ECN Forum Forums General Topics of the Trade General Discussion Area 208, 240, & 277 volt questions

Just George;


Two Transformers NORMALLY indicates an Open Delta "Vee" configuration.
It can be either a 3 Wire or 4 wire type, and for 3 Wire ones, they can be either Grounded or Ungrounded.

Same goes for "Full / Closed" Deltas, which use Three individual Transformers. All "Flavors" are available too!

Typically if the setup is a 4 Wire Delta, the "Center Transformer" on Closed Deltas is larger than the outer ones, and for Open Deltas, one Transformer is larger.

The "Larger" Transformer is the one used for the 1 phase 3 wire portion of the system - and utilizes the center tap on its secondary winding for the Grounded Neutral.

With that being said, I do need to include a DISCLAIMER regarding Two Transformers on a single pole, vs. the type of system created.

TWO TRANSFORMER DISCLAIMER

The usage of Two individual Single Phase Transformers on a Single Pole, may also be one of the following connections:

<OL TYPE=A>


[*] Single Phase 3 Wire - Parallel Systems: whereas each Transformer's Secondary feeds a separate service,


[*] Single Phase 3 Wire - Parallel Transformers: to increase the ampacity available from only a single Transformer. I have only seen it done 3 times. Each Transformer's Primary and Secondary are connected together in Parallel,


[*] 3 Phase Open Delta "Tee" - Typically used on dry type Transformers of 15 KVA and less, these may be 3 or 4 wire secondaries and typically are configured to "Represent" a 208Y/120V 3 Phase 4 Wire system - although they may be setup in many ways,


[*] 2 Phase Open Delta "Tee" - similar to that described above in "C", these include the "Scott 'T' Connection", but typically when used for 2 Phase systems, they are connected as 3 and 4 wire (no "Neutral") systems,


[*] 3 Phase 3 Wire "Open Wye" - typically has a conductor which is "Common" to both sides (Primary and Secondary), which is connected to the common star point of the ends of each windings' pair.
</OL>

Scott35

edited for spelling blunders

[This message has been edited by Scott35 (edited 05-08-2006).]

Thanks for that explanation, Scott.

Ok, on the Delta 3 phase transformer with a ground center tap. I see how you get the 208v going to a roughly phase and a half. But then it seems if you measured from phase to ground on the other 2 phases it would be something less than 120v.
Also, in this type of 3 phase panel wouldn't you have the 208v every 3rd breaker down?

Yes, you do. (Have 208 on every 3rd breaker.) That's why it was suggested that you have two panels--one for the three-phase loads, and a second, single-phase three-wire panel connected A-neutral-C for the single-phase, 120/240 loads.


[This message has been edited by SolarPowered (edited 05-09-2006).]