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Three-Phase Imbalance

Compute voltage or current imbalance percentage per IEEE 1159 / NEMA MG-1, check it against utility and equipment thresholds, and report the motor derating factor.

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Imbalance
0.87%
Within 2% threshold
Tick = 2% threshold
NEMA motor derating
98.3%
Apply when sizing motors on this bus
Average
230.00 V
Arithmetic mean of three phases
Max deviation
2.00 V
Furthest phase from average
Definition
Max-deviation method
(max |phase − avg|) ÷ avg × 100

Phasor view

Spoke length is the reading. The dashed circle is the balanced reference at the average.

ABC
  • A 230 V
  • B 228 V
  • C 232 V

NEMA vs true (IEC) unbalance

1.00%

Voltage unbalance factor (negative ÷ positive sequence) from the three magnitudes — the CIGRE closed form used by IEC 61000-4-30. Exact for line-to-line readings, approximate for line-to-neutral. The NEMA max-deviation figure above is 0.87 %; the two normally agree within a few tenths.

Why imbalance matters

Voltage imbalance disproportionately stresses three-phase motors — even a 3.5% imbalance can double the temperature rise on the most-stressed phase winding. Current imbalance on a panel usually points to a load-distribution problem that's easy to fix: move some single-phase circuits to the least-loaded phase. Persistent voltage imbalance with balanced current is a utility issue.

Catch imbalance before it kills the UPS bypass.

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Two definitions, one number

Three-phase imbalance has two competing definitions in practice. NEMAuses the maximum deviation from average divided by the average — the simple form used here, which is what most field meters report and what NEMA MG-1 §14.36 references for motor derating. IEC / IEEE uses the symmetrical-components form (ratio of negative-sequence to positive-sequence), which is more rigorous but requires instantaneous phase-angle data. For routine field use, the NEMA form is correct to within a few tenths of a percent.

Why 2% is the typical limit

ANSI C84.1 sets the utility distribution voltage tolerance bands; the harmonised number for steady-state imbalance is 2% over a three-second window. IEC 61000-2-2 sets 2% as the compatibility level for LV distribution. Above 2%, motor manufacturers require derating. Above 5%, motors must not be operated at full load — the negative-sequence current causes a reverse-rotating magnetic field, doubling rotor losses and tripping thermal protection.

Voltage vs current imbalance

Voltage imbalance is the property of the supply — caused by unequal single-phase loads on a distribution feeder, asymmetric transformer impedances, or open-delta connections. Current imbalance is the property of the load — caused by uneven distribution of single-phase loads across the three phases inside the building. A small voltage imbalance (say, 1%) can cause a much larger current imbalance (5–10%) on running motors because of the negative-sequence impedance — and the resulting heat is what shortens motor life. Fix the load distribution first; if voltage imbalance persists with balanced load, escalate to the utility.

Application to UPS bypass

Online double-conversion UPS isolate the load from utility imbalance during normal operation, but the bypass path is direct: an out-of-tolerance grid passes straight to the load on bypass. Capturing the bus imbalance at install gives you a baseline; large drift over time is the cue to investigate before a fault forces the UPS to bypass into an unhappy grid.

Questions engineers ask

Which definition does the headline number use?

The NEMA MG-1 max-deviation form: the largest deviation of any phase from the average, divided by the average. It is what field meters display and what the motor-derating curve is referenced to. The true symmetrical-components factor (VUF) is shown alongside for voltage readings.

What is the 2 % threshold based on?

ANSI C84.1 recommends supply unbalance under 3 % and most specifications tighten that to 2 %; IEC 61000-2-2 sets 2 % as the LV compatibility level. Motor makers begin requiring derating above 1 % and prohibit full-load operation above 5 %.

My voltages balance but the currents don’t. What does that mean?

A load-distribution problem inside the installation: too many single-phase circuits on one phase. Move circuits to the lightly loaded phases. The reverse case, balanced current with unbalanced voltage, points upstream to the utility, the transformer or a loose connection.

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Engineering disclaimer: Power Stack provides this calculator as a general engineering estimate. Final design must be verified by a qualified electrical engineer and reconciled with manufacturer datasheets, the applicable national wiring regulations (NEC, BS 7671, IEC 60364, or your local equivalent), and site-specific conditions. Power Stack accepts no liability for design decisions made from this output.