Calculating maximum demand load current

In IEEE 519, the maximum demand load current (IL) represents your site’s long‑term peak demand at the point of common coupling (PCC).

IEEE 519 uses IL with the short‑circuit current (Isc) to calculate the short circuit current ratio (Isc/IL). This ratio, and the defined bus voltage at the PCC, are used to find your allowable current‑distortion limits at the PCC.

How you calculate IL depends on the IEEE 519 edition that your site follows.

Equation

Where:

  • N is the number of months included (typically 12).

  • Im is the current corresponding to the monthly peak demand for month m (15- or 30-minute demand interval) measured at the PCC.

IEEE 519‑2014: Calculate IL​ from 12 months of demand history

IL is the average of the currents associated with each month’s peak demand (15- or 30-minute window) over the previous 12 months, measured at the PCC.

Prerequisites

  • PCC voltage (for converting demand in kVA to current)

  • Monthly peak demand (kVA) for each of the last 12 months, measured at the PCC

Steps

To calculate the maximum demand load current for IEEE 519-2014:

  1. Record the monthly peak demand at the PCC for the last 12 calendar months (15- or 30-minute demand interval).

  2. Convert each month’s peak demand to current.

  3. Calculate the average of the 12 currents to get IL.

IEEE 519-2022: Calculate IL​ with full, partial, or projected history

The 2022 edition keeps the same monthly‑average concept, adds clarity for limited history, and prescribes projected demand for new sites.

Prerequisites

  • PCC voltage (for converting demand in kVA to current)

  • One of the following:

    • Monthly peak demand (kVA) for the last 12 months, measured at the PCC, or

    • Monthly peak demand (kVA) for the available months (fewer than 12), measured at the PCC, or

    • Projected monthly peak demand (kVA) for each month of the first year after commissioning (new installation)

Steps

To calculate the maximum demand load current for IEEE 519-2022:

  1. If you have 12 months of data:

    1. Convert each month’s peak demand to current.

    2. Calculate the average of the 12 currents.

  2. If you have fewer than 12 months of data:

    1. Convert each available month’s peak demand to current.

    2. Calculate the average of the available currents.

  3. If you have no measured history (new installation):

    1. Define projected monthly peak demand for the first year.

    2. Convert each month's peak demand to current.

    3. Calculate the average of the 12 currents.

    4. Recalculate IL when measured data becomes available.