Electrical equipment, especially motors and controllers, will not operate reliably on unbalanced voltages in a 3-phase system. Generally, the difference between the highest and the lowest voltages should not exceed 4% of the lowest voltage. Greater imbalances may cause overheating of motors and intermittent shutdown of motor controllers. And to make matters worse, most motor starter overload relays can't sense the overheating and won’t trip. Also, many of today’s solid-state motor controllers and inverters include components that are especially sensitive to voltage imbalances.

Four percent of the lowest voltage (230V) is 9.2V (230V x 4%= 9.2V). The difference between the highest voltage (240V) and the lowest voltage (230V) is 10 volts. Therefore, these voltages have too great an imbalance. Why? Because the 10 volt difference is greater than four percent of the lowest voltage (9.2V).

There is a more precise procedure for calculating voltage imbalance that you can use. The first step is to calculate the average voltage by adding all three phases and dividing by 3. In our example, the average is 235V: add the voltages to get your total (230V + 235V + 240V = 705V) and then divide by the number of phases for the average (705V / 3 = 235V).

Next, add up the absolute differences between each phase voltage and the average voltage. In this case, the difference between the average voltage and 230V is 5V. The difference between the average and itself is 0V; and the difference between the average and 240V is 5V. Adding up the differences, we get 10 volts. And that 10 volts is what we call the total imbalance.

Finally, divide the total imbalance in half to get an adjusted imbalance. Half of 10V is 5V. Finally, divide the adjusted imbalance into the average voltage to get a percentage imbalance (in this case: 5V / 235V = 0.021). That is a 2.1% imbalance.

Reliable, long-term operation of most electrical equipment requires a voltage imbalance of less than two percent. In this example the system has too much imbalance.

If you find voltage imbalances in your facility, the first place to look is not necessarily at the power company’s incoming line. Instead, look for electrical distribution systems in which one leg of a 3-phase supply powers both single-phase and 3-phase loads. You may find that single phase loads are not evenly balanced across the phases. If your facility employs the use of in-line reactors to correct imbalances, check the settings on them. These reactors usually have taps for adjustment, and somebody may have adjusted them. Or, the imbalance they originally corrected for may have shifted over time. Circuits with tapped reactors rarely stay in balance indefinitely.

Now, here's a practice question. You go out onto the factory floor and find a 460V motor with phase voltages of 458V, 465V, and 480V. Is there a problem or not? Let’s check.

Lowest voltage is 458V. First, calculate your four percent (458 x 4% = 18.32). The difference between the highest voltage (480V) and the lowest voltage (458V) is 22V (480-458). Since the difference (22V) is greater than 4% of the lowest voltage (18.32V), the phases are too far out of balance to ensure continued, reliable operation of electrical equipment.

Lastly, remember that voltage levels within your facility are dynamic; you should do spot checks periodically. It might also be a good idea to install voltage monitoring relays on your motor starter equipment. These inexpensive little devices can shut down motor starters if phase imbalance, single-phasing, or other anomalies occur, thus protecting the motor. Also, keep in mind that additions and replacement of equipment in your plant as well as new or removed circuits can alter the electrical environment and ultimately lead to what would otherwise be preventable costly repairs.