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I think the phases are 90 degrees apart, or something like that. They taught us in school, you see it a lot in older farms and that's pretty much it.
 

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What is the reasoning behind this....is there a practical reason?
Easy conversion from 3 phase to 2 phase. The first power systems in the US were 2 phase. Latter as networks upgraded to 3 phase some method had to be devised to convert 3 phase to 2 phase for customers who already had 2 phase motors and switchgear. 2 phase power is an extreme rarity, most systems being at least 100 years old.


Of note, a lot of people even a collage professor on U tube was calling split phase 2 phase power claiming 2 phase power is 180 degrees apart. Its not, that's split phase or center tap power. 2 phase is 90 degrees apart. Power can be distributed a number of ways 3, 4 or even 5 wire. When run as a 3 wire the common wire acts as a partial neutral but not a real one since balanced loads wont give you zero like split or 3 phase.
 

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Used to be real common back in my grandpa's day. I think there are still places in the northeast U.S. that have 2 phase generation
I think some are now even museums. Most hydro electric if I remember.

Of note, the T connection is actually not a rarity. Its still used to step down 3 phase power where economy is desired. A scot T transformer can be configured with the right taps to convert 2 phase to 3 and latter 3 phase to 3 phase. TT connected windings are used in many low cost 3 phase pole pigs. One of those connections like an open delta, not perfect for heavy loads but light 3 phase doesn't care.
 

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Philadelphia is I think the only place still using true 2-phase power in the US now, and only in the really old industrial sectors. Niagara used to have it too, but I recently heard that it's only in a museum now. Tesla's first power generators at Niagara Falls were 2-phase.

In Europe by the way, they use the term "2 phase" all the time, but for them it means 2-out-of-3 phases, i.e. 2 pole. Confused the heck out of me when I worked for Siemens, I thought it was weird that they sold so many different products that could only be used in Philadelphia!
 
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There are still two villages here with their own generation plants and utilities running two phase 5 wire services in ancient buildings. I upgraded one to 3 ph.
 

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Philadelphia is I think the only place still using true 2-phase power in the US now, and only in the really old industrial sectors....
There are still some plants in New England that are almost exclusively 2 phase. One of our customers is even 2Ø 40Hz. The whole plant runs off a massive DC-link inverter yard because it was cheaper than upgrading all the specialized gear in the plant itself.
 
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