What it calculates
Three-phase voltage variation can be summarized with different mathematical metrics. This calculator makes the measurement basis explicit so a line-magnitude statistic is not confused with a symmetrical-component ratio.
Use the line-to-line method when you have three simultaneous line-pair RMS magnitudes. Use the sequence method when you have three phase-to-neutral fundamental RMS phasors, with magnitude and angle on one common reference.
Methods and examples
Line-to-line maximum deviation: calculate the mean of the three line-pair RMS magnitudes. Divide the largest absolute difference from that mean by the mean and multiply by 100. For 600 V, 585 V and 609 V, the mean is 598 V, the largest deviation is 13 V and the result is approximately 2.174%.
Fundamental sequence ratio: resolve the three phase-to-neutral fundamental RMS phasors into zero, positive and negative sequence components. Report 100 × |V₂| ÷ |V₁|. The same phasors also produce three line-to-line magnitudes, from which this page reports a separate maximum-deviation percentage. For a balanced set with equal magnitudes and 120° separation, |V₂| is zero and the sequence ratio is zero.
Both formulas use magnitudes and angles from one consistent measurement condition. Enter readings as supplied by a meter or analysis system; the calculator does not correct instrument, wiring, sampling or time-window settings.
Limits and interpretation
The line-magnitude deviation and |V₂|/|V₁| sequence ratio are distinct metrics. A set of line-to-line magnitudes does not contain the phase-angle information required to compute symmetrical components. Do not compare one metric to a limit stated for the other.
The sequence method requires a measurable positive-sequence component. It reports no ratio if |V₁| is zero or numerically unresolved. Its phasors must be phase-to-neutral fundamental components measured at a common point and time; the tool does not confirm utility-specific connection conditions or measurement intervals.
No threshold, motor derating, protection choice, equipment acceptance or jurisdictional code result is applied. Consult the applicable equipment and utility requirements for the actual installation.
Common questions
Which method should I use?
Use the line-to-line method for three RMS magnitude readings when only the maximum deviation from their mean is required. Use sequence ratio when you have three phase-to-neutral fundamental phasors and need the negative-to-positive sequence ratio.
Are the two results the same percentage?
No. They use different inputs and mathematical definitions. The page labels each result; do not interchange them.
Does a low percentage mean the supply or motor passes?
No. The calculator applies no acceptance threshold and does not select a motor, derating factor or utility criterion.
Related tools
These tools describe different three-phase quantities.
Sources
Natural Resources Canada, Electric Motors: Energy Efficiency Reference Guide, section 7.5. Describes the line-voltage maximum-deviation-from-average percentage in a motor-operation context. The source's motor derating advice is not implemented here.
EPCOR Distribution & Transmission Inc., Power Technical Requirements: Interconnection Guide, section 4.3, release November 28, 2022, effective March 31, 2023. Defines the fundamental negative-to-positive sequence voltage ratio under its distribution interconnection context. This calculator implements the ratio arithmetic only, not EPCOR's interconnection conditions or limits.
These references describe different metrics. This tool reports them separately and makes no standards or compliance claim.
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