---
title: "Transformer Impedance (%Z): How to Specify It | Entogo"
description: "Transformer percent impedance (%Z) sets how much fault current the secondary can deliver and how far voltage sags under load. Lower %Z means higher…"
url: https://entogo.ca/insights/transformer-impedance-percent-z-selection/
lang: en
type: article
image: https://entogo.ca/_astro/transformer-impedance-percent-z-selection.CNeq_Jkf.jpg
datePublished: 2026-08-19
dateModified: 2026-08-19
site: https://entogo.ca/
llms: https://entogo.ca/llms.txt
---

Home › Insights › What transformer impedance should you specify? How %Z shapes fault current and voltage drop

Power & Distribution

# What transformer impedance should you specify? How %Z shapes fault current and voltage drop

Entogo August 19, 2026

![Power transformer at an electrical substation, where nameplate percent impedance sets fault current and voltage regulation](https://entogo.ca/_astro/transformer-impedance-percent-z-selection.CNeq_Jkf_24aj2F.webp)

In short

Transformer percent impedance (%Z) sets how much fault current the secondary can deliver and how far voltage sags under load. Lower %Z means higher available fault current and better regulation; higher %Z limits fault duty but drops more voltage. Here is how to choose it and what to specify.

## Why one nameplate number drives so many downstream decisions

Percent impedance — the **%Z** stamped on a transformer nameplate — is often treated as a formality, yet it quietly sets the boundaries of the system around the transformer. It determines how much fault current the secondary can deliver into a bolted short, how far the output voltage sags between no load and full load, how two units share load when paralleled, and what short-circuit rating the switchgear and breakers downstream must carry. Choosing it deliberately is a design decision; accepting whatever comes back on a quote is not.

## What percent impedance actually measures

**Impedance voltage** is measured by shorting the secondary and raising the primary voltage until rated current flows. The voltage required, expressed as a percentage of rated primary voltage, is the transformer”s %Z. A 6% transformer needs 6% of rated voltage to push full-load current through a shorted secondary; the internal opposition that number represents is what limits current when a real fault occurs. Because it is a per-unit quantity referred to the transformer”s own rating, %Z carries cleanly into short-circuit and load-flow studies without unit conversions.

## How impedance sets available fault current

The relationship every specifier should know is inverse and direct. Ignoring source impedance — the conservative **infinite-bus** assumption — the secondary short-circuit current is approximately full-load current multiplied by 100 divided by %Z. A 2% transformer therefore delivers roughly three times the bolted-fault current of a 6% unit at the same rating — the multiplier 100 ÷ %Z works out to about 50 times full-load current versus about 17 times. Short-circuit current is inversely proportional to impedance, which means %Z is not only a transformer parameter — it dictates the **interrupting rating (AIC)** of the breakers and the **short-circuit current rating (SCCR)** of the panels, [metal-clad switchgear](https://entogo.ca/products/metal-clad-switchgear/), and busway fed from the secondary. Selecting and rating that downstream gear against the calculated fault current is a separate exercise; the point here is that the transformer”s %Z sets the number those ratings must clear. Under-specify %Z and the fault duty can outrun affordable downstream gear; the infinite-bus estimate is intentionally worst-case, because it ignores the utility and cable impedance that would reduce the real number.

## The trade-off you are actually buying

Lower impedance is not free. The same internal impedance that limits fault current also drives the **voltage regulation** drop between no load and full load. A low-%Z transformer holds output voltage tightly and starts motors with less dip, but hands a large fault current to the switchboard. A high-%Z transformer softens fault duty and can ease coordination, but drops more voltage under load and may need a wider **tap range** or an on-load tap changer to hold setpoint.

- Lower %Z: higher fault current, better voltage regulation, lower motor-start dip
- Higher %Z: lower fault current and downstream SCCR, poorer regulation, more voltage support needed
- The right value balances protective-device ratings against voltage quality for the specific load

## Standard values, tolerance, and when to depart from them

Product standards under **ANSI/IEEE C57.12** publish preferred impedance values by rating, and most transformers ship near them:

| Transformer size | Typical %Z range |
| --- | --- |
| Up to ~150 kVA | about 2.0–4.5% |
| ~150–300 kVA | about 4.0–5.0% |
| ~300–600 kVA | about 5.0–5.75% |
| ~600–2,500 kVA | about 5.75–6.5% |
| Above 2,500 kVA | about 6.0–7.0% |

These ranges are governed by **IEEE C57.12.00 and C57.12.01**, which also fix a manufacturing tolerance — commonly about plus or minus 7.5% for two-winding units, and about plus or minus 10% for three-winding or autotransformer types — so a nameplate 5.75% may test anywhere in a narrow band. Fault studies should use the low end of tolerance for maximum current and the high end for minimum. A non-standard %Z can be specified when the application demands it, but it becomes a design input the factory engineers to, not a value pulled from a catalog. Distribution units such as a [three-phase pad-mounted transformer](https://entogo.ca/products/three-phase-pad-mounted-transformer/) or a [dry-type distribution transformer](https://entogo.ca/products/three-phase-dry-type-distribution-transformer/) sit toward the lower kVA rows; a [36 kV oil-immersed power transformer](https://entogo.ca/products/oil-immersed-power-transformer-36kv/) or a [compact secondary unit substation](https://entogo.ca/products/compact-secondary-unit-substation/) carries the higher impedances typical of larger ratings.

## Paralleling and system coordination

When two transformers feed a common bus, load divides in inverse proportion to their impedances, so the lower-%Z unit takes more than its share and can overload before the other is full. Dependable paralleling requires matched rated voltage, the same **vector group** and phase displacement, and impedances held close — within tolerance, not merely within the same catalog line. The **X/R ratio** matters as well: it governs how the fault contribution splits during a short circuit, which relay coordination depends on. These constraints shape substation and distribution layout as much as the ratings themselves.

## Where a deliberate impedance choice pays off

- **Data centers** and other fault-sensitive sites: a slightly higher %Z can keep downstream AIC and SCCR within reach, trading a little regulation for coordinated, economical gear
- Long secondary feeders or voltage-sensitive process loads: lower %Z preserves voltage quality
- Large motor starting: lower %Z reduces starting voltage dip
- Paralleled or redundant banks: impedances matched on purpose so load and fault current split predictably

## What a buyer should specify

- Rated %Z and the acceptable tolerance, referencing **ANSI/IEEE C57.12.00** as the governing standard
- The downstream SCCR and AIC the secondary fault current must not exceed
- The voltage-regulation target and tap range (or on-load tap changer) needed to hold it at full load
- Vector group and any paralleling requirement, so units are electrically compatible
- Whether a non-standard impedance is required, stated as a design value the manufacturer builds to

Because %Z is engineered into the winding geometry, it is set on the factory floor rather than adjusted in the field — which makes a manufacturer”s ability to build to a specified value part of the specification itself. Entogo”s vertically integrated factory builds transformers and [substation and power-distribution](https://entogo.ca/solutions/substations-power-distribution/) packages to a stated impedance, designed and built to ANSI/IEEE C57; UL (cULus) or CSA certifiable on request, and supports the fault and coordination studies behind the number for [industrial EPC](https://entogo.ca/solutions/industrial-epc/) and [data center](https://entogo.ca/solutions/data-centers/) projects. Teams weighing a value can start from a [transformer quote](https://entogo.ca/products/transformer-quote/) or reach the engineering group through [contact](https://entogo.ca/contact/).

- Transformers
- Power & Distribution
- Fault Current
- Short-Circuit Rating
- Voltage Regulation

Glossary: [Percent impedance](https://entogo.ca/glossary/#percent-impedance) [Interrupting rating vs. short-circuit current rating](https://entogo.ca/glossary/#interrupting-rating-sccr) [Metal-clad switchgear](https://entogo.ca/glossary/#metal-clad-switchgear) [Switchboard](https://entogo.ca/glossary/#switchboard-ul-891) [Taps](https://entogo.ca/glossary/#taps) [Pad-mounted transformer](https://entogo.ca/glossary/#pad-mounted-transformer)

FAQ

## Common questions

- **What is transformer percent impedance**: Percent impedance is the voltage needed to drive full-load current through the transformer with its secondary shorted, expressed as a percent of rated voltage. It fixes how much fault current the secondary can deliver and how far voltage drops under load.
- **Does lower impedance mean higher fault current**: Yes. Available short-circuit current is roughly full-load current times 100 divided by percent impedance, so a 2 percent unit delivers far more fault current than a 6 percent unit at the same rating. Downstream gear must carry a matching short-circuit rating.
- **What is a typical transformer impedance value**: Small distribution units often run about 2 to 4.5 percent, mid-size units near 5 to 5.75 percent, and larger units 5.75 percent and up. IEEE C57.12 governs standard values and a manufacturing tolerance of about plus or minus 7.5 percent for two-winding transformers.
- **Can you parallel two transformers with different impedance**: You can, but load divides in inverse proportion to each unit''s impedance, so mismatched %Z overloads the lower-impedance unit before the other is full. Match rated voltage, vector group, and impedance closely, and keep X over R ratios similar.
- **How does impedance affect voltage regulation**: Higher impedance drops more voltage between no load and full load, so a high-%Z transformer needs more tap range or voltage support to hold output. Lower impedance improves regulation but raises fault current, which is the core trade-off to specify.

Keep reading

## Related insights

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Project inquiry

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_Markdown twin of https://entogo.ca/insights/transformer-impedance-percent-z-selection/ — generated at build time from the same content as the HTML page. Company facts, catalogue index and contact: https://entogo.ca/llms.txt_