Transformer Inductance
Calculator
Calculate transformer leakage reactance and inductance from kVA, voltage, and % impedance. Also computes rated current and available short-circuit current.
Leakage Reactance & Fault Current
The base impedance relates per-unit values to ohms. For any transformer: Z_base = V² / (kVA × 1000). This normalises the transformer's rated voltage and capacity into a reference impedance from which all actual values are derived.
%Z (percentage impedance) is the fraction of rated voltage needed to circulate rated current through the short-circuited transformer. Leakage reactance XL = (%Z/100) × Z_base in ohms. Most of the transformer impedance is inductive, so %Z ≈ %XL for design purposes.
Inductance L = XL / (2π × f). For a 500 kVA, 480 V transformer at 5% impedance, this is approximately 2.9 mH. This value is used in filter design, drive input reactor sizing, and harmonic current calculations.
The maximum available fault current at the transformer secondary is: Isc = I_rated / (%Z/100). For a 500 kVA / 480 V unit at 5% impedance, rated current ≈ 601 A and Isc ≈ 12,000 A. Protective devices must have an interrupting rating exceeding this value.
Leakage inductance (also called leakage reactance when expressed in ohms) represents magnetic flux that links only one winding and does not transfer energy to the other. It acts as a series impedance that limits fault current, causes voltage regulation under load, and determines the transformer's short-circuit impedance (%Z).
%Z determines the maximum available fault current at the transformer's secondary terminals. Lower %Z means higher fault current, requiring protective devices (breakers, fuses) with higher interrupting ratings. Higher %Z limits fault current but increases voltage regulation at load. Typical values: 2–4% for distribution, 5–7% for power transformers.
Leakage reactance XL = (Z%/100) × (V²/kVA×1000) in ohms. Inductance L = XL / (2π × f) in henries. These values are used in harmonic filter design, reactor sizing, cable impedance calculations, and short-circuit studies per IEC 60909 or ANSI C37.
Enter the secondary line-to-line voltage (e.g. 480 V, 600 V, 4160 V). The base impedance formula V²/(kVA×1000) uses line-to-line voltage for three-phase systems and gives per-phase results on a per-unit base. The short-circuit current uses line current (= kVA/(√3×V)).
Need %Z or Loss Data for a Specific Unit?
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