We noted previously that voltage imbalance usually arises from within the facility. Two causes we already discussed are improper allocation of single phase loads and poor connections in your premises wiring.
In an actual case, the problem was the supply conductors were different sizes. This isn’t common, but it does happen. Differing conductor lengths can also cause imbalance.
Be sure you check for poor connections elsewhere on the load side of the disconnect (including the motor starter and inside the motor weatherhead). You need to do more than visually inspect these connections; measure their impedance. High impedance across a connection can be due to any number of factors, including poor assembly or material incompatibility. If you find a high impedance connection, don’t just tighten it down, replace it.
It’s not just connections that can present high impedance on one phase. This can also happen with the motor control devices. For example, if you find a worn or burnt contactor on a motor starter you’ve probably found your voltage imbalance cause.
