Three line-to-line voltages are rarely identical. The calculation problem is deciding which difference belongs in the numerator: the largest deviation from the average, not the largest gap between two readings. This original study example is for 313A candidates reviewing motor and electrical troubleshooting; it is not a recalled exam question or a field measurement procedure.
Which three readings and formula do you need?
For a three-phase motor supply, start with three line-to-line voltage values taken under comparable conditions. Fluke's voltage-unbalance guide, checked 2026-09-28, gives the study sequence: average the three readings, find each absolute deviation from that average, then divide the largest deviation by the average and multiply by 100.
Percent voltage unbalance = (largest absolute deviation from average ÷ average voltage) × 100. Keep volts in both parts of the fraction so the final answer is a percentage. Do not use phase-to-neutral numbers in one row and line-to-line numbers in another; the inputs must describe the same set of measurements.
How does the arithmetic work with three voltages?
Use a made-up study set of 478 V, 482 V and 480 V. The average is (478 + 482 + 480) ÷ 3 = 480 V. The absolute deviations from 480 V are 2 V, 2 V and 0 V. The largest is 2 V, so the unbalance is (2 ÷ 480) × 100 = 0.42% after rounding.
Notice that the highest-to-lowest gap is 4 V, yet the formula uses 2 V because it asks for deviation from the average. For a self-check, try 477 V, 480 V and 483 V: average 480 V, largest deviation 3 V, result 0.625%. Write all three deviations before selecting one; this prevents choosing the largest line reading by mistake.
What does the percentage tell you about a motor?
It describes how unequal that set of supply voltages is at a point in time. Fluke explains that voltage unbalance can produce a much larger current unbalance and contribute to heat and poor motor performance. It also notes that a manual calculation is a point-in-time result. A single percentage does not identify the faulty connection, motor condition or the source of the imbalance.
For study purposes, separate the observation from the diagnosis: the three voltages are unequal is supported by the arithmetic; this specific motor is defective requires more evidence. Compare the distinction between electrical input and output in the motor power-factor example. For the official scope, see 313A exam information and the current-standard guide.
When should a study calculation stop short of field advice?
This page teaches arithmetic from supplied readings. It does not tell a candidate to open or test energized equipment. CCOHS lockout guidance, checked 2026-09-28, addresses hazardous-energy control, training and assigned authorization. Actual testing requires the workplace procedure, suitable equipment and qualified personnel. A calculated percentage alone is not a safety clearance, equipment rating or repair instruction.
Explain the formula aloud, then repeat it with a new three-value set. If you want a quick baseline across the broader study areas, use the free five-question sample. It is original, unofficial draft practice pending the existing certified-mechanic review and does not predict a certification result.
Sources
- Fluke: What is unbalanced voltage and current unbalance?Three-step formula, consequences and point-in-time limit; checked 2026-09-28
- Fluke: How to troubleshoot motors and drives at the inputsIndependent worked formula and need to interpret supply conditions; checked 2026-09-28
- CCOHS: Lockout/Tag outHazardous-energy controls and authorized personnel; checked 2026-09-28
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.