In IEEE 519-2022, IL is the average of the monthly maximum demand current over the previous twelve months at the PCC, and Isc is the maximum short-circuit current at that point. Their quotient, Isc/IL, decides which row of the harmonic current tables applies to the installation.
Short answer
IL is the maximum demand current of the installation at the PCC. IEEE 519-2022 defines it as the sum of the RMS currents of the 15- or 30-minute maximum demand of each of the previous twelve months, divided by 12. Isc is the maximum short-circuit current at the same point. The short-circuit ratio (SCR) is Isc divided by IL, and that number selects the row of the harmonic current limit tables.
What IL is
IL appears in the definition of clause 3.1 as maximum demand load current. It has three rules:
- Installation with a full history: for each of the previous twelve months, take the RMS current of the 15- or 30-minute maximum demand. Add the twelve and divide by 12.
- Installation with less than twelve months in service: the standard allows adding the monthly maximum demand (15- or 30-minute apparent power) of the months available and dividing by the number of those months.
- New installation: IL is based on the projected monthly maximum demand, 15 or 30 minutes, during the year following the start of operation of the harmonic-producing loads listed in the service request.
Two details avoid confusion. First, IL is established at the PCC and not at the service entrance of a piece of equipment or at a load’s panel. Second, it is the maximum demand current, not the rated current of a transformer nor the installed load: a plant that almost never reaches its capacity has an IL far below its nameplate current.
IL is the reference for TDD. That total demand distortion is expressed as a percentage of IL (definition of TDD in 3.1), and so it is not inflated when the load drops, as current THD is. The guide THD: What It Is, How It Is Calculated and IEEE 519 Limits explains that difference.
What Isc is and how the ratio is reached
The current tables of the standard (2, 3 and 4) define Isc as the maximum short-circuit current at the PCC and IL as the maximum demand load current at the PCC under normal load operating conditions. The definition of short-circuit ratio in 3.1 is the same idea: at a given location, the quotient between the available short-circuit current, in amperes, and the maximum demand load current, in amperes.
The value of Isc is provided by the utility or obtained with a short-circuit study of the system. The standard does not explain how to calculate it. That calculation is not part of IEEE 519: it is covered in the harmonic study, which starts from the network model.
- Maximum demand of each month (15 or 30 min). RMS current at the PCC over twelve months
- Average of the twelve months. Result: IL
- Maximum short-circuit current at the PCC. Data: Isc
- Quotient Isc / IL. Result: short-circuit ratio
- Row of the limit table. By ratio range, according to voltage level
Illustrative example
The values below are made up to show the calculation. They are not from a real installation.
A plant is supplied at 13.2 kV. The maximum demands of the previous twelve months, measured at the PCC, were (in amperes): 410, 395, 420, 430, 440, 450, 460, 455, 440, 435, 425 and 420. The sum is 5,180 A and the average is IL = 431.7 A.
The utility reports a maximum short-circuit current at the PCC of 9,000 A. The ratio is 9,000 / 431.7 = 20.8. That ratio falls in the “20 to less than 50” range of Table 2 (systems from 120 V to 69 kV), whose TDD limit is 8.0 %.
If the plant doubled its demand and IL rose to 863 A with the same network, the ratio would drop to 10.4 and the installation would move to the “less than 20” row, with a TDD limit of 5.0 %. Growing without any change in the network makes the limits stricter.
Why a weak network gets stricter limits
Short-circuit current measures how “strong” the network is at the PCC. A strong network has low impedance: the harmonic current the user injects produces a small voltage drop. A weak network has high impedance: the same harmonic current distorts more the voltage received by all users connected at the point.
The IEEE 519 current tables capture that idea with a scale of ratios. The lower Isc/IL is, the lower the permitted percentages. In Table 2 the TDD limits go from 5.0 % (ratio below 20) to 20.0 % (ratio above 1,000). Clause 5 summarizes it in its basic assumption: by limiting users’ harmonic currents, voltage is kept below objectionable levels.
| Isc/IL range | Limit tendency |
|---|---|
| Less than 20 | Strictest (TDD 5.0 % in Table 2) |
| 20 to less than 50 | Low intermediate |
| 50 to less than 100 | Intermediate |
| 100 to less than 1,000 | High intermediate |
| Greater than 1,000 | Most relaxed (TDD 20.0 % in Table 2) |
Table 3 (above 69 kV up to 161 kV) uses the same five ranges and Table 4 (above 161 kV) has three. The full values are detailed in Harmonic Current Limits and TDD by Isc/IL.
What the ratio changes over the life of a plant
- Growing the load without reinforcing the network lowers Isc/IL and can move you to a stricter row.
- A network that is reinforced (new substation, more nearby generation) raises Isc and relaxes the row.
- A demand that varies by season is reflected in IL through the twelve-month average, which smooths the peaks of a single month.
- Generation: the tables indicate that generation facilities are limited to the values of the lowest-ratio row, regardless of their actual Isc/IL, unless another standard with applicable scope covers them.
The IL figure belongs to the installation; the Isc figure belongs to the network. Whoever obtains them separately must confirm that both refer to the same PCC.
Both data are established at the coupling point: see PCC in IEEE 519: Where Harmonic Limits Apply. To calculate the TDD of a spectrum with your IL you can use our free harmonic distortion THD and TDD tool, which compares up to 69 kV. The short-circuit current comes from the short-circuit study or from the utility.
We help with the data and the study
At Tiza Group we carry out power quality studies, in Colombia, or elsewhere on request: we obtain the maximum demand and the short-circuit current at your PCC, calculate the ratio and verify which IEEE 519-2022 row applies to you. If your plant is going to grow, we show you in advance how your limits change.
Related guides
- IEEE 519: Harmonic Limits and Key Concepts
- Harmonic Current Limits and TDD by Isc/IL
- PCC in IEEE 519: Where Harmonic Limits Apply
- Short-Circuit Study: IEC 60909 and ANSI/IEEE
- IEEE 519 Harmonic Study
References
- IEEE Std 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems: clause 3.1 (definitions of maximum demand load current, short circuit ratio, TDD); clause 5 (basic assumption); clauses 5.3 to 5.5 and Tables 2, 3 and 4 (definition of Isc and IL and limits by Isc/IL ratio).
Frequently asked questions
Is IL the transformer rated current or the installed load current?
Neither. IL is a measured value: the average of the RMS currents of the 15- or 30-minute maximum demand of each of the previous twelve months, taken at the PCC. A new installation uses the projected demand of its first year.
What do I do if the plant has been in service for less than twelve months?
The definition allows averaging the months available: take the maximum demand of each month on record and divide by the number of months. For a new installation, IL is based on the projected monthly maximum demand of the year following start-up.
Why does a weak network get stricter current limits?
Because with low short-circuit current the network impedance is high, and the same harmonic current produces more voltage distortion. So the lower Isc/IL is, the lower the permitted harmonic current percentages.