How to Calculate Three Phase Voltage Imbalance Description

When you think of three-phase equipment, remember that there are three phases of power out of phase with each other. However, unlike split-phase voltage, these phases are not 180 degrees out of phase with each other. As 360/2 equals 180 (the 2 comes from the split-phase nature of the equipment), 360/3 equals 120, so the phases are 120 degrees out of phase with each other.

Voltage imbalance may occur due to voltage drop on one of the phases, especially due to issues with breakers or disconnects. However, the unit’s internal controls are a more likely source of voltage imbalance (e.g., the contactor).

When you calculate voltage imbalance, you measure all three phases together, add them, and find the average. Then, you calculate the percentage of difference based on the average. Practically, you would measure between each leg of power on the contactor (if the system is under load). Multiply the three together and divide by three. Then, compare each leg to the average to come up with the percentage of difference. Do that on the L side and then the T side (under load as well). If there’s a significant difference between the two, then you can conclude that the contactor is the source of the voltage imbalance.

You can also use your voltmeter as a voltage drop device. You can just test the control by putting the legs of your meter on L1 and T1 to see if you have any voltage drop. Repeat for the other legs (L2 to T2 and L3 to T3). In many cases, anything around or above 1 volt is typically a problem.

Per the U.S. Department of Energy, voltage imbalance should ideally be less than 1%. However, other sources claim that up to 4% is considered acceptable.