Does an electric water heater tank, on a well system which already has an expansion tank on the system, still need a separate expansion tank on the cold line?
What I think you wanted to say was “already has a pressure tank on the system”. A separate thermal expansion device is only needed with a well system when a hot water circulation pump is present. This is due to the check valves that are used with these circulating systems. When the hot water loop is activated after being off due to various reasons such as the timer setup, the water in the loop heats up and expands rapidly due to the blockage created by the check valves. A thermal expansion device can be a bladder/tank or another correctly rated relief valve in the system. The TPR valve that is already on the water heater does not count, that will only vent when the pressure is already too high.