Big John, on the west coast, [CA/OR/WA] there are thousands of utility-owned 4-wire open-delta-secondary [two-transformer] banks in service—primarily for smaller farms (house, barn and water pump), isolated pump motors, dairy barns and small, individual retail firms. The ANSI-standard voltage designation for both delta (three transformer) and open delta (two transformer) is 240/120-volt 4-wire, [3ø is implied in the designation] where 120/240 3-wire is the common residential 1ø designation.
The tradeoff between the two configurations for electric utilities is purely economic—open-delta banks are usually limited to loads of ±20 horsepower.
For open-delta services, the primary-winding configuration may be either open-delta or open wye. (A slang term for open-wye primary is “v-phase,” and must also include a primary-side neutral conductor connected to both transformers.) An easy way to differentiate 2-transformer, crossarm-mounted-bank primaries is by the number of fused cutouts—two means open-wye—three indicates open-delta.
