Voltage unbalance calculator

Field use cases

Voltage unbalance work usually begins with a record, not a blank formula. A technician may have three readings written beside a main switchboard test sheet. An electrician may be looking at a plant-room complaint where one motor starter has been running hotter than expected. An engineer may receive a power-quality report and need a quick arithmetic check before deciding whether the matter belongs with load allocation, network supply, equipment selection or further logging.

This calculator is built for that first calculation record. It takes one comparable set of three voltages and turns it into an average, a maximum deviation, a spread and an entered-threshold result. It does not decide the cause. It does not say that a DNSP must act. It does not turn a spot reading into a formal network-performance finding. Its value is that it makes the record readable enough for the next technical decision.

The most useful reading is not the percentage alone. It is the full pattern: which voltage is highest, which is lowest, how far apart they are, and whether the threshold used for the record has a clear source. A small percentage with a vague threshold is weaker than a larger percentage with a well documented record basis.

Measurement basis

The first discipline is to keep the measurement set clean. Line-to-line mode means the three entries are Vab, Vbc and Vca. Phase-to-neutral mode means the three entries are Van, Vbn and Vcn. The calculator deliberately does not let those labels become a mixed set. A mixed set may still contain real measurements, but it is not a valid basis for this arithmetic record.

Australian low-voltage work commonly uses 400 V line-to-line and 230 V phase-to-neutral context, but the fields remain editable because site records, instrument reports and project documents must control the entered values. The nominal voltage field is not used to decide the unbalance percentage. It is shown to keep the average-voltage deviation visible beside the unbalance result.

Result interpretation

The formula compares each voltage with the average of the three. The voltage with the largest absolute deviation controls the percentage. That makes the result simple to read: if one phase pair or one phase-to-neutral reading is pulling away from the other two, the number rises.

If the result is above the entered threshold, the record needs review. That does not automatically identify the cause. Possible directions include load allocation, loose or high-resistance connections, source impedance, generator regulation, inverter export condition, transformer loading, motor starting, equipment sensitivity or network-side conditions. The right direction depends on the measurement location and the operating condition recorded with the voltages.

If the result is within the entered threshold, the record is still only a record. It may be good evidence that the three readings were not materially uneven at that moment or over that report window. It does not prove that the site has no power-quality issue, and it does not replace manufacturer requirements for motors, drives, medical equipment, lifts, controls, refrigeration or other sensitive plant.

Review path

Begin by naming the record boundary. A value from the main switchboard tells a different story from a value at a motor control centre or a tenancy distribution board. Then confirm whether the numbers are spot readings, logged averages or report values. That matters because a short measurement can miss a recurring condition, while a logged record may already represent a specified interval or event filter.

The result often leads to a separate calculator or technical review. If the voltage issue appears during motor starting, the motor voltage dip calculator is usually closer to the event. If the concern is uneven single-phase load allocation, the phase balancing calculator is the better tool because it uses currents rather than voltages. If transformer loading or source context is part of the question, transformer current and kVA can provide the surrounding load picture.

Difference from phase balancing and motor voltage dip

Voltage unbalance, phase balancing and voltage dip can appear in the same discussion, but they are not the same calculation.

A board can have even current allocation and still show a voltage issue. A board can also have uneven current allocation without enough information to call it voltage unbalance. Treat the calculations as related evidence, not interchangeable labels.

Worked field example

A plant-room record shows 412 V between A and B, 398 V between B and C, and 386 V between C and A. The entered threshold is 2% from equipment data. The average voltage is 398.67 V. The maximum deviation is 13.33 V because Vab sits that far above the average. The voltage spread is 26 V between Vab and Vca.

The record is above the entered threshold, so the practical outcome is a review of the measurement basis and the equipment context. A motor complaint may need manufacturer data, current measurements, temperature records, start events and possibly a logged power-quality record. A distribution-board record may need load allocation, connection condition or upstream supply review. The calculator does not choose between those causes; it gives a clean arithmetic record that keeps the investigation honest.

Stop points

• The three voltages do not belong to the same measurement set.
• The readings were taken from different times but treated as one record.
• A phase-to-neutral set is being compared with line-to-line results.
• The threshold has no stated project, equipment, DNSP/report or engineering basis.
• The result is being used as a formal supply-quality finding without instrument and measurement review.
• The record is being used to adjust equipment or inverter settings without manufacturer and network requirements.
• The voltage concern is actually a motor-start dip, current imbalance, harmonic issue, loose connection or protection issue that needs a different test.

When one of those stop points appears, the calculator is still useful. It shows that the arithmetic has reached the edge of its authority. That is often the most important output on a power-quality page.