In a transmission line, sag is defined as the vertical difference in level between points of support (most commonly transmission towers) and the lowest point of the conductor. Sag is mandatory in transmission line conductor suspension to prevent the conductor from excessive tension. The sag span curve is parabolic, and the tension in each point of the conductor acts always tangentially. The tension at supports is nearly equal to the tension at any point in the conductor. The formula to calculate sag changes based on whether the support levels (i.e. the transmission towers holding up the overhead conductor) are at equal levels or not. The value of sag is calculated based on the weight of the conductor, the span length of the conductor, and the working tension on the conductor. Sag is an important indicator of the operational health of a transmission line, and its timely measurement is of great significance.