IEEE Transformers Categories

There are four (4) categories of transformers according to IEEE Std. C57.12.00 - IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.

Categories of Transformer Rating

Category Single Phase (kVA) Three Phase (kVA) Short Circuit Test Duration
Inote-1 5 to 500 15 to 500 Refer to Equation (1)
II 501 to 1,667 501 to 5,000 1.0 second
III 1,668 to 10,000 5,001 to 30,000 0.5 second
IV above 10,1000 above 30,000 There is no long duration test for category IV transformers.
1. Category I shall include distribution transformers manufactured in accordance with ANSI C57.12.20 [B3] up through 500 kVA, single phase or three phase. In addition, autotransformers with equivalent two-winding kVA of 500 or less, which are manufactured as distribution transformers in accordance with ANSI C57.12.20 [B3], shall be included in Category I, even though their nameplate kVA may exceed 500.
2. All kVA ratings listed are minimum nameplate kVA for the principal windings.

Equation 1

Test duration for Category I transformers

t = test duration (s)
I = symmetrical short-circuit current in multiples of normal base current(I=I_sc/I_rated)