This doesn't look rare to me, it looks extremely common:
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You almost never see that for a | residential service, except maybe for a very large house with a (3 | phase) whole house A/C system, or a business in the home with a 3 phase | load. | | The typical US system has one single phase transformer, with its secondary | center tapped. The center tap is grounded, and there is 120V between | each end of the secondary and the neutral (center tap) and 240V | between the two ends. | In other words two phases are across the horizontal leg with the centre tap and the other two legs are absent.
| Go back to your incorrect diagram at
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eliminate | the L3 terminal and the two transformer windings connected to it, and the | remaining winding is the typical US service, L1 and L2 are the two hots. | Exactly. So Don Klipstein should have written: "In USA, homes generally get two phases of the 3 phases, not one" instead of "In USA, homes generally get one phase of the 3 phases, not two."
The problem with discarding the L3 terminal as you suggest is that it is used with L1 or L2 further up the street to keep the load balanced, so the diagram should have shown two more grounded centre taps, N2 and N3, like this:
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| >| Two 120V legs 180 degrees out of phase with each other comprise one | >| phase, not two. | | >Two phases out of 3 are 120 degrees apart, not 180. | | The two ends of a center tapped transformer are 180 degrees apart from | each other w/respect to the center tap.
They are indeed.
| For 120 degrees, you need 2 | transformers from 2 phases, at minimum.
The grounded centre tap has created two transformers from one, as you can clearly connect a 110 light bulb to either one. For 120 degrees you need the third phase.
| | >Two 220V legs 120 degrees out of phase with each other comprise two | >phases, not one, even with a centre tap at 110V below one and | >above the other, and grounded. | | It's a single phase from 1 transformer.
It's two phases 180 degrees apart from two transformers wound on a common iron core.
Any two phases of a 3 phase star/ | wye service will be 208 volts, not 120V. (the typical 3 phase service | is *not* that at
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)
The typical US 3 phase service *IS* that at
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with two more centre taps added.
------------------------ I bloody well hope not. That is a recipe for disaster unless the center taps were completely isolated from each other- and not to a common ground. Since the secondary neutrals are grounded, in practice , there is no way of avoiding this problem. For single phase on a 3 phase system , generally a
208V/120V Y secondary is used. For normal domestic single phase loads, the primary of the transformer may be between two of the 3 phases (if delta) or from one of the phases to neutral (grounded neutral) if Y. The secondary is then centre tapped single phase( yes- it can justifiably be called 2 phase 180 degrees apart. It isn't called that in North America where it is used but rather single phase 3 wire or single phase center tapped or the Edison system because he developed this for DC (omitting the transformer). In some cases, only one phase and neutral of the primary is run down a street. This is also common in rural areas. Effectively only single phase primary service is provided in these areas. Your picture appears to be a delta/delta setup in which case, there is no center tap involved on any of the transformers and is providing 3 phase service. The secondary voltage may be 240V (the LV terminals look it) and certainly the service leads look that way. It isn't a common case here but may be more common in the southern states where air-conditioning loads are high.
Don Kelly cross out to reply