In short
To get an accurate distribution transformer quotation, state the rating (kVA or MVA), HV and LV voltages, phases and frequency, vector group, tapping arrangement, type and cooling, any loss or efficiency requirements, the applicable standard such as IEC 60076, site conditions, terminations and accessories, quantity and destination.
Why a complete specification matters
Two transformers with the same kVA rating can differ widely in price, size, losses and lead time. When a specification leaves gaps, suppliers fill them with their own assumptions, and the quotations you receive stop being comparable. A complete specification avoids that, and it shortens the back-and-forth before a firm price is possible.
Rating, voltages and frequency
Start with the basics: the rated power in kVA or MVA, the primary (HV) and secondary (LV) voltages, the number of phases and the system frequency (50 or 60 Hz). State voltages exactly as your network uses them, for example 11 kV / 433 V or 33 kV / 415 V, because a small difference in the secondary voltage changes the design.
Vector group and impedance
The vector group describes how the windings are connected and the phase shift between HV and LV. Dyn11 is common for distribution transformers that need a neutral on the LV side, but your network may use a different arrangement.
Percentage impedance affects the prospective fault level downstream and how the transformer shares load. If the new unit will run in parallel with an existing transformer, the vector group must match and the impedance should be as close as possible, so include the existing unit's nameplate data.
Tapping arrangement
Most distribution transformers have an off-circuit tap changer on the HV winding, often with a range such as ±2 × 2.5%, adjusted only when the transformer is de-energised. Where voltage must be regulated while on load, an on-load tap changer is needed, which is a significant cost and design difference. Say which you need and the tapping range.
Type and cooling
Specify whether you need an oil-immersed or dry-type unit, and the cooling class. Oil-immersed transformers are commonly ONAN (oil natural, air natural) or ONAF (with fans). Dry-type units are typically AN, or AF with forced air. If you are unsure which type suits the site, see our guide to oil-immersed versus dry-type transformers.
Losses and efficiency requirements
No-load and load losses drive running cost over the transformer's life, and many markets regulate them. Examples include the EU Ecodesign requirements for transformers, energy-efficiency labelling of distribution transformers in India, and US Department of Energy efficiency standards. If a minimum efficiency or maximum loss level applies in the destination country or in your tender, state it, along with any loss capitalisation values used to evaluate bids.
Standards and tests
Name the standard the transformer must comply with, such as the IEC 60076 series or IEEE C57.12 in North America, plus any utility specification. Say which reports you need: routine test reports are produced for every unit, while type test reports demonstrate a design. If you need witnessed testing at the factory or third-party inspection, say so at the enquiry stage, since it affects price and schedule.
Site and installation conditions
Standard ratings assume normal service conditions, including limits on ambient temperature and an altitude up to 1,000 m. Higher altitudes, high ambient temperatures, coastal or polluted air, or seismic requirements can all change the design. State whether the unit is for indoor or outdoor use and describe any restrictions on space, weight or access.
Terminations, protection and accessories
Describe how cables or busbars will connect: open bushings, cable boxes or a direct connection to switchgear. List the accessories and protective devices you expect, such as temperature indicators, a pressure relief device, or a gas-actuated relay on conservator-type units, plus wheels, lifting arrangements and any monitoring requirements.
A checklist to copy into your enquiry
| Item | Example of what to state |
|---|---|
| Rating | kVA or MVA, continuous |
| Voltages | HV / LV, for example 11 kV / 433 V |
| Phases and frequency | Three-phase, 50 Hz |
| Vector group | For example Dyn11 |
| Impedance | Percentage, or the existing unit's value if paralleling |
| Tapping | Off-circuit or on-load, and range |
| Type and cooling | Oil-immersed ONAN, or dry-type AN |
| Losses | Limits or efficiency class, if regulated |
| Standard | IEC 60076, IEEE C57.12 or utility specification |
| Site conditions | Indoor or outdoor, altitude, ambient temperature |
| Terminations and accessories | Bushings or cable boxes, protection devices |
| Quantity and delivery | Units, destination country, required date |
Partial information is still useful. If some items are not yet decided, say so, and they can be clarified before quotation.