Sorry but the voltage from the grounded point on the lighting (larger, center tapped) transformer to the B phase is 208 volts nominal, not 265 volts. This configuration is normally used as an open wye/open delta and each transformer serves 57.7% of the three-phase load. The lighter serves the entire single-phase load.
The voltages from A to ground to C are usually very stable and the voltage from the common end of the open delta to the high phase are also usually very stable. In both cases, there is a real transformer in the circuit. From the other end of the lighter to the high phase, the voltage is usually very stable as long as the motor load is not too great.
In this case, I am betting that either the bank is very close to a substation or a capacitor bank is on top of the installation (the utility's capacitor bank). Of course it could be both.
Most of the state public service commissions require the investor owned electric utilities to maintain their voltages to about 5% of nominal. It may be time to call them if you haven't been able to get through to the correct people to get their problem taken care of. I highly recommend that you get through to their power quality engineer or at least a distribution engineer first.
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Charlie Eldridge, Indianapolis,
Utility Power Guy
