Transformer K-Factor From the Harmonic Spectrum (UL 1561)

K = sum(Ih^2 x h^2) / sum(Ih^2) with the harmonics entered as per-unit of the fundamental (UL 1561 / IEEE C57.110), then rounds up to the next standard K-rating (K-1 standard, then K-4 / K-9 / K-13 / K-20 / K-30 / K-40). The measured spectrum and the manufacturer govern.

K = sum(Ih^2 x h^2) / sum(Ih^2) over h = 1,3,5,7,9,11,13 with harmonics in per-unit of the fundamental; round up to the next standard K-rating (K-1, K-4, K-9, K-13, K-20, K-30, K-40).

Transformer K-factor for nonlinear loads, UL 1561 and IEEE C57.110, by name.

The K-factor weights each harmonic by the square of its order; a near-linear load (K close to 1) needs no K-rated transformer. The measured spectrum and the manufacturer govern the final selection.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: i1, i3, i5, i7, i9, i11, i13, k_factor, recommended_k_rating

• K-factor definition sum of (per-unit harmonic current squared times harmonic order squared) divided by the sum of (per-unit harmonic current squared)UL 1561 / IEEE C57.110
• Standard K-ratings the tabulated ladder K-1 (standard) / K-4 / K-9 / K-13 / K-20 / K-30 / K-40UL 1561