Three-Phase Voltage Imbalance: Causes, Effects and How to Check It

On a three-phase supply, the three line voltages should be nearly equal. When they are not, the supply is “unbalanced”. It is easy to miss, because the motor still runs. But it runs hotter, wastes energy and wears out sooner. This guide explains how to calculate the imbalance, why even a small percentage matters, what causes it, and how to find the source.

Safety first: Measurements on live equipment must only be done by a qualified person, with suitable instruments, following local rules and your plant’s procedures. Lock out the power before you touch connections, fuses or terminals.

What is voltage imbalance?

Voltage imbalance (or unbalance) means the three phase-to-phase voltages are not equal. For example, a nominal 400 V supply might read 400 V, 408 V and 392 V. It is expressed as a percentage.

How to calculate voltage unbalance

Use this method:

1. Measure the three line-to-line voltages at the motor terminals, with the motor running under load.
2. Calculate their average.
3. Find the largest difference between any one reading and the average.
4. Divide that difference by the average and multiply by 100.

Example: 400 V, 408 V and 392 V give an average of 400 V. The largest deviation is 8 V, so the unbalance is 8 ÷ 400 × 100 = 2 %.

As a guide, aim for less than 1 %. Many motor standards require the motor load to be reduced above 1 % and advise against running above 5 %. Check your motor manufacturer’s documentation for its own limits.

Why a small imbalance matters?

The key problem is that current is much more sensitive than voltage. As a rule of thumb, the current imbalance can be about 6 to 10 times the voltage imbalance. A 2 % voltage unbalance can therefore mean a current unbalance of 12 % to 20 %. The most heavily loaded winding gets hot, and as a rule of thumb, a 3.5 % voltage unbalance can raise the temperature of the hottest winding by roughly 25 %.

The effects are:

• Higher winding temperature and faster ageing of the insulation
• Lower efficiency and higher energy cost
• Vibration and extra noise
• Nuisance tripping of overload relays, and early motor failure

If you have a motor that runs hot with no obvious reason, see our guide on motors running hot.

Common causes

1. Unequal single-phase loads

Lighting, heaters and small machines connected to only one or two phases pull those phases down.
Fix: Redistribute the single-phase loads so each phase carries about the same.

2. Loose or corroded connections

A poor connection adds resistance on one phase, which creates a voltage drop and heat.
Fix: With the power locked out, check and retighten terminals, lugs and busbar connections. Look for discoloration from heat.

3. A blown fuse or a worn contact

A blown fuse, a worn contactor contact or a damaged breaker pole on one phase unbalances the supply to the motor, and in the worst case causes a missing phase (see our guide on motors that won’t start).
Fix: Check fuses, contactors and breakers on all three phases and replace damaged parts.

4. Faulty power factor correction capacitors

A blown fuse in a capacitor bank, or a failed capacitor, can unbalance the supply.
Fix: Check the capacitor bank, its fuses and its controller.

5. Supply or transformer problems

Unequal transformer taps, an open-delta connection or an imbalanced utility supply can produce unbalance before it reaches your equipment.
Fix: Measure at the source. If the imbalance is already present there, involve the electrical supplier or the transformer maintenance team.

6. Unequal cables

Long runs, unequal lengths or different cable sizes on the three phases cause unequal voltage drops.
Fix: Check the cable sizes and lengths against the design, and replace damaged cables.

Step-by-step troubleshooting

1. Measure the three line voltages at the motor, with the motor running (qualified person).
2. Calculate the unbalance percentage.
3. Measure the current on each phase and compare it with the average and the nameplate (qualified person).
4. Compare the voltage at the source with the voltage at the motor, to see where the imbalance appears.
5. With the power locked out, check connections, fuses and contacts.
6. Check the loads on the supply, including single-phase loads and capacitor banks.
7. Use the lead rotation test, described below, to separate a supply problem from a motor problem.

The lead rotation test

This test tells you whether the imbalance comes from the supply or from the motor.

1. Measure and note the current on each of the three leads, with the motor running (qualified person).
2. Lock out the power. Move each supply lead one terminal over, in the same direction, for example L1 to T2, L2 to T3 and L3 to T1. This keeps the direction of rotation unchanged. Never swap just two leads, because this reverses the motor.
3. Restart and measure the current on each lead again.
4. If the highest current follows the supply lead (it moved to a different motor terminal), the cause is in the supply. If it stays on the same motor terminal, the cause is in the motor.
5. Restore the original connections afterward.

Mistakes to avoid

• Measuring only at the panel. The imbalance may appear only at the motor, after a bad connection or a long cable.
• Measuring with no load. Many problems show only under load.
• Relying only on the overload relay. It may not trip until the damage is done. Consider a phase failure and unbalance monitoring relay.
• Ignoring “small” numbers. Even a 2 % to 3 % voltage unbalance matters in the long term.

Quick reference

Voltage unbalance	What to do
Below 1 %	Good. Keep monitoring.
1 % to 2 %	Find the cause and fix it at the next opportunity.
2 % to 5 %	Reduce the motor load as the manufacturer advises, and fix the cause soon.
Above 5 %	Do not run the motor until the cause is found.

When to call a specialist?

If the imbalance is present at the source, if you can’t find the cause, or if you are not sure how to work safely, stop and call a qualified electrician or your supplier. The cost of a visit is far lower than a failed motor.

Have you found a surprising cause of voltage imbalance? Share it in the comments below.

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