120, 208 and 240 volts From Distribution Transformer Bank?

[WoodyM]

A client inquired as the following connection.
Connect three 120/240v distribution transformers in 120/208 volt configuration. The secondary windings of two transformers would be paralleled for 120 volts with one end of each paralleled winding connected to ground and the other ends to phase (A and B). The third transformer would be left in a 120/240v configuration. One end would be connected to phase C. The center tap would be connected to ground. The other of the winding would be used along with phase C to obtain single phase 240 volts. The third transformer would be larger than the first two to compensate for the smaller capacity of the winding in the three phase configuration. The purpose is to feed a three phase 120/208v load and a 120/240v residence from one transformer bank, Is this feasible?

 

[dpc]

I don't think so, but even if it were feasible, it's a terrible idea. Setting yourself up for all kinds of future misery with a bastardized one-off installation that will be a safety hazard. The real "purpose" is simply to be clever. You won't save any money in the end and you'll still need two meters.

Just do it right and put in another transformer. Even if you have to set another pole, it's still what I would do.

 

[stevenal]

Sounds feasible to me. Lots of residences are served 120/208, though. An open wye connection is used with a form 12S meter and five jaw base.

 

[Mbrooke]

Three options exist with the last two more along the lines of what you are looking for.


1. 120/240 3 phase 4 wire delta

2. 3 to 6 phase diametrical transformer bank

3. 120/208/240 3 phase 5 wire wye. I will post the bank hook up:

 

[Mbrooke]

Other half of the pic: