If I understand you correctly, you are describing a "wild-leg" delta system. They are nothing but trouble.
You start with a three-phase delta connection, with the phase-to-phase voltage of 240V. So far, so good.
Now, you center tap ONE of the transformer secondary windings and ground it. This gives you 120V from each adjacent corner of the delta to the center tap (or ground). So you really have a three-phase 240V system, plus two 120V hot legs tacked on.
So, the ability of the transformer to handle 120V loads is quite limited, because you are working with only 1/3 of the total transformer.
This is often used for irrigation systems and pump stations where the predominant load consists of the three-phase motors (230V) and 120V load is limited to a few control devices, lighting, etc. If you have mostly 120V loads, you don't want to use this.
If you need three-phase power, and have a choice, take service at 480V and buy a little "uni-center" to step down to 120V if necessary. Then you *could* have, 480V (1-phase and three-phase), 277V single-phase, 208V (1-phase, and three-phase) and 120V single phase.
Utilities sometimes encourage these wild-leg systems because they can use three pole-mounted single-phase transformers that they probably have on hand in their yard. 480v transformers are usually not on hand.
NEC has some special requirements for the wild-leg systems.
I'm of an older generation, where this was a quite common approach, especially for light industial applications, or commercial applications (where there was a major need for 240 volts, three phase power, and a minimal need for 120/240 volt, 1 phase, 3 wire power).
In fact, in older light industrial areas, this remains a common service arrangement.
DPC is correct in that the utilities' service transformers were frequently an unusual mix. For example: 2-75 KVA transformers and 1-100 KVA transformer center tapped.
Some utiliies really economized by using 2 transformers in an open delta system,(for example 1-75 KVA transormer and 1-100 KVA transormer center tapped at 120/240 volt). Amazingly enough, there were (are) very few complaints about the unbalanced three phase voltages and their effects on motor loads.
While you can still order a dry transfomer with a 120/240 volt, 3 phase, 4 wire secondary, you MUST (underscore MUST) be careful. Most such transformers today have a limited 120/240 volt capacity, due to a minimal neutral bringout. You should be sure that your vendor knows what your intended use is.
Figures 12 and 15 for delta- connected transformer-bank primaries. Also, figures 19 and 21 apply to wye-connected primaries.
In some regions, this voltage is served to dairy barns and gas stations, where there is typically a single large motor like an air compressor or vacuum pump, and various small single-phase loads. One correction...you referred to the configuration as "120 volt A and C phase, and 240 B phase." The B-phase voltage to ground is 208 volts.
ok , thanks , that answered my question. i did mean a 208 v b phase though- with 240 v phase to phase- , or a CENTER TAP GROUNDED DELTA- WITH A "WILD" or "STINGER" LEG...The reason i needed it is , a guy at work on friday told me i was nuts when i said that there is a 3 phase delta system with 120 volt A & C phase , and 208 volt B phase.He is from russia , so i guess thats why he thought i was nuts. Anyway we made a $200.00 bet on this, so i really want to thank you guys, and will definately visit this most helpful site more often. jc3333
Your timing at winning the bet is perfect! Tomorrow is my birthday and I was saving up for Aunt Nadine to pay for her first set of dentures. She's 97, and finally deserves some of the better things in life.
