I just underwent a major renovation and now when my water heater is maxed out, it trips the washing machine breaker on its own dedicated circuit. I upgraded to 400A service with one of the 200A panels supplying only a Rheem tankless electric water heater (240V 150A using 4x 40A), dryer (240V 30A), and washer (120V 20A Square D PON AFCI/GFCI).
The washer works fine by itself, and all is good when the water heater is only serving a light load. But if I set the heater too high, or fill a tub faster than the heater can work, it shows “E5” (can’t keep up with demand) and the washing machine circuit trips.
Interestingly, when the water heater is fully taxed, there is a slight electric hum at the breaker panel.