A 3-phase overhead line is an electrical system that uses three alternating currents of the same frequency, but each current is out of phase with the others by 120 degrees. This staggered timing ensures that power delivery is constant and uniform, unlike a single-phase system where power fluctuates.

The primary reason for using a 3-phase system is its superior efficiency. It can transmit significantly more power with less conductor material than an equivalent single-phase system, making it the most economical choice for long-distance transmission and heavy-duty applications.