Why does the transmission voltage level only indicate the phase to phase voltage number?

The reason the transmission voltage level typically indicates only the phase-to-phase voltage is primarily related to the configuration of the electrical system used in high-voltage transmission lines. In many high-voltage transmission systems, particularly those using three-phase systems without a grounded neutral, the phase-to-phase voltage values are the most relevant for understanding the operation and efficiency of the transmission system.

In a three-phase system without a neutral connection, phase-to-ground voltages can vary and are less useful for determining the overall effectiveness of the transmission network compared to the phase-to-phase voltage. This configuration also simplifies the design and installation of the transmission lines, as the focus is on the balance and performance of the phases relative to each other.

Using phase-to-phase voltage not only provides a consistent standard for specifying transmission levels but also aligns with how electrical equipment, such as transformers and motors, are rated. Thus, indicating only the phase-to-phase voltage effectively communicates the voltage level being utilized in the system.