Were the heat strips coming on because of a voltage drop, or because the compressors were unable to keep up or maintain efficiency due to the insufficient current? How many AMPs of service are you set up for? 200? 400?
I like the idea of a dedicated forum for geothermal discussion.
In forced air system the heating strips come on when room temperature is several degrees less than SP. In example it might happen when the program increases the SP in the morning. The strips can heat the air to much higher temperature than the Heat pump so that way the system is able to increase the room temperature rapidly (take the chill out). Other possibility is to size the heat pump for about 80% of maximum (it is cheaper) and cover the deficit by the heat strips. The reasoning is that the extreme heating demand is relatively short and it is more cost effective to use cheaper heat pump and pay little for direct heat.
To answer your question: The heat strips use a lot of power and therefore the combined load of heat strips and the heat pump overloaded the transformer causing voltage drop.
