A modest 1.5% voltage unbalance between three phases — well under the 5% NEMA ceiling, but still enough to call for a small motor derating.
| Phase A-B voltage | 415 V |
| Phase B-C voltage | 408 V |
| Phase C-A voltage | 420 V |
Using the published NEMA MG1 / ANSI C84.1 curve points (1% -> 0.98 p.u., 2% -> 0.95 p.u.), linearly interpolated for 1.53%.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Voltage unbalance | ≤ 5% (beyond which the curve doesn't apply) | 1.53% | ✓ PASS |
| Recommended motor derating | n/a (this is the result) | 0.964 p.u. | ✓ PASS |
Key insight: Even a seemingly modest voltage unbalance disproportionately stresses a motor, because unbalance produces negative-sequence current that circulates in the rotor and generates extra heat roughly in proportion to the square of the unbalance percentage — which is why NEMA's derating curve drops fairly steeply (to 0.75 p.u. at just 5% unbalance) even though 5% doesn't sound like a large number.
Every input in this example is editable in the live calculator — free, no signup.
Open Voltage Unbalance & Motor Derating calculator →NEMA MG1's published guidance explicitly stops at 5% unbalance and recommends against operating motors above that level without direct manufacturer consultation — beyond that point, the heating effects become severe enough, and specific enough to each motor design, that a generic derating curve is no longer considered a reliable guide, and this calculator deliberately declines to extrapolate a number for it.
Common causes include unevenly distributed single-phase loads across the three phases of a supply, an open or high-resistance connection on one phase, or unbalanced impedance somewhere upstream in the distribution system — checking and rebalancing single-phase loads across phases is often the simplest and most effective fix, before assuming a motor needs permanent derating.