What is the difference between THD and TDD?

THD expresses harmonic content as a percentage of the fundamental current at the moment of measurement, so it changes whenever the load changes. TDD expresses the same harmonic current as a percentage of the maximum demand load current, which is a fixed reference. IEEE 519 uses TDD for current limits precisely because it does not drift with load.

Where is the point of common coupling?

The point of common coupling is the point on the public supply where other customers' loads could be connected — in practice, usually the utility's metering point or the main breaker of the installation. IEEE 519 limits apply at that point, not at the terminals of the distorting load. This distinction matters because a filter installed at the load may satisfy the load but still leave the PCC above limits if other loads also distort.

Does IEEE 519 apply to my plant outside the United States?

IEEE 519 is a United States standard, but it is widely referenced internationally in specifications and tender documents. In Europe the equivalent requirements appear in EN 61000-2-2 and EN 61000-2-4, in the UK in Engineering Recommendation G5/5. The numeric limits differ, so check which standard the specification actually cites.

Is IEEE 519 a legal requirement?

No. It is a recommended practice, not law. It becomes binding when written into a utility interconnection agreement, a consultant's specification, or a tender document. In practice most large installations are held to it through one of those routes rather than through legislation.

Compliance

IEEE 519 harmonic limits explained

The tables most specifications cite, what each column actually constrains, and the three places projects usually get the calculation wrong.

IEEE 519-2014 limits harmonic voltage distortion at the point of common coupling to 8% THD on systems at or below 1 kV, and harmonic current distortion according to the ratio of short-circuit current to load current, expressed as TDD.

Voltage limits apply to the utility, which controls the supply. Current limits apply to the customer, who controls the loads. The standard separates the two because responsibility is divided at the metering point.

Updated 22 October 20267 min readIEEE Std 519-2014

Voltage limits at the point of common coupling

These are the numbers a utility or consultant will quote when they say the supply is out of specification. The limit tightens as system voltage rises, because high-voltage networks serve more customers and a given distortion level affects more equipment.

IEEE 519-2014 Table 1. Limits apply at the PCC, measured over a period of at least one week.
Bus voltage at PCC	Individual harmonic (%)	Total THD (%)
V ≤ 1.0 kV	5.0	8.0
1 kV < V ≤ 69 kV	3.0	5.0
69 kV < V ≤ 161 kV	1.5	2.5
V > 161 kV	1.0	1.5

Two conditions sit alongside the table and are easy to miss. The limits apply to the worst weekly condition, not the average, and they are evaluated at the PCC rather than inside the installation. A plant can be fully compliant at its own main board while contributing to a PCC violation shared with neighbouring customers.

Current limits, by short-circuit ratio

Current limits are not a single number because the same harmonic current causes different voltage distortion depending on how stiff the supply is. A 100 A harmonic current on a 2 MVA transformer barely moves the voltage; the same current on a 200 kVA transformer produces severe distortion.

IEEE 519 handles this with the short-circuit ratio, Isc/IL, where Isc is the available short-circuit current at the PCC and IL is the maximum demand load current.

IEEE 519-2014 Table 2, for systems rated 120 V to 69 kV. Limits are percentages of IL, the maximum demand load current.
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

Even harmonics are limited to 25% of the odd harmonic limits shown. That is not a formality: DC offset in a rectifier, or a half-wave asymmetry, produces even orders that the odd-harmonic columns do not constrain.

Getting the short-circuit ratio right

This single number decides which row of the table applies, and it is where most compliance calculations go wrong. Two errors dominate.

Using transformer kVA instead of fault level

Isc is the fault current available at the PCC, not the transformer rating. For a transformer of rating S and impedance Z%, the approximate fault level is S ÷ (Z/100). A 1000 kVA transformer at 6% impedance gives roughly 16.7 kA at the LV terminals.

# short-circuit current at transformer secondary
I_sc = S / (√3 × V_LL × (Z/100))

# 1000 kVA, 400 V, 6% impedance
I_sc = 1000 000 / (1.732 × 400 × 0.06) ≈ 24 000 A

# ratio against a 1200 A load current
I_sc/I_L ≈ 20 → boundary of the first two rows

Using measured current instead of maximum demand

IL is the maximum demand load current, taken as the average of the twelve highest monthly demand readings over the preceding year — not the current flowing when the analyser happened to be connected. Using a light-load reading inflates the ratio and moves the installation into a more permissive row.

The PCC question decides who pays

If a compliance study places the PCC at the utility metering point, then every harmonic source in the plant contributes to the same limit, and a single large drive can push the whole installation out of compliance. If the PCC is taken at the customer's main breaker, the calculation is narrower.

In practice the difference determines whether the fix is one filter at the main board or several smaller filters distributed around the plant. Distributed filtration usually costs more in hardware but less in cable and installation, and it avoids the resonance risk of concentrating compensation at one point.

When the specification is not IEEE 519

Standard	Region	How it differs
EN 61000-2-2 / 2-4	Europe	Sets compatibility levels rather than emission limits; the emission limits sit in EN 61000-3-2 and 3-12 for equipment.
EREC G5/5	United Kingdom	Assessment based on the number and type of harmonic sources, with staged limits above 5th harmonic.
GB/T 14549	China	Voltage THD limit of 5% at 0.38 kV, 4% at 6–10 kV. Tighter than IEEE at low voltage.
IEC 61000-4-7	International	Measurement method, not a limit. Defines how harmonics are grouped and aggregated.

If a tender document cites one of these instead, the row you need will differ, and the filter rating derived from an IEEE 519 table will be wrong in either direction. Confirm which standard the acceptance test will use before sizing anything.

Need a compliance calculation for a tender?

Send the transformer rating and impedance, the maximum demand, and the harmonic spectrum. We will return the applicable row, the TDD margin, and the filter rating required to stay inside it.

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