Transformers
Distribution transformers
A distribution transformer steps a medium voltage network down to the low voltage a building, a plant or a process runs on. Ratings run from 100 to 2500 kVA in the standard series.
- Ratings
- 100 to 2500 kVAother ratings on request
- Primary voltage
- commonly 6.6 to 33 kVset by the network at the connection point
- Secondary voltage
- 400 V, 415 V or 433 V480 V on 60 Hz systems
- Construction
- liquid immersed or dry typeset by where the unit stands
- Standards
- IEC 60076-1, IEC 60076-11 for dry typeIEEE C57.12.00 on North American specifications
A distribution transformer is the last voltage step before the loads. Its ratings follow a standard series, so manufacturers build a known set of designs and offers compare like with like. A rating between two standard values usually costs more and takes longer for no technical gain. The practical decision is which standard value the load sits under.
The voltage pair is not a free choice either. Continental European networks commonly run at 10 or 20 kV and supply 230/400 V, the value IEC 60038 lists for three phase systems. In the UK, 11 kV is the common distribution voltage. One network operator's specification for ground mounted units lists the ratio 11000/433-250 V from 25 to 1000 kVA, a no load secondary chosen, the operator notes, to help provide the nationally declared low voltage. On 60 Hz systems the low voltage is commonly 480 V.
The rated current on the low voltage side follows from the rating and the secondary voltage. That current sizes the incomer, the busbar and the cables, and the ratings table below gives it at 400 and 433 V for every standard step.
What gets specified
| Parameter | Typical options | What decides it |
|---|---|---|
| Rating | 100 to 2500 kVA in R10 steps | The diversified load and its planned growth, rounded up to the next step |
| Primary voltage | 6.6, 10, 11, 13.8, 20, 22 or 33 kV | Whatever the network operator runs at your connection point |
| Secondary voltage | 400 V; 433 V no load on many UK specifications; 480 V at 60 Hz | The declared supply voltage of the destination market |
| Vector group | Commonly Dyn11 or Dyn5 | The neutral the low voltage side must provide, and any unit it will run beside in parallel |
| Construction | Liquid immersed or cast resin | Outdoors or in a plant room, against inside an occupied building |
| Windings | Copper or aluminium | Mass, price and preference; the loss ceilings are the same for both |
| Losses | Tier 2 ceilings of Regulation (EU) No 548/2014, or lower by agreement | Where the unit is placed on the market, and how the owner values lifetime running cost |
Ratings from 100 to 2500 kVA
| Rating | Full load current at 400 V | Full load current at 433 V | Next step |
|---|---|---|---|
| 100 kVA | 144 A | 133 A | Request a quote |
| 160 kVA | 231 A | 213 A | Request a quote |
| 200 kVA | 289 A | 267 A | Request a quote |
| 250 kVA | 361 A | 333 A | Request a quote |
| 315 kVA | 455 A | 420 A | Request a quote |
| 400 kVA | 577 A | 533 A | Request a quote |
| 500 kVA | 722 A | 667 A | Request a quote |
| 630 kVA | 909 A | 840 A | Rating page |
| 800 kVA | 1155 A | 1067 A | Request a quote |
| 1000 kVA | 1443 A | 1333 A | Rating page |
| 1250 kVA | 1804 A | 1667 A | Request a quote |
| 1600 kVA | 2309 A | 2133 A | Rating page |
| 2000 kVA | 2887 A | 2667 A | Request a quote |
| 2500 kVA | 3608 A | 3333 A | Rating page |
Rated current is S / (√3 × U). Each linked rating carries its own engineering decisions, and any rating without a link is quoted straight from its row.
Applicable standards
| Standard | What it covers |
|---|---|
| IEC 60076-1 | The general requirements a unit in this range is designed and routine tested to |
| IEC 60076-11 | Takes over for cast resin units, with its own classes for climate, environment and fire |
| Regulation (EU) No 548/2014, as amended | The loss ceilings for units placed on the EU market, which also remain law in Great Britain |
| EN 50708-1-1 and EN 50708-2-1 | European additional requirements for medium power transformers, in force since EN 50588-1 was withdrawn in 2020 |
| IEEE C57.12.00 | The general requirements behind North American specifications for liquid immersed units |
What a distribution transformer request needs
- The voltage pair as the network operator states it, with a 433 V no load secondary where your specification asks for one
- The standard step you expect, or the diversified load and the largest motor if the step is still open
- Liquid immersed or cast resin, decided by where the unit will stand
- The loss level: the Tier 2 ceiling, or a lower figure with the capitalisation values behind it
Specification template
Fill it in on screen or by hand, then upload it with your request.
Questions buyers ask
Should the secondary be 400 V or 433 V?
400 V is the IEC 60038 value for 230/400 V systems. Many UK specifications ask for 433 V at no load instead; one distribution network operator lists its ratio as 11000/433-250 V. In Qatar, Kahramaa's low voltage network runs at 240/415 V. Use the value your network operator or your designer states, because the low voltage board is rated around it.
How much headroom should a rating carry?
Enough for the growth that is actually planned and for the starting behaviour of the largest motor, and no more. An oversized unit carries higher no load losses for its whole life, and those are paid every hour whether the load is there or not.
Is a 33 kV unit covered by the same loss limits?
Not directly. The EU loss tables apply where one winding is at most 24 kV Um. A 33 kV system uses a 36 kV Um primary, which the regulation corrects under a separate table, and the offer states the limit for the configuration quoted.