Electrical Sub Panel

[gfretwell]

It is just a garden variety single phase 120/240 transformer that they add a second one to and trick motors into thinking it is 3 phase. Generally they don't use the 3d transformer to make a true delta. If they wanted to spend that much money you would get 3p wye. You see it in light industrial areas where most of the load is single phase on a big 1p transformer and a second only supplies the "red" leg for the 3p load. (actually identified orange) As long as you never put L/N loads on the 208v red leg it works fine.

[Don Young]

I have seen a lot of the two transformer open delta and three transformer wye services, but I have also seen quite a few three transformer closed delta systems. It seems to depend on the relative loads. The open delta seems to be used where the three phase load is fairly light and the closed delta where there is a heavy three phase load but 120/240 single phase service is also required. It may be a regional thing and I think the delta connection is becoming rarer and is being replaced with a separate single phase transformer where 120/240 is required.

Don Young

[Bud--]

Open delta lowers the transformer power factor so the the transformers have to be derated - probably why open delta is generally light loads. Wye is easier to balance which is a good reason to use it instead of delta. (You can also get wye with 2 transformers in a Scott or T connection.)

-- bud--

[Mark Lloyd]

That reminds me that there's a lot about 3-phase systems that I don't know.