In system single-line diagrams, transformers are often depicted using the symbol for a single-phase air-core transformer. This representation does not necessarily mean the transformer is an air-core type or single-phase. Single-line system diagrams are meant to provide general functional information, similar to what is presented on a P&ID for piping systems. For more detailed information, further investigation is required. In diagrams of three-phase systems, a small symbol may be placed next to the transformer primary and secondary to indicate the type of transformer windings used.

The accompanying image shows commonly used symbols to indicate phase connections in three-phase windings and examples of how these symbols appear in a three-phase single-line diagram.