The Collaborative International Dictionary of English v.0.48:

T connection \T connection\ (Elec.)
   The connection of two coils diagrammatically as a letter T,
   chiefly used as a connection for passing transformers. When
   the three free ends are connected to a source of three-phase
   current, two-phase current may be derived from the secondary
   circuits. The reverse arrangement may be used to transform
   from two-phase. -- T"-connected, a.
   [Webster 1913 Suppl.]