IEEE 519 Harmonic Limits — What the Tables Actually Mean
IEEE 519 limits are measured at the PCC, not at individual loads. The Isc/IL ratio determines which row of Table 2 applies. Here's how to read it correctly.
February 26, 2026
The Most Misunderstood Standard
IEEE 519 is probably the most frequently misapplied electrical standard. The two key misunderstandings:
- Limits apply at the Point of Common Coupling (PCC), not at individual equipment or branch circuits
- Current limits are percentages of the load current (IL), not the system capacity
Where to Measure: The PCC
The PCC is defined as the point where the utility supply meets the customer's installation. In practice:
- For a commercial building: the main service entrance
- For an industrial plant: the utility metering point
- For a facility within a campus: the sub-distribution point where the facility connects to the shared network
Individual VFDs, LED drivers, and other harmonic sources measured at their terminals will almost always exceed IEEE 519 limits. This is expected and acceptable. The standard cares about the aggregate effect at the PCC, where harmonic cancellation between diverse loads typically reduces total distortion.
Current Distortion Limits (Table 2)
The current limits depend on the ratio of short-circuit current to load current at the PCC:
| Isc/IL | h<11 | 11≤h<17 | 17≤h<23 | 23≤h<35 | 35≤h | TDD |
|---|---|---|---|---|---|---|
| <20 | 4.0% | 2.0% | 1.5% | 0.6% | 0.3% | 5.0% |
| 20–50 | 7.0% | 3.5% | 2.5% | 1.0% | 0.5% | 8.0% |
| 50–100 | 10.0% | 4.5% | 4.0% | 1.5% | 0.7% | 12.0% |
| 100–1000 | 12.0% | 5.5% | 5.0% | 2.0% | 1.0% | 15.0% |
| >1000 | 15.0% | 7.0% | 6.0% | 2.5% | 1.4% | 20.0% |
Isc = maximum short-circuit current at the PCC (from utility data or calculation) IL = maximum demand load current at the PCC (15-minute demand) TDD = Total Demand Distortion (like THD but referenced to IL, not fundamental)
A "stiff" system (high Isc/IL) can tolerate more harmonic current because the system impedance is low and harmonic voltage distortion is minimal. A "weak" system (low Isc/IL) has tighter limits.
Voltage Distortion Limits (Table 1)
| Bus Voltage | Individual Harmonic | THD |
|---|---|---|
| ≤1 kV | 5.0% | 8.0% |
| 1–69 kV | 3.0% | 5.0% |
| 69–161 kV | 1.5% | 2.5% |
| >161 kV | 1.0% | 1.5% |
Voltage limits are the utility's responsibility. If the customer meets the current limits and the utility still has excessive voltage distortion, the utility must strengthen the system (lower impedance).
Common Misapplications
- Measuring at the VFD output: A single VFD will show 80%+ current THD. This is NOT an IEEE 519 violation — the standard doesn't apply at this point
- Using THD instead of TDD: THD is referenced to fundamental current. TDD is referenced to maximum demand current. At light load, THD can be high even when absolute harmonic current is trivially small
- Applying limits to voltage AND current simultaneously: Current limits are the customer's responsibility. Voltage limits are the utility's. They're measured at the same point but allocated to different parties
- Ignoring the Isc/IL ratio: Using the most restrictive row (<20) when the actual ratio is 200 dramatically over-constrains the installation
Assess your harmonics: Calculate TDD and verify compliance with the Harmonics Analysis Calculator.
Frequently Asked Questions
What are harmonics and why are they a problem?
Harmonics are distorted current/voltage waveforms at multiples of 50/60Hz. They cause transformer heating, neutral conductor overload, and equipment malfunction. IEEE 519 sets distortion limits per voltage level.
Related Articles
- Harmonics Analysis - Interactive calculator with standards compliance