A 400 V bus with a real-world harmonic spectrum, checked against both the voltage distortion limits (Table 1) and the load-referenced current distortion limits (Table 2) — passing both, with the fifth harmonic as the closest call.
| Bus voltage | 0.4 kV |
| Isc/IL ratio band | 20–50 (moderately stiff supply relative to this load) |
| Measured current | 200 A |
| Maximum demand load current (IL) | 250 A |
| Individual voltage harmonics | 2nd: 0.3%, 3rd: 1.5%, 5th: 2.0%, 7th: 1.0%, 9th: 0.5%, 11th: 0.4%, 13th: 0.3% |
| Individual current harmonics (% of fundamental) | 2nd: 0.5%, 3rd: 3.0%, 5th: 5.0%, 7th: 2.5%, 9th: 1.0%, 11th: 1.5%, 13th: 0.8% |
| Check | Limit (≤1 kV) | Actual | Result |
|---|---|---|---|
| VTHD | 8.0% | 2.8% | PASS |
| Worst individual harmonic (5th) | 5.0% | 2.0% | PASS |
IEEE 519 current limits are deliberately referenced to IL (maximum demand load current), not to the instantaneous measured current — this prevents a lightly loaded system from appearing artificially compliant.
| Check | Limit (20–50 band) | Actual | Result |
|---|---|---|---|
| ITDD | 8.0% | 5.33% | PASS |
| Worst individual harmonic, TDD basis (5th) | 7.0% | 4.0% | PASS |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Voltage THD | ≤ 8.0% | 2.8% | ✓ PASS |
| Worst individual voltage harmonic | ≤ 5.0% | 2.0% (5th) | ✓ PASS |
| Current TDD | ≤ 8.0% | 5.33% | ✓ PASS |
| Worst individual current harmonic (TDD basis) | ≤ 7.0% | 4.0% (5th) | ✓ PASS |
Key insight: Current distortion limits in IEEE 519 are referenced to maximum demand current (IL), not to whatever current happens to be flowing at the moment of measurement — this is exactly why the calculator asks for both a measured current and a separate maximum demand current, and why running below full load doesn't automatically make a harmonic-heavy installation look artificially compliant.
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Open Harmonic Analysis calculator →A higher Isc/IL ratio means the point of common coupling has a 'stiffer' supply relative to the size of this particular load — the load's harmonic currents are a smaller disturbance to a stiff system than to a weak one, so IEEE 519 permits looser current distortion limits at higher Isc/IL bands (a small non-linear load on a strong utility feeder gets more headroom than the same load on a weak, high-impedance feeder).
Even harmonics are far less common in typical three-phase non-linear loads (rectifiers, VFDs) because symmetric waveform distortion naturally cancels even-order components — a significant even-harmonic content usually points to an unusual or faulted condition (like a half-wave rectification fault or a DC offset), which is why the standard treats them more strictly rather than assuming they behave like ordinary odd harmonics.