Transformer impedance is the total internal opposition a transformer presents to current flow, made up of the resistance in its copper windings and the reactance caused by magnetic flux that leaks outside the core. It’s typically expressed as a percentage of the transformer’s rated voltage, and it plays a central role in determining how much fault current a transformer will let through, how well it maintains voltage under load, and whether two transformers can safely operate side by side.
What Makes Up Transformer Impedance
Two physical properties combine to create transformer impedance. The first is winding resistance, which comes from the copper (or aluminum) wire in the primary and secondary coils. This is straightforward electrical resistance, and in most power transformers it’s relatively small compared to the second component.
That second component is leakage reactance, and it’s usually the dominant part of the impedance. Not all magnetic flux produced by one winding successfully links to the other winding through the iron core. Some flux “leaks” into the air or structural parts around the windings. This leakage flux creates an opposition to alternating current that behaves differently from simple resistance. It depends on the physical dimensions of the windings, the number of turns, and the geometrical arrangement of the coils relative to each other. Because resistance is comparatively small in most transformers, impedance values are often treated as roughly equal to leakage reactance alone for practical calculations.
How Percentage Impedance Works
Transformer impedance is almost always stated as a percentage rather than in ohms, because a percentage value lets you instantly compare transformers of different sizes and voltage ratings. A typical distribution transformer might have an impedance of 4% to 6%, while large power transformers can range higher.
The percentage is determined through a short-circuit test. You short-circuit the secondary winding (connect its terminals together), then gradually increase the voltage applied to the primary side until rated current flows through the secondary. The voltage you had to apply, expressed as a percentage of the transformer’s rated primary voltage, is the percent impedance. If a transformer rated at 1,000 V primary needs 50 V applied to push rated current through a short-circuited secondary, its impedance is 5%.
This test also reveals real power losses in the windings, since any watts consumed during the short-circuit test come almost entirely from resistive heating in the copper. The combination of voltage, current, and power measurements from this single test gives engineers everything they need to model the transformer’s internal losses and voltage drops.
Limiting Fault Current
The most critical job of transformer impedance is controlling how much current flows during a short circuit on the secondary side. When a fault occurs downstream, the transformer’s internal impedance is often the only thing standing between the power source and a dangerously high current surge.
The relationship is inversely proportional. A transformer with 5% impedance will allow a maximum fault current of about 20 times its rated full-load current (100 ÷ 5 = 20). Drop that impedance to 2.5%, and the available fault current doubles to 40 times rated current. The actual calculation used by engineers accounts for the resistance and reactance components separately, combining them as the square root of the sum of their squares to find total impedance. But the percentage shortcut gives a fast, reliable estimate.
This matters enormously for selecting circuit breakers, fuses, and bus bars downstream of a transformer. Every piece of protective equipment must be rated to handle the maximum fault current the transformer can deliver, and that number flows directly from the impedance value on the nameplate.
Effect on Voltage Regulation
Impedance creates an internal voltage drop every time current flows through a transformer. Under no load, the secondary voltage sits right at its rated value. As you add load, current increases, and voltage drops across the internal impedance pull the output voltage down. The higher the impedance, the more the output voltage sags under load.
This is the fundamental tradeoff in transformer design. A higher impedance does a better job of limiting fault currents, which simplifies protection and improves safety. But that same higher impedance causes larger voltage swings between no-load and full-load conditions, making voltage regulation worse. Transformers with lower impedance maintain more stable output voltage as load changes, but they let through larger fault currents that downstream equipment must be built to handle.
For applications where voltage stability is critical, such as sensitive electronics or precision manufacturing, designers lean toward lower impedance values. For installations where fault current levels are already pushing the limits of available protective equipment, a higher impedance transformer may be the practical choice even at the cost of some regulation performance.
Why Impedance Matters for Parallel Operation
When two or more transformers are connected in parallel to share a load, their impedance values need to match. If they don’t, the transformers won’t divide the load proportionally to their ratings. The lower-impedance unit will grab more than its fair share of the current, potentially overloading while the higher-impedance unit runs underloaded.
Worse, mismatched impedances cause circulating currents between the transformers even when no external load is connected. These currents flow in a loop through both sets of windings, generating heat and wasting capacity without doing any useful work. The general guideline is that circulating current from any combination of mismatched ratios and impedances should not exceed 10% of the full-load rated current of the smaller transformer. In practice, this means the per-unit (percentage) impedance values of paralleled transformers should be as close to identical as possible. Transformers with similar per-unit impedances will naturally share load in proportion to their kVA ratings, which is exactly the behavior you want.
Typical Impedance Ranges
Impedance values aren’t arbitrary. They follow patterns based on transformer size and application:
- Small distribution transformers (up to about 500 kVA): typically 3% to 5.75%. These lower values prioritize voltage regulation for residential and commercial loads.
- Medium power transformers (500 kVA to several MVA): typically 5% to 8%. The higher values help manage the larger fault currents these units can deliver.
- Large power transformers (tens of MVA and above): can reach 10% or higher. At these power levels, limiting fault current becomes essential because the available short-circuit current would otherwise exceed what switchgear can safely interrupt.
These ranges exist because impedance is partly a consequence of physical construction. Larger transformers have bigger gaps between windings for insulation, which increases leakage flux and pushes reactance (and therefore impedance) higher. Designers can adjust impedance within limits by changing winding geometry, but the physics of insulation and cooling set boundaries.
Reading the Nameplate
Every power transformer has its impedance printed on the nameplate, usually labeled as “%Z” or “%IZ.” This value is always referenced to a specific kVA rating and temperature. If a transformer has multiple cooling ratings (for example, different kVA ratings with fans on versus fans off), the impedance may be listed for each rating.
When you see a nameplate reading of, say, 5.75% impedance on a 1,000 kVA transformer with a 480 V secondary, you immediately know three things. The output voltage will drop by roughly 5.75% between no load and full load (somewhat less in practice, depending on the power factor of the load). The maximum fault current available at the secondary terminals is about 100 ÷ 5.75 = 17.4 times the full-load current. And any transformer you want to parallel with this one should also have an impedance very close to 5.75% at its own rated kVA.