From Cliff K9QD
220, 221- Whatever it takes. Electric Utility Service Drop…and why the service drop to the consumer has a center-tapped transformer grounded to earth.
Wiring diagram of today’s typical single-family service entrance panel:
Note the Utility Transformer’s center-tap is grounded at the pole. Also, the consumer’s service entrance panel and Neutral/Equipment bonding bars are also grounded to earth.
Origin of ‘grounding’ and why utilities in the early 1900s quickly changed to (for single phase service) center-tapped transformers at the customer’s drop.
In the early 1900s, as electrification spread to more and more consumers, the new advantages of Alternating current allowed higher voltage transmission than Direct Current and saved a ton of copper [Source: Hawkins Electrical Guide Theo. Audel & Co 1917].