C.Urch
I suspect the PoCo has a chance of being involved in this one. How far away is the transformer and what are the wires (Al or Cu? and gauge?). As I read your question, the domestic HWH load is causing a visible voltage sag out on the 200 Amp service center branch cicuits. That means the sag, or voltage drop is in the conductors between the 200 Amp bus and the supply transformer. Verify the connections on the service entrance conductors and talk to the PoCo customer service rep.
Al
