Load Unbalance

On a three-phase wye system where the dominant loads are single phase, the neutral carries the unbalanced amount of current. Even though the loads may have been balanced at one time, normal changes in the system may cause a large unbalance, leading to a large neutral current.

Many times, when sizing and distributing loads within a panel, only the full load current is considered, not how often that load is on or off. At any given time there may be a significant unbalance due to loads being on or off.

Whether because of load shifts and changes, or the diversity of loads being on or off, neutral current from unbalance can be high, but is seldom excessive. Problems happen when the neutral wire has been undersized and high currents occur -- possibly leading to a burnt neutral. Quite often this is aided by illegal wiring practices.

High neutral currents from unbalance are fairly easy to resolve. Loads need to be powered from appropriate phases to ensure balancing, so relocate or rewire particular loads to bring balance about.

If diversity is the problem, then use a power flow monitor such as the BMI PowerProfiler to determine the current swings of loads and when the shifts occur. Then rewire or relocate to create a better balance.

In both cases, keep the neutral conductor at lease the same size as the phase conductors. For those who really want to think ahead, make the neutral one size larger. This will help with harmonics concerns.