GuideCalculators and sizing8 min read
Transformer impedance, explained
A transformer’s impedance, written as a percentage (%Z) on the nameplate, is the share of rated voltage that has to be applied to the primary to push rated current through a shorted secondary. A 5.75% impedance means 5.75% of 12,470 V, about 717 V, drives full-load current into a dead short. It sets three things a buyer cares about: the fault current the transformer lets through (full-load amps × 100 ÷ %Z), how much the voltage sags under load, and whether the unit can be paralleled with another.
The short version
- Fault current at the secondary, with an infinite source: full-load amps × (100 ÷ %Z). A 1,000 kVA, 480 V, 5.75% unit: 1,203 A × 17.4 = 20,900 A.
- Lower impedance: stiffer voltage, more fault current. Higher impedance: less fault current, more voltage drop and bigger dips when motors start.
- Typical values: 2 to 4% below 150 kVA, 4 to 5% to 500 kVA, 5.75% for 750 to 2,500 kVA pad-mounts, 6 to 8% on substation transformers.
- Transformers in parallel should have impedances within about 7.5% of each other (5.5% and 5.9%, say), the same ratio and taps, and the same vector group; load divides in inverse proportion to impedance.
- The nameplate %Z is measured at a reference temperature and on the rated tap; it’s a little different on other taps.
What percent impedance means
Short the secondary of a transformer, then raise the primary voltage from zero until rated current flows. The voltage you reach, as a percentage of the rated primary voltage, is the impedance. On a 1,000 kVA, 12,470 V unit at 5.75%, that’s 717 V. The test is done in the factory (IEEE C57.12.90) and the result goes on the nameplate, with the temperature it was corrected to.
Physically it’s the leakage reactance between the windings, plus the small resistance of the copper or aluminum. The reactance is a design choice: more space between the windings, more leakage flux, higher impedance. Makers set it to a standard value for each size, and buyers can order it higher or lower within IEEE C57.12.00’s tolerance of ±7.5% of the specified figure.
%Z = (voltage that drives rated current into a short ÷ rated voltage) × 100
Fault-current calculator
The rating, secondary voltage and impedance give the fault current at the terminals with an infinite source behind the transformer, the figure used to rate the switchgear.
20,918A, available fault current
1,203 A full load × (100 ÷ 5.75) = 20,918 A symmetrical at the secondary terminals, with an infinite source; about 20.9 kA, so the switchgear needs a rating above that.
Fault current, and the switchgear rating
If 5.75% of rated voltage drives rated current, then 100% of rated voltage, which is what a bolted fault on the secondary sees, drives 100 ÷ 5.75 = 17.4 times rated current. That’s the transformer’s contribution to a secondary fault, and because the transformer is usually the largest impedance between the utility and the fault, it’s most of the answer.
Fault current ≈ full-load amps × 100 ÷ %Z (infinite source)
| kVA | Typical %Z | Full-load amps | Fault current | Switchgear rating needed |
|---|---|---|---|---|
| 150 | 3.5% | 180 | 5,200 A | 22 kA |
| 300 | 4.5% | 361 | 8,000 A | 22 kA |
| 500 | 5% | 601 | 12,000 A | 22 kA |
| 750 | 5.75% | 902 | 15,700 A | 22 kA |
| 1,000 | 5.75% | 1,203 | 20,900 A | 22 kA |
| 1,500 | 5.75% | 1,804 | 31,400 A | 42 kA |
| 2,000 | 5.75% | 2,406 | 41,800 A | 42 kA |
| 2,500 | 5.75% | 3,007 | 52,300 A | 65 kA |
| 3,750 | 6% | 4,511 | 75,200 A | 100 kA |
With the utility’s source impedance included, the figures fall by 10 to 30%; with motors on the bus contributing, they rise. A fault study (IEEE 141) does it properly; this table says which rating to expect.
The pattern matters when buying: a 2,500 kVA unit at 480 V delivers about 52 kA, which needs 65 kA switchgear; a 2,000 kVA unit about 42 kA, within a 42 kA rating; a 1,500 kVA about 31 kA. Going one size up in transformer can mean going one rating up in switchgear, and a listing that gives a higher-than-standard impedance (7%, say, on a 2,500 kVA) may be the better buy for an existing 42 kA lineup.
Typical impedances by size
| Transformer | Typical %Z |
|---|---|
| Single-phase pole-mount or pad-mount, 10 to 167 kVA | 1.5 to 3% |
| Three-phase, 45 to 150 kVA | 2 to 4% |
| Three-phase, 225 to 500 kVA | 4 to 5% |
| Three-phase pad-mount, 750 to 2,500 kVA | 5.75% (ANSI standard) |
| Three-phase, 3,000 to 5,000 kVA | 6 to 7% |
| Substation and power transformers, 5 to 30 MVA | 6.5 to 10% |
| Dry-type, 600 V class, 15 to 500 kVA | 3 to 6% |
| Dry-type, medium-voltage cast coil, 500 kVA to 3 MVA | 5 to 8% |
Ranges from manufacturers’ catalogs and IEEE C57.12.34; a given unit’s nameplate is the figure to use.
Small transformers have low impedance because their windings are close together; large ones are designed with more, on purpose, to keep fault currents within what switchgear and cables can take. A transformer ordered with a non-standard impedance, higher to limit fault current or lower to stiffen a motor bus, says so on the nameplate and usually in the listing.
Voltage regulation and motor starting
The same impedance that limits fault current drops voltage under load. Regulation, the rise in secondary voltage from full load to no load, depends on the impedance and the load’s power factor: at unity power factor only the resistive part matters and regulation is around 1%; at 0.8 lagging the reactance counts and a 5.75% transformer regulates at roughly 4%. A transformer feeding motors runs its secondary a few percent low at full load, which the taps on the primary are there to correct (the tap changer guide).
Motor starting is the same effect in a burst. A motor started across the line draws about six times running current, and the dip at the bus is close to %Z times the ratio of the motor’s starting kVA to the transformer’s kVA: a 600 kVA start on a 1,000 kVA, 5.75% unit dips the bus 3.5%; on a 300 kVA unit, 11.5%. The sizing guide has a table of the transformer needed to keep the dip under 10% for each motor size.
Paralleling transformers
Two transformers on one secondary bus share the load in inverse proportion to their impedances (adjusted for kVA). If the impedances match, each carries its share of the total; if they don’t, the lower-impedance unit carries more than its share and reaches full load first, while the other still has capacity. The rule that keeps the mismatch harmless:
- Impedance
- Within 7.5% of each other, as a proportion: 5.5% and 5.9% parallel well; 4.5% and 5.75% don’t.
- Ratio and taps
- The same voltage ratio, set to the same tap. A tap difference circulates current between the units even at no load.
- Vector group
- The same angular displacement: two delta-wyes, or two wye-wyes, never one of each (the delta-wye guide).
- kVA
- Any sizes, but the rule of thumb is within 3:1 of each other, since the impedance is a percentage of each unit’s own rating and the division of load follows the rating.
- Polarity and phasing
- Checked with a TTR set and a phasing stick before the tie is closed.
The practical case is a double-ended substation, two transformers with a tie breaker, run with the tie open and closed only during switching. Momentary paralleling needs the same conditions; the circulating current from a mismatch flows for the seconds the tie is closed, and the breakers are rated for it.
Reading and checking the figure
The nameplate gives the impedance as “%Z 5.75 at 85 °C” (liquid-filled; dry-types reference 75, 100 or 120 °C) on the rated tap, and sometimes on every tap. On a used unit the figure can be confirmed with a short-circuit impedance test, which is part of a full diagnostic set and a good idea on any unit that will be paralleled: a change from the nameplate value means a winding has moved, usually from a through-fault, and that unit shouldn’t go on a bus with another until it’s understood. The testing guide covers the rest of the set.
Questions people ask
What does transformer impedance mean?
The percentage of rated primary voltage needed to circulate rated current with the secondary short-circuited. It’s measured in the factory (IEEE C57.12.90), printed on the nameplate as %Z or %IZ at a reference temperature, and it describes how much the transformer resists a change in current. Nearly all of it is reactance from leakage flux between the windings; the resistive part is under 1% on a distribution transformer.
How do you calculate fault current from transformer impedance?
Divide the secondary full-load current by the impedance as a decimal. A 1,500 kVA transformer at 480 V has 1,804 A full load; at 5.75% impedance the fault current at its terminals is 1,804 ÷ 0.0575 = 31,400 A, assuming an infinite source. The utility’s own impedance lowers it: with 100 MVA available on the primary, the same transformer delivers about 31,400 × 100 ÷ (100 + 1.5 ÷ 0.0575) = 24,900 A. The switchgear must be rated above the result.
Is high or low impedance better in a transformer?
Neither; it’s a trade. Low impedance (3 to 4%) gives better voltage regulation and lets motors start with less dip, but lets through more fault current, which can push the switchgear to a higher rating. High impedance (6 to 8%) limits fault current and lets cheaper gear be used, at the cost of more voltage drop. Standard values, 5.75% for a pad-mount of 750 kVA and above, are the compromise the industry settled on.
What impedance do you need to parallel two transformers?
Within about 7.5% of each other, as a proportion (5.5% and 5.9% are fine; 4% and 6% are not), along with the same voltage ratio, the same tap positions and the same vector group. Load divides in inverse proportion to impedance, so the lower-impedance unit takes more than its share; the 7.5% rule keeps the mismatch small enough that neither unit overloads before the pair reaches its combined rating.
What is a typical transformer impedance?
About 2 to 4% for single-phase and small three-phase units under 150 kVA, 4 to 5% from 225 to 500 kVA, 5.75% for 750 to 2,500 kVA three-phase pad-mounts (the ANSI standard figure), 6 to 7% at 3,000 to 5,000 kVA, and 6.5 to 10% on substation and power transformers, where limiting fault current matters more.
Does the impedance change on different taps?
A little. The impedance on the nameplate is measured on the rated-voltage tap; on the extreme taps it shifts by a few percent of its value, which matters for a fault study only at the margins. Some nameplates list the impedance on each tap.
Sources: IEEE C57.12.00 (impedance tolerances and standard values); IEEE C57.12.90 (impedance test); IEEE 141 (the Red Book: short-circuit calculations); IEEE C57.12.34 (pad-mount transformers).