The most common type of distribution transformer used in North America is the single-phase oil-filled, pole-mounted transformer, normally referred to as a "pole-top" or "overhead" transformer. Differences in overhead transformers generally relate to their physical size and the way they are connected to one another to produce different secondary voltage outputs.
The basic components in these transformers is the core and coil assembly inside the enclosure. In fact, the core and coil assembly is the transformer; it is all that is needed to step down the distribution voltage to levels that customers can use. All other devices are simply accessories designed to improve its operation and/or protect it. The core and coil assembly are immersed in a cylindrical steel tank filled with high-grade mineral oil especially refined for electrical use. This oil provides both electrical insulation and cooling to the windings of the transformer.