American style electrical systems (Some exist outside the America) have more variations--and cause more confusion.
(1) Three-phase power is delivered at either 460, 230, or 208 volts (480 and 240 are basically the same as 460 and 230--as mentioned above, there are regional differences).
The differences are not because of "regional" . In the same electrical distribution system we have e.g. 240 and 230 V. First value is for power supply units/elements as transformers (they have on their Nameplates .../240 V, and second value is on the load (appliances, motors) nameplates. Again it is most often net/grid assumption that the existing drop voltage should not exceed 5 %. Read a basic books about feeders + circuits designing.
(2) Single-phase power is either nominal 115 (110-120) volts, or nominal 230V (220-240) volts.
These values apply to loads, e.g. lights (115V) or greater loads as oven/ranges , air conditioners (230 V)
(3) The three-phase transformers (Often three single-phase transformers), can be wired in three different methods: (a) WYE or STAR with a center tap, (b) Closed delta, or (c) Open Delta (Using only two transformers).
Transformer's windings are connected in three possible ways: Wye/Star, Delta or Zigzag, Three phase feeders (from transformers to Load Centers) are wired using three or (more popular) four wire systems (three "hot voltage" conductors + one neutral/grounded or grounding). One of the reason residential transformers are single-phase is that Primary terminals (H1 & H2) are connected directly to "hot" phases of overhead middle voltage lines, and secondary windings have middle (center) connection to N (neutral usually grounded) and 120 V "hots" are connected inside to x1 and x2 terminals. Service (drop to house) has three wires (sometime multi-conductors). Entering Fuse/Circuit Breakers Center / also Load Centers terminals are marked L1-N-L2. This solution leads to more single-phase motors produced in the USA comparing to Europe where more Load Centers are 3-phase.
By the way in Europe terminals are marked: U-V-W / X-Y-Z ("hot" = phase) and O and N (neutral/earth "zero")
(4) All three arrangements can deliver reliable three-phase power. By using only two of the three terminals, all three can deliver single-phase power at the same voltage. However only two of them can deliver single-phase power at a different voltage.
Even the description is correct, the better way to educate readers is to use diagrams. Of course better understanding diagrams require at least taking any of EET basic classes about Alternating Current _ Three Phase Systems chapter. E.g. why x-formers winding connections are described as "Delta", "Star/Wye", "Zig-zag"?. Electricians in USA must know so many different primary/ secondary windings connections and how then recognize them outside by using voltmeters on x-former terminals.
(5) A 460V or 230V WYE transformer delivers an unusable (57%) single-phase voltage. However, a 208V WYE transformer delivers a usable 120V between all three legs and the center tap. These are often used to supply both three-phase and single-phase power to a small commercial building.
Everybody understood?
(6) A 230V Closed Delta can have a center tap (May be called Neutral) between two terminals (Say L1 and L2), and deliver 115V single-phase power between L1 and Neutral, and also L2 and Neutral. A connection from L3 to Neutral creates an unusable "Wild Leg" voltage of about 190 volts.
From the basic definition of Supply Voltage you may not use the value of 230 V here. It should be 240V because closed delta apply in this case to the transformer (x-former - tech slang), right? Everybody knows what is L1-L2-L3-Neutral?
(7) An open delta can actually deliver three-phase power using only two single-phase transformers (Impressive!). Single phase power is available, if you chose to connect to only two of the terminals, however, the single-phase voltage will be the same as the three-phase voltage (Nominal 460 or 230).
Nominal (rather European term)/ Rated (USA term) value should be here 480 or 240
Open Delta delivers (or must be selected for power demand accordingly) two-third of three single x-formers connected in full Delta. If power supply is composed from three single-phase transformers it is possible to still deliver two-third of the rated power is the time when only two x-formers are transferring higher voltage system to low voltage distribution one
(8) As mentioned above, most residences receive nominal 230V single-phase only. By wiring the transformer L1---N---L2, the customer can get 230V (L1 to L2) for large appliances, and 115V (L1 to N, and L2 to N) for small appliances.
Again, check in Electricians Books: L1 (should have measured 120V to N) - N (should have almost zero volts to the Ground /Earth in UK) - L2(another 120 V). After 5% drop loads could have measured 115 or 230 V as stated above. By the way motors have terminals marked: T1-T2-T3.
Anyway, I hope my second step after first step explanation of USA systems from California, I did compared to East Coast and Europe (they also have some differences) system. In Europe and some places here transformers' secondary winding are connected is so called Zig-Zag. This configuration lets overcome some problems created by uneven power drawn from each phase.
Interesting topics is also "Harmonics". But leave it to these who will find time and appetite to read Electrician's Handbooks.
