A standard single phase (120/240) service actually has 2 hot legs and a neutral.

A standard 3 phase service (208 or 480) is 3 hot legs and a neutral.

 

Yes I understand 3 phase has three hot legs but what I do not understand is where or how are the three hot legs getting 120 degree phase separation with only two high voltage lines to the two transformers. One of the transformers does have a center tap but that would provide the split phase at 180 degrees apart.

The building does have 3 hot legs but I'm not seeing how there could be the 120 phase separation that 3 phase AC would have.

???

 

Your probably seeing an open wye primary / open delta secondary.
The high side uses only 2 hots and the primary neutral

 

I think this is what you are looking at. It gets you 3phase with only two primary lines and the nuetral. View attachment 129680

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Is this corner grounded delta

 

It's a open delta, and it may or may not be in open star on the primary. Open deltas are kinda on the way out in part to the acceptance of 208 stars. A quick internet search, with pictures, will yield more info on open and closed deltas than I could ever type.

 

Start with 3 phase. Let’s say it’s 4160 V phase to phase. Now if we put a 4160:208 V transformer between say A and B we have 208. Now do the same between B and C so you get 208 again. What’s the voltage from A to C? Since we started balanced, it’s still 208 V. Now going a little further we can ground B (and it typically is) which makes it corner grounded delta. The advantage is the transformer is super cheap since you only need 2 sets of coils instead of 3. This is commonly used in substation metering and some very light duty commercial installations. If the high side is also corner grounded delta, the corner is a ground/neutral. There are big advantages to 4 wire wye industrially so that’s what’s in the text books but there are about a dozen different variations.

Another common commercial system which you might have is 120/240 high leg delta and could also be what you see. In this system start with a single phase tapped transformer so you get 120/240 with the center tap as a grounded neutral. Now add another single phase transformer between either of the phases feeding the 240/120 transformer and the third leg with a 240 V output. This gives you 240/120 and 240 delta outputs again with an open (2 coil) delta. Optionally if the 240 service is higher demand a third transformer can be added or a true three phase transformer with both sets of outputs.

None of this looks like nice neat text book work and it’s not super efficient but when you just have say an HVAC load, cost trumps efficiency.


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I think this is what you are looking at. It gets you 3phase with only two primary lines and the nuetral.

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^^Bingo^^

 

Forge Boyz's diagram is a good one however the OP wanted to know why are only 2 primary phases are used to get 3 phase secondary. Here in northern and central CA converting from 12kv delta to 21kv wye means 2 hots and the primary neutral to existing open delta banks. Trns. would still need 12 kv to grounded neutral-not line to line.

 

No.

The center-tap is grounded.

That's no corner.