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. |
| NOTES 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. |
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Equation 1
Test duration for Category I transformers
where:
t = test duration (s)
I = symmetrical short-circuit current in multiples of normal base current(
)