This is 4 wire delta or center tapped delta
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The so called "wild leg" is not used for 120V service, it runs variously somewhere around 180V to neutral. The wild leg can be used with either of the other two legs in some cases for 230V single phase equipment, and obviously all 3 for 3 phase 230
For 120V singe phase, phase A or C to the center tap (neutral) gives you separate feeds of 120V just like your household power. It is my belief that this lashup started because some industrial outfits may have started with one big single phase transformer, and later wanted 3 phase, so all th epower co. did was add the other two for the other two phases
