electrical current is what is produced by power plants and offers an advantage over single-phase because, by offsetting three separate oscillations, there will always be one sine wave near the peak charge. This results in a smoother current. Three-phase current requires more energy and is simply more than is needed for common electrical needs aside from running industrial-sized motors or high-energy welding equipment.
AC Advantages Over DC
Households use AC rather than DC simply because it is easier and more efficient. Large electricity generators, like power plants, naturally generate an alternating current. For households to use DC, the extra conversion would require equipment and result in a net loss of energy. The conversion of AC to DC is easier and cheaper than converting DC to AC, and electrical transformers necessary to the power grid can only operate on alternating current.