For a transmission line, the positive-sequence impedance (Z₁) and negative-sequence impedance (Z₂) are typically considered equal. They represent the opposition to the flow of balanced three-phase currents.

The zero-sequence impedance (Z₀) is fundamentally different because the three-phase currents are in-phase and return through the ground and ground wires. This creates a different magnetic field pattern and, as a result, Z₀ is usually 2 to 3.5 times greater than Z₁ and Z₂. This difference is critical for understanding and calculating asymmetrical fault currents.