The decision to standardize a distribution network on 10kV, 20kV, or 35kV is usually made once, early in a project's planning, by whoever is designing the distribution system layout — and then lived with for decades by everyone who specifies, maintains, and expands equipment on that network afterward. It's a decision that gets far less attention than kVA sizing or cooling class, despite having a bigger long-term impact on how many substations a network needs, how far power can travel before losses become significant, and how much flexibility exists for future load growth.

This guide compares 10kV vs. 20kV vs. 35kV transformers from a distribution planning standpoint — what actually changes between these voltage classes, and how to decide which one fits a given project.

What Changes Between 10kV, 20kV, and 35kV Class Transformers

Beyond the obvious difference in primary voltage rating, moving between these classes changes several practical specification details:

• Insulation level and clearances scale with voltage class, meaning a 35kV-class transformer requires more internal insulation distance and generally a larger physical footprint than a 10kV unit of comparable kVA rating.
• Typical short-circuit impedance shifts by class and cooling type. Oil-immersed units in the 20kV class commonly run around 5.5–6% impedance, while dry-type units in the same class often run closer to 6%, figures worth confirming against your specific protection coordination study rather than assuming a generic value applies.
• Vector group conventions remain broadly consistent across classes (commonly Dyn11 or Yyn0 for medium-voltage distribution transformers), so voltage class alone doesn't change how paralleling or grounding is approached — but any equipment mixed across classes still needs the same vector group, ratio, and impedance verification covered in standard paralleling practice.

Why 20kV Delivers More Than "Just a Higher Number"

It's easy to treat 20kV as simply "20kV instead of 10kV" — a bigger number that requires more expensive insulation for no obvious benefit unless a load genuinely demands it. In practice, stepping up a distribution network's voltage class delivers benefits that compound at the system level, not just the individual transformer level:

• Supply radius extends meaningfully. For a given load density, a 20kV distribution class can serve roughly 1.26 times the supply radius of an equivalent 10kV network, which directly reduces how many substations a given service area needs.
• Coverage area per substation grows further still, with a 20kV network able to cover roughly 1.6 times the area of a 10kV network at the same load density — a meaningful factor when planning substation count and siting for a large campus, industrial park, or expanding urban area.
• Line losses drop substantially at the same load level. At comparable loading, a 20kV distribution network can see line losses around a quarter of what an equivalent 10kV network would experience, translating into lower energy cost over the life of the network, not just a one-time equipment consideration.

When 35kV Makes Sense: Large Loads and Reduced Substation Count

Stepping up further to 35kV-class equipment makes the most sense for:

• Large industrial loads and campuses where a single point of supply needs to serve substantial aggregate demand without a proliferation of intermediate substations.
• Projects with significant future load growth expected, where standardizing on a higher voltage class up front avoids a costly network-wide voltage class conversion later — a project few operators want to undertake once a distribution network is built out and energized.
• Sites with long feeder runs to the point of use, where 35kV's lower per-unit-distance loss characteristic compared to 10kV or 20kV becomes economically significant over the feeder's operating life.

The trade-off is a larger physical footprint and typically higher upfront equipment cost for comparable kVA capacity, which is why 35kV isn't the default choice for smaller commercial or light industrial projects where 10kV or 20kV already meets the load and growth requirements comfortably.

Retrofit and Compatibility Considerations: Why You Can't Simply Swap Voltage Classes

A distribution transformer's primary voltage rating is fixed by design — a transformer built for 35kV input cannot simply be substituted onto a 10kV feeder or vice versa without the equipment being severely underutilized or, in the reverse direction, subjected to overvoltage that risks insulation failure. This has a direct practical consequence for any project considering a future voltage class change: it's not a transformer swap, it's a distribution network redesign, since switchgear, protection settings, and often the physical substation layout are all built around the class chosen at the outset.

For projects genuinely uncertain about long-term load growth, this argues for erring toward the higher voltage class when the cost difference is modest, since converting a 10kV network to 20kV or 35kV years later is a far larger undertaking than the incremental cost of specifying the higher class from day one.

A Decision Framework for New vs. Expanding Projects

A Practical Checklist

Final Thoughts

Choosing between 10kV, 20kV, and 35kV isn't primarily a transformer specification decision — it's a distribution network planning decision that happens to determine which transformer class you'll be specifying for decades afterward. Weighing supply radius, substation count, and loss characteristics against your project's actual growth expectations, rather than defaulting to whatever voltage class a legacy network happens to use, is what prevents an expensive network-wide conversion years down the road.

Once you've settled on a voltage class and received equipment, our guide on how to read a transformer nameplate covers how to confirm the unit's actual ratings match your specification, and our comparison of on-load vs. off-circuit tap changers is worth reviewing if your project's voltage class decision also involves choosing a voltage regulation approach.