The answer first: for a small voltage change an autotransformer needs roughly half the copper — but it gives you no galvanic isolation
An autotransformer (often written “auto transformer”) is a transformer built on one continuous winding instead of two. A tap divides that winding into a common section shared by input and output and a series section carrying the difference — and that shared conductor is exactly why it is smaller, lighter, cheaper and more efficient than a two-winding isolation transformer of the same throughput. The catch is structural: because input and output share a conductor, there is no electrical (galvanic) isolation between them. The money question — whether the saving is worth losing isolation — is decided by the voltage ratio. The size, weight, copper and loss advantage scales as (1 − 1/a), where a is the transformation ratio: for a 2:1 change (400 V to 230 V) you save about 50%; for a 5:1 change it falls to ~20%; and by 10:1 the saving is down to ~10%, at which point the isolation argument wins. Here is the full trade-off, the sizing rule and what to put on the order (IEC 60076-1 / IEEE C57.12.10 / GB 1094).
What an autotransformer actually is
A standard (isolation) transformer has two separate windings on a common core: primary and secondary. Energy transfers from one to the other entirely by magnetic induction, and the windings touch only through the magnetic field — hence “galvanic isolation”. An autotransformer removes the second winding: it has one winding with a tap, and the load is connected across a portion of it.
- Common section — the part of the winding shared by the supply and the load; it carries only the difference current.
- Series section — the remainder of the winding; it carries the full load current.
Only a fraction of the total power is transferred by induction; the rest flows through by direct conduction. That conduction path is why an autotransformer of a given rating is physically smaller than its two-winding equivalent.
Why it is smaller: the (1 − 1/a) copper rule
The savings come down to a single ratio. Call the voltage ratio a (primary ÷ secondary, where a > 1). The core and windings only have to carry the “equivalent kVA”, not the full throughput:
Equivalent kVA = Throughput kVA × (1 − 1/a)
Worked example: a 100 kVA autotransformer changing 400 V to 230 V has a = 1.74, so 1 − 1/a = 0.425. The unit is built as if it were a ~42.5 kVA transformer, not 100 kVA — about 42% of the copper and iron. Copper loss and no-load loss shrink in roughly the same proportion, so the autotransformer is not only cheaper to buy but cheaper to run. At a = 2 the advantage is 50%; at a = 5 it is 20%; at a = 10 it is only 10% — and a large ratio also means a large voltage stress between the series winding and earth, which eats into the saving.
The trade-off: no galvanic isolation
The saving has a real cost, and it is a safety one:
- A fault on one side appears on the other. A surge, short circuit or earth fault on the supply is conducted straight to the load — there is no magnetic barrier to attenuate it.
- Open common-section failure. If the common part of the winding breaks, the full primary voltage can appear across the output — a dangerous failure mode a two-winding transformer does not have.
- No ground-loop or noise isolation. An autotransformer cannot break a ground loop or block common-mode noise, which is why control, instrumentation, medical, marine and IT supplies use isolation transformers.
Standards reflect this: IEC 60076-1 covers power transformers including autotransformers and gives their auto-connection symbols (e.g. YNa0d11 for a transmission autotransformer with a delta tertiary), while IEEE C57.12.10 is the dedicated US standard for autotransformer requirements.
When an autotransformer is the right choice
- Interconnecting networks of similar voltage — e.g. 220 kV ↔ 400 kV transmission autotransformers, or a 6.6 kV ↔ 11 kV industrial tie. Ratios are small, so the savings are large.
- Voltage adaptation where isolation is not required — 400 V to 230 V, 208 V to 240 V, or adapting a motor or receptacle between close voltages.
- Reduced-voltage motor starting — autotransformer starters apply a tapped fraction of line voltage and reduce inrush current.
- Variable voltage for test and laboratory work — a variable transformer (variac) is a continuously adjustable autotransformer.
- Voltage regulation — automatic voltage regulators use autotransformer tap-changing to hold output within a tight band despite input swings.
When you need an isolation transformer instead
- Stepping down from medium or high voltage to a safe low voltage — a 10 kV to 400 V distribution transformer must be two-winding; galvanic separation between the MV network and the consumer is a code requirement, not a preference.
- Safety-critical loads — medical, marine, control and instrumentation circuits where isolation protects people and equipment.
- Breaking ground loops or common-mode noise — an isolation transformer is the standard fix.
- Large voltage ratios — beyond roughly 3:1 to 5:1 the saving is small and the isolation you give up matters more.
If you are buying a conventional distribution or industrial transformer rather than an interconnecting unit, a two-winding design is almost certainly what you want — see our oil-immersed vs dry-type selection guide to pick the insulation system first.
How to specify an autotransformer (what to put on the order)
- Rated throughput (kVA) — the full load you will transfer, not the smaller equivalent kVA.
- Primary and secondary voltages and frequency (50 or 60 Hz).
- Connection and ratio — the auto-connection symbol and the exact tap positions.
- Impedance (%Z) — determines short-circuit current and voltage regulation; specify a target, as it also affects parallel operation.
- Insulation class, cooling and temperature rise — e.g. ONAN cooling, Class A/F insulation, 65 °C or 75 °C rise.
- Applicable standard — IEC 60076-1, IEEE C57.12.10 or GB 1094, plus any purchaser’s country code.
Reading the nameplate works the same way as any transformer — the vector group and connection symbol tells you at a glance whether the unit is an auto- or isolation design.
Bottom line
Use the ratio test: if the voltage change is small (roughly 3:1 or less) and isolation is not required, an autotransformer is cheaper, lighter and more efficient. If you need galvanic separation or a large step-down, buy a two-winding isolation transformer. For a distribution or industrial transformer sized to your load, start with our engineering calculators to build a bill of materials and an FOB Qingdao estimate, or contact us with your voltages and kVA for a specification-matched quotation.