Transformer base (T-base) poles are similar to shoe base poles, but the base plate is sized to fit on top of a transformer base, which is bolted to a foundation. T-base poles are designed to break away when hit, and should be used in locations inside the clear zone where they might be struck by a vehicle.

The transformer base originated in the days when the ballast (or transformer) for the luminaire was placed in the base of the pole, and the fixture housing and lamp were at the top of the pole. When luminaires began using internal ballasts, the T-base was kept for its breakaway characteristics even though it was no longer needed to house the ballast.

A transformer base is a breakaway support made of aluminum. TxDOT T-bases are designed to have the strength to support a 50 foot pole with two 12 foot arms, and also to be easily broken on impact (frangible). Transformer bases must meet AASHTO requirements and also must be impact tested and approved by the FHWA. TxDOT's approved T-bases are listed under Item 610 on the MPL for Roadway Illumination and Electrical Supplies.

Transformer base poles do not typically have a hand hole in the pole, but the electrical connections are made up in the T-base. TxDOT T-bases have a non-metallic door to discourage metal theft. The RID standards require a two-pole fused breakaway connector for the internal conductors to be placed in the T-base. If the pole is hit and breaks away, the breakaway connector will safely separate the pole from the electrical circuit.