A transformer fits the job when its voltage, capacity, cooling, and insulation line up with the load and the site — not when the nameplate looks impressive. Start with the primary and secondary voltages, size the kVA to about 75–80% of the continuous load, then weigh oil against dry, short-circuit impedance, and the environment. Everything else is detail built on those five.

1. Voltage rating: match both windings to the system

The transformer's rated voltages have to match the system on the primary side and the equipment on the secondary. In North America that usually means a primary around 12.47 kV or 13.8 kV (occasionally 34.5 kV) stepping down to 480Y/277 V for three-phase plants, or 600Y/347 V in Canada; single-phase services commonly land at 120/240 V.  The source's example of 10 kV / 400 V is the common Chinese distribution pairing; the U.S. equivalents are the values above.

For the full liquid-filled and dry-type range we build, see the products page.

2. Capacity (kVA): size to the load, not the wish list

Capacity is set by the load's real and reactive power, not a round number. Undersize and you get voltage sag, overheating, and premature failure; oversize and you burn capital and run at poor efficiency. The working rule is to pick a unit loaded to roughly 75–80% of its rating under continuous full load, which leaves headroom for inrush and modest growth without running the tank hot. Add the reactive component (motors, ballasts, VFDs) — a load that looks like 100 kW at 0.8 pf draws 125 kVA, and that's the number that sizes the transformer.

3. Winding configuration: single-phase vs three-phase

Single-phase units serve small, single-phase-dominated loads — think residential taps and light commercial. Three-phase serves three-phase supplies and anything with real power draw. The source also mentions multi-winding transformers for systems needing several secondary voltages; those show up in converters, rectifiers, and traction supplies where one primary feeds two or more secondaries. Pick the configuration the load actually presents; forcing a three-phase unit onto a single-phase job just adds cost and a dead winding.

4. Core material: silicon steel vs amorphous alloy

Most cores are grain-oriented silicon steel — good permeability, proven, and cheap enough to be the default. Amorphous alloy cores cut no-load (iron) loss by roughly 40–50%, which pays back on lightly loaded or always-on units (rural feeds, night base load). The trade is higher material cost and a bit more noise. If the unit sits near bedrooms, classrooms, or a hospital, the quieter silicon-steel design with vibration isolation often beats amorphous on the metric that actually gets complained about. 

5. Cooling method: oil vs dry, and what ONAN/AN really mean

Cooling codes tell you how the unit sheds heat. Oil-immersed transformers run ONAN (oil natural, air natural) or ONAF (oil natural, air forced with fans); dry-types run AN (air natural) or AF (air forced). Oil cools better and scales to larger kVA, but you take on oil handling, containment, and more maintenance. Dry-types are cleaner and simpler to maintain and go where fire and explosion risk can't be tolerated — indoor commercial, hospitals, underground. The choice between oil and dry is usually made by the site, not the spec sheet; we lay the trade-offs out in our oil-immersed vs dry-type guide.

6. Short-circuit impedance: the trade-off you can't ignore

Impedance (the %Z on the nameplate) sets two things at once: a higher %Z limits the fault current the transformer feeds into a short, which eases switchgear duties, but it also lets the secondary voltage sag more under load. Distribution units typically sit around 4–6%; larger or special units run higher. [Confirm the value against the system's available fault current and the voltage-regulation budget — pick it with the utility or the protection engineer, not off a table.] A number that's wrong here either trips breakers you didn't expect or lets the bus sag under motor start.

7. Insulation level and temperature class

Insulation has to survive the system's transients and age gracefully in the field. That means a basic insulation level (BIL) matched to the system, plus a temperature-rise class the environment can live with — oil units commonly 55 °C or 65 °C rise, dry-types 80/100/125/150 °C classes. [Confirm the BIL and rise class with the local environment and the equipment it feeds.] Skimp here and you trade a few percent of cost for a shorter life and a higher failure rate.

8. Overload capability

Real loads spike. A transformer's overload rating — how long it can carry 120% or 150% before damage — depends on its cooling and thermal design. If your process has short, hard peaks (crane lifts, compressor starts), size for the peaks or pick a design with a documented overload curve, rather than assuming the continuous rating covers them. 

9. Operating environment: temperature, location, ventilation

Heat, cold, humidity, and corrosive air all shorten a transformer's life. High ambient pushes you to a higher temperature class or forced cooling; freezing matters for oil viscosity and the drainage around the base. Put the unit where air moves and away from moisture and corrosive fumes — a sealed, well-ventilated enclosure beats a damp corner every time. For siting and the foundation underneath, our resources library covers drainage and clearance details.

10. Efficiency and reliability (and the "95%" myth)

A distribution transformer that's genuinely 95% efficient is broken — real units run 98–99.5% at full load, because the losses are small relative to the power moved. If a spec quotes "above 95%," check what it's actually measuring; it may be a tiny unit or a composite figure. What matters for operating cost is the loss evaluation: no-load loss runs all the time, load loss scales with loading squared. Reliability is the other half — buy from a supplier with test data and a track record, because a cheap unit that fails costs more than the saving. 

11. Size, weight, and transport limits

Space and how you get it there rule the form factor. Pad-mounted and unit-substation designs, compact dry-types, and small distribution rooms all cap what fits. Weigh the crate against the door, the elevator, and the crane path before you commit — a unit that won't fit the corridor is the most expensive mistake on this list. For tight sites, see how these constraints played out in our projects archive.

12. Price and lifetime maintenance cost

Two numbers, not one. The sticker price differs by brand, rating, and design; the lifecycle cost is dominated by losses (energy over 20+ years) and maintenance. Oil units are cheaper to buy large but cost more to maintain and insure; dry-types cost more up front in some ratings but need little beyond inspection. Run the total-cost comparison on your duty cycle, not the quote. For export or spec-writing, our IEC 60076 vs ANSI/IEEE guide maps the ratings to the standards your reviewer will check.

13. A practical selection checklist

Before you issue the purchase spec, confirm:

1. Primary and secondary voltage, confirmed with the utility and the load. 
2. kVA sized to ~75–80% of continuous load including reactive power.
3. Phase configuration matches the load (single vs three; multi-winding only if required).
4. Core choice: silicon steel default, amorphous if lightly loaded and loss-sensitive.
5. Cooling and insulation decided by site (oil vs dry; ONAN/ONAF/AN/AF; BIL and temperature class). 
6. Short-circuit impedance set with the protection engineer against available fault current. 
7. Overload curve documented for process peaks. 
8. Environment classified (ambient, humidity, corrosion, ventilation).
9. Efficiency and guaranteed losses stated on the nameplate. 
10. Physical size and transport path verified.
11. Total cost of ownership, not just price, compared.

14. FAQ

What kVA transformer do I need for my load?

Size to the load's apparent power, which includes reactive power. A 100 kW load at 0.8 power factor draws 125 kVA, and that's the figure that sets the rating. A good rule is to land at about 75–80% loading under continuous full load, leaving headroom for inrush and growth. 

Is 95% efficiency good for a power transformer?

No — for a distribution or power transformer, 95% would be a failure. Real units run roughly 98–99.5% at full load because the losses are tiny next to the power moved. If a spec sheet shows "above 95%," find out what it's measuring; it's likely a small unit or a composite number. Judge the unit by its guaranteed no-load and load losses instead. 

Oil-immersed or dry-type — which should I pick?

Let the site decide. Oil cools better and scales to larger kVA but needs oil handling, containment, and more upkeep. Dry-type is cleaner, simpler to maintain, and goes where fire or explosion risk can't be allowed — indoor commercial, hospitals, underground. If you're stuck between them, the trade-offs are laid out in our oil-vs-dry comparison.

What do ONAN, ONAF, AN, and AF mean on the nameplate?

They're cooling codes. ONAN is oil natural, air natural; ONAF adds fans (oil natural, air forced). AN is air natural for dry-types; AF adds forced air. Forced cooling raises the continuous rating but adds noise and a fan that can fail, so size for the forced rating only if you accept running the fans.

Why does short-circuit impedance matter so much?

Because it sets two opposing things at once. A higher %Z cuts the fault current the transformer feeds into a short, which protects downstream gear, but it also lets the secondary voltage droop more under load and during motor starts. Pick it against the system's available fault current and your voltage-regulation budget, with the protection engineer. 

Single-phase or three-phase?

Match the load. Single-phase fits small, single-phase-dominated services (residential, light commercial). Three-phase fits three-phase supplies and real power draws. If the load needs several secondary voltages, a multi-winding unit may be the answer. Don't buy a three-phase transformer for a single-phase job just because it's "more capable" — you pay for a winding you won't use.

Does ambient temperature change the rating?

Yes. High ambient reduces the margin before the winding hits its limit, so hot sites need a higher temperature class or forced cooling; cold matters for oil viscosity and base drainage. Classify the environment — ambient, humidity, corrosion, ventilation — before you finalize the rating.