Imbalanced loads

Balanced load

Imbalanced loads - What members should consider


NECA Technical have been receiving calls relating to some quite obscure electrical situations, which members have been experiencing.

We have written the following article to assist our members in understanding why balanced loads are important.

An imbalanced load can be described as deviation in voltage and current waveforms from perfect sinusoidal.  In terms of their magnitude or phase shift the waveform is termed as imbalanced.

With perfect conditions i.e. with only linear loads connected to the system, the phases of power supply are 120 degrees apart in terms of phase angle and magnitude of their peaks should be the same. 

On a distribution level, any load imperfections cause current imbalance which can travel to a transformer and cause imbalance in the three phase voltage. Minor imbalance in the voltage at the transformer level disturbs the current waveform significantly on all the loads connected to it. Not only in the distribution side but through the transformer - this can affect the high voltage system as well.

What does this mean?


Imbalance can decrease motor efficiency by causing extra heating in a motor. The heat generated can also affect a motors life by decomposing the grease or oil in the bearings and de-rate motors windings.

Uninterruptible power supplies (UPS) or inverter supplies also perform with poor efficiency and inject more harmonic currents.

Imbalances can cause the de-rating of power cables by increasing I2R losses (power loss caused by the flow of a current I through a resistance R) in the cable. For distribution cables the de-rating factor represents the part of total current giving effective outcomes.

The causes of imbalance can be attributed to:


Three phase equipment such as an induction motor with imbalance in its windings. If the reactance of the three phases is not the same, it will result in varying current flowing in the three phases.

A large single phase load, or a number of small loads connected to only one phase, cause more current to flow from that particular phase causing voltage drop. Switching of three phase heavy loads can result in current and voltage surges which cause imbalance in the system.

Methods to reduce imbalance include:

  • Single phase loads should be distributed on a three phase system so that they put equal load on three phases.
  • Identifying and reducing harmonics - this can be done by installing reactive or active filters. These filters reduce the negative phase sequence currents by injecting a compensating current wave.