Power quality guide
Phase imbalance: when three phases stop sharing the load.
A three-phase supply works best when all three phases carry the same voltage and current. When they drift apart, motors run hotter, one phase’s cable and breaker carry more than their share, and neutral current rises. Here is how to measure it and where it usually starts.
How unbalance is calculated
The common method, used for motors, takes the largest deviation of any phase from the average of the three, as a percentage of that average:
Unbalance % = largest deviation from the average ÷ average × 100
| Example | R / Y / B | Average | Largest deviation | Unbalance |
|---|---|---|---|---|
| Voltage (line to line) | 415 / 410 / 400 V | 408.3 V | 8.3 V | 2.0% |
| Current | 100 / 95 / 80 A | 91.7 A | 11.7 A | 12.7% |
The same formula works for voltage and current, but the limits are very different. A few percent of voltage unbalance matters; current unbalance of 10% or more is common on mixed loads.
The limits
| Reference | What it says |
|---|---|
| NEMA MG 1 (motors) | Derate a motor when voltage unbalance is above 1%; operation above 5% is not recommended |
| EN 50160 (supply) | Voltage unbalance on public supply normally within 2% |
| Rule of thumb | Current unbalance runs several times the voltage unbalance in a motor |
Small voltage unbalance causes large current unbalance in a motor, and the extra current turns into heat. That is why a 2% voltage unbalance, which looks harmless, can shorten winding life.
What causes it
- Single-phase loads piled on one phase: lighting, offices, welding sets, heaters, single-phase machines.
- Supply-side unbalance: the DISCOM transformer or line is already unbalanced before it reaches you.
- A failing component: a loose joint, a burnt contactor tip, a blown fuse in one leg of a capacitor bank.
- An unequal capacitor bank or a step that has lost capacitance on one phase.
Finding where it starts
- Check voltage unbalance at the incomer. If it is already high there, the cause is upstream. Record it and raise it with the DISCOM.
- If incomer voltage is fine, look at current on each feeder. The feeder with the highest current unbalance is where single-phase loads need spreading.
- Watch it over time. Unbalance that appears only on one shift points to the loads on that shift; unbalance that slowly grows points to a failing component.
- Check neutral current where the meter reports it. High neutral current on a balanced-looking panel can also mean harmonics. See power quality.
Does it affect the bill?
Not as a line item. It costs through motor heating and failures, losses in overloaded cables and transformers, and breakers tripping on one phase while the others have headroom.
Phase imbalance in KONDUKTOR® EMS
- Voltage and current unbalance on every three-phase meter, live and in history.
- Per-phase voltage and current with R, Y and B shown side by side.
- An alert when unbalance crosses the limit you set, on any meter.
- Hourly minimum and maximum with the time each occurred, to match unbalance to a shift or a load.
Common questions
What is an acceptable voltage unbalance?+
For motors, NEMA MG 1 recommends derating above 1% and avoiding operation above 5%. EN 50160 expects public supply within 2%, so a reading above 2% at your incomer is worth raising with the DISCOM.
Why is current unbalance higher than voltage unbalance?+
A motor offers much lower impedance to the unbalanced part of the voltage than to the balanced part, so a small voltage unbalance drives a much larger current unbalance. Mixed single-phase loads add more. Current unbalance of 10% or more on a mixed feeder is common; on a single motor it points to a problem.
Does phase imbalance increase the electricity bill?+
Not directly as a charge. It costs through extra heat and losses in motors, cables and transformers, more motor failures, and breakers tripping on one overloaded phase.
See unbalance on every panel, before a motor fails.
Book a live demo and see per-phase voltage, current and unbalance on meters like yours.