Total Demand Distortion Limit Check (IEEE 519-2022)
The distortion limit that loosens as the supply gets stiffer, the number a utility actually holds you to at the PCC.
Example
You enter
- Short-circuit current at the PCC (A) 10000
- Maximum demand load current (A) 400
- Measured TDD (%) 6
You get
- Short-circuit ratio Isc/IL 25
- Applicable TDD limit 8%
Checked against published worked examples from: Elspec.
Details, formula, and sources
ScopeIEEE 519 limits total demand distortion (harmonic current over the maximum DEMAND load), not THD or a flat 5%. ratio = Isc/IL sets the limit: < 20 -> 5%, 20-50 -> 8%, 50-100 -> 12%, 100-1000 -> 15%, > 1000 -> 20%. Isc 10,000 A / IL 400 A is a ratio of 25 -> 8% limit, so a measured 6% TDD PASSES; on a soft service (Isc 6,000 A -> ratio 15) the limit tightens to 5% and the identical 6% FAILS. The distortion did not change; the supply stiffness did. Even/individual sub-limits also apply. A screening aid; the utility agreement and a measurement study govern.
Formula or tableratio = Isc / IL; limit = ratio < 20 ? 5.0 : ratio < 50 ? 8.0 : ratio < 100 ? 12.0 : ratio <= 1000 ? 15.0 : 20.0 (%); pass = measured_TDD <= limit.
Source and editionThe IEEE 519-2022 Table 2 current-distortion limits (total demand distortion at the point of common coupling), by name; the utility agreement and a measurement study govern.
Free accessThe IEEE 519 TDD limit bands by Isc/IL ratio are published in the standard; the short-circuit and demand currents come from the facility's service data.
What governsEstimate. AHJ and licensed professional govern.
API field namesisc_a, il_a, measured_tdd_pct, ratio, limit_pct
- TDD not THD the limit is on total demand distortion (over the maximum demand load), not THDIEEE 519-2022
- Ratio-dependent the limit loosens as Isc/IL rises because a stiffer supply absorbs more harmonic currentIEEE 519-2022 Table 2
- Sub-limits individual-harmonic and even-harmonic sub-limits also apply and are not checked hereIEEE 519-2022