open delta 3 phase voltages - 2 single phase transformers

[Designer_82]

What would be the 3 phase secondary voltage from this configuration?
Would it be 208v or 240v 3phase?

Thanks

IMG 3398 — Postimages

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[JJ_Roy]

Probably some very bad voltage. There are lots of things wrong with the drawing including xfmr on the right appears to have only one secondary connection and the "X" bushing nomenclature doesn't look right. But the drawing is probably trying to go for a 3ph 240V open-delta maybe?

This is a good reference for xfmr connections but it doesn't have everything.
https://www.eaton.com/content/dam/e...es-and-connections-information-tc202001en.pdf

The old GE Distribution Transformer Manual handbook is probably the best if you can find one!!!!

 

[jraef]

That looks to be a transformer configured for an open delta 240/120 3 phase 4 wire “high leg” system, where one pot is center tapped to provide 120/240 single phase for lights and outlets, then the other pot is straight 240, with the open delta providing 240V 3 phase for motors. Generally in this configuration, the 120 capacity is limited to 5% of the total kVA rating.

There would technically be 208V from one leg of the 240V-only pot to ground, which is required to be the B phase when it enters the service panel and marked with orange, but that 208V should not be used. So to avoid potential confusion and equipment destruction in the field by people who don’t understand, they don’t sell a single pole breaker rated for straight 240V (which is what that would require). Most panelboard mfrs have a limitation to not allow 1 pole breakers on B phase, or force you to have to use a 480V rated single pole breaker if you want to try.

So to summarize, you get 120V L-N from A or B phase, (typically limited to 5% of the kVA of the bank), 240V single phase from any 2 phases, and 240V 3 phase anywhere.

You also have an “artifact” voltage of 208V single phase from B to N, but that really shouldn’t be used. People do, but it’s risky for someone who comes along later who doesn’t understand and if they tap into it thinking it is 120V because it is L-N, they fry something.

 

[JJ_Roy]

It might be a "high leg" as jraef pointed out..... Especially with the four secondary conductors. But I'd probably ask what the intent is.

 

[waross]

Where did you get that POS drawing.
The numbering is wrong according to the transformer terminal locations.
Going by standard terminals locations:
120/240 Volt single phase. One line, a voltage to no-where.
If you add the missing jumper from X0-T2 to either of the outside terminals of T1, you will have an open delta, 120/240, High Leg 240 Volt three phase.
Trust me. I used to have a couple of these open delta banks on my system.