The primary causes of energy loss in transmission lines are:

• Joule Heating (I2R Loss): This is the most significant source of loss. As current (I) flows through a conductor, the conductor's inherent electrical resistance (R) causes some of the electrical energy to be converted into heat. This loss is proportional to the square of the current, making it the most important factor to manage. This is why electricity is transmitted at very high voltages to reduce the current and, thus, minimize this heat loss.

• Corona Effect: In high-voltage transmission lines (typically above 230 kV), the strong electric field around the conductors can ionize the surrounding air. This causes a partial discharge of electrical energy, creating a visible glow, a hissing noise, and power loss. This effect is mitigated by using bundled conductors and designing lines with smooth surfaces to reduce the electric field gradient.

• Skin Effect: In AC systems, the current tends to flow more on the outer surface of the conductor rather than being distributed evenly throughout its cross-section. This reduces the effective conductive area and increases the resistance, leading to higher losses.