pauluk—This continent has a penchant for lots of 2:1 voltage ratings [and fewer 1.7320508:1 ratings.] The idea allows for series/parallel operation of (the coil pairs in) motors and transformers...hence 120:240:480 ratios.

And it gets weirder yet. Used to be that one service voltage--common for overhead service to dairy barns, gasoline stations and convenience stores--was 240/120-volt 3ø 4-wire ∆ service. It measures 240V ø-ø, and 120V ø-ground/neutral on A and C phases, with 208V ø-ground/neutral on Bø. Years ago it was common to have two meters—a 1ø 3-wire meter for “lighting” (typically with a lower electric rate,) and a 3ø 3-wire meter for “power” (motors.) The “drop’ (overhead span of either 6 or 4 leads) was served via three 1ø center-tapped transformers with secondaries delta connected, or two 1ø center-tapped transformers in a 4-wire open-delta secondary configuration; yielding the same voltages. The open-delta arrangement saved money for the utility where the 3ø load was small; possibly even a single ~3-7½hp [refrigeration, air-compressor or vacuum-pump] motor. Nowadays with only one power rate a single meter handles the 4-wire ∆ service, with up to a 400-amp {fuse or circuit-breaker} building service. [Old habits die hard, they say.]

My best guess for the multiple service voltages over here was, with a lower population density in some areas, loads are generally farther apart, making it customary for more but smaller transformers. I understand that in parts of Europe and Australia many homes and businesses may often be served by the same transformer; with what we would term 400Y/230V 3ø 4-wire {50Hz} for businesses and homes with relatively large-kW non-storage water heating, and 2-wire 230V for most other residences.

In the US, a single-phase service tapped from 3ø 4-wire secondary is relatively rare for single-family dwellings, with generally 3-wire 208Y/120V ‘network’ service for multiply-metered apartment/condo buildings. Primates of 120/240V 3-wire secondary transformers may be served ø-ø or ø-neutral from the medium-voltage distribution system.

Sometimes 120/240V 3-wire service—say, for an isolated small office in a plant or warehouse—
is derived from 480V electrical systems can use the 1ø “dry-type” transformer. 208Y/120V 3ø service is most often used in smaller offices. On rare occasions, two or three 1ø 480-120/240V transformers may be used to serve localized 3- or 4-wire Δ loads. Here, 480Y/277V in a residence would surely give electrical inspectors an aneurysm.


[This message has been edited by Bjarney (edited 05-11-2002).]