Without getting into a lengthy discourse on electricity, the simple answer is that any water heater internally wired in a 3 phase open delta configuration is unbalanced, meaning that the amperage drawn by one of the three legs is high and unequal to the other two legs. This unbalanced water heater can have a detrimental effect on a buildings power system and therefore needs to be carefully understood before you install a water heater configured for 3 phase open delta. It should be noted that the branch circuit protection (i.e. circuit breaker) servicing a 3 phase open delta water heater needs to be sized for the high amperage leg. The Hubbell model E is available up to a maximum of 12kw in 3 phase open delta wiring. Alternatively, the Hubbell model SE is a balanced 3 phase water heater, meaning that all three legs draw equal amperage. The Model SE is available up to 58kw click here details Model SE