A common sight in these parts, especially in older, light-industrial neighborhoods, is a utility pole with two distribution transformers (often referred to as "pots" or "pigs"), one large and one small. I got curious about these and it turns out that they supply the infamous high-leg delta service. The big pot supplies the (typically) 120/240 V single-phase part just as if it were single phase service. Its secondary's center tap is connected to the neutral and grounded. The small pot has one end of its 240 V secondary connected to one end of the big pot's secondary and the other end supplies the high leg or stinger. So the three-phase part is an open delta configuration. There is no third transformer and the entire load is carried by just two of the primary-side phases.
More trivia: The big number painted on the side of almost every distribution transformer in the US is its capacity in kVA (kilo Volt Amperes). It's interesting to me how small the kVA rating can be in relation to the notional size of the service. For example, our vacation place in the Sierra Foothills has 200 A service. At 80% load that works out to 38.4 kVA. The lone transformer on the pole has a big "15" on it. I guess PG&E figures we won't turn on everything at once, at least not for very long. Question: does anyone know what, roughly, the thermal time constant of typical small distribution transformers is?
In leafy suburbs, like Palo Alto, the utility connects the secondaries of multiple distribution transformers in parallel. In this case undersizing the transformers is a pretty safe bet since the the larger the number of customers sharing the transformers the less likely it is that they all fire up their plasma cutters at once. I suppose it also increases reliability since a transformer failing open won't cause an outage. On the other hand, if the primary side fails open between transformers you could end up with quite an interesting circuit. I guess the utilities have thought through this scenario