Electrical designers assume that any current in the neutral leg of a three-phase circuit doesn’t incur any energy usage. This current happens when the three phases are not carefully balanced, and the balance has to be made up by the neutral phase. When the phases do not balance, in other words, there’s current in the neutral.
Any time there’s current flow in a conductor, there’s energy wasted, even if the current isn’t used to operate a motor or appliance. The current flow encounters resistance to free flow through the wiring, and this resistance is manifest as heat. Though the voltage drop may be 2 percent or less, the losses occur 24 hours a day. The cumulative total losses over a year can be far from negligible.
A simple solution is to rewire the electrical panel so that the three phases are more closely matched. Even though many of the connected loads have to be estimated, any kind of balance is better than a random connection of loads to the circuits.
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Excerpt from Energy Conservation Projects.