Distribution transformer loss classes — Ecodesign Tier 2 and why no-load loss dominates a lightly loaded transformer

The guide on §8-1 energy efficiency in NEN 1010 covers the general energy-efficiency chapter of the installation standard itself. This article covers a related but separate piece of EU product regulation that applies before a transformer is even installed: the Ecodesign minimum-efficiency requirements for power transformers, and the distinction between the two loss mechanisms those requirements regulate.

What the regulation covers

Regulation (EU) No 548/2014, implementing the Ecodesign Directive for power transformers, sets minimum efficiency requirements for power transformers with a rated power of 1 kVA or more used in 50 Hz transmission and distribution networks or for industrial applications. Regulation (EU) 2019/1783 tightened those requirements with a second, stricter stage — commonly referred to as "Tier 2" — which has applied since 1 July 2021. Distribution transformers are responsible for a significant share (on the order of a fifth) of total losses in the electricity grid, and because a transformer typically stays in service for decades once installed, the regulation treats this product group as a priority for lifetime energy savings.

Two separately regulated loss mechanisms

A transformer loses energy through two physically distinct mechanisms, and the regulation classifies and limits each one independently, rather than through a single combined efficiency percentage:

No-load loss (core loss, iron loss): the loss in the transformer's magnetic core caused by hysteresis and eddy currents, present continuously whenever the transformer is energised, entirely independent of how much load current is actually flowing — even a transformer supplying zero load still dissipates its full no-load loss, 24 hours a day.
Load loss (copper loss, winding loss): the resistive I²R loss in the primary and secondary windings, which scales with the square of the load current — negligible at light load, but the dominant loss mechanism as the transformer approaches its rated capacity.

Because these two loss mechanisms behave so differently with load, the regulation defines separate minimum-efficiency (maximum-loss) classes for each, and manufacturers must declare both figures — not just a single "efficiency at rated load" percentage, which on its own cannot distinguish a transformer with low no-load loss and higher load loss from one with the opposite balance.

Why the balance between the two matters for the actual application

A single combined efficiency percentage, quoted at rated (100%) load, can look identical for two transformers with a very different loss balance — yet those two transformers behave very differently once installed in a real network:

A transformer that runs near its rated capacity most of the time (for example a heavily loaded industrial feeder) accumulates most of its lifetime losses as load loss, so the load-loss class matters most for its actual energy cost.
A transformer that is oversized relative to its typical demand — common in practice, for example an agricultural or horticultural connection sized for a future or seasonal peak that is rarely reached — spends most of its life lightly loaded. For that transformer, the continuously present no-load loss, not the load loss, dominates the lifetime energy bill, because it is incurred around the clock regardless of how little current is actually flowing.

Amorphous-core transformers, whose core material has inherently lower hysteresis losses than conventional silicon-steel cores, can reach the lowest available no-load-loss classes and are therefore particularly relevant where a transformer is known in advance to run lightly loaded for most of its life.

Practical relevance

When replacing an ageing distribution transformer, or comparing offers for a new one, it is not enough to compare a single quoted efficiency percentage: the no-load loss and load loss figures should be requested and compared separately, and weighted according to the expected loading pattern of the specific installation — a transformer that will run lightly loaded for most of its service life benefits far more from a low no-load-loss class than from a marginally better load-loss class, and vice versa for a continuously heavily loaded one. Confirming that a transformer meets the Tier 2 minimum requirements of Regulation (EU) 2019/1783 (rather than only the older Tier 1 level) is also relevant when assessing how recently a given transformer model was placed on the market.

Common mistakes

Comparing transformers on a single "efficiency at rated load" percentage alone, without separately checking the no-load and load loss figures — two transformers with the same headline efficiency can have very different lifetime energy costs depending on the actual loading pattern.
Assuming load loss dominates for every installation — for a transformer that is structurally oversized relative to its typical demand, the continuously present no-load loss is usually the larger contributor to lifetime energy cost, not the load loss.
Not checking whether a transformer meets the Tier 2 requirements of Regulation (EU) 2019/1783 (in force since 1 July 2021) when assessing a recently manufactured unit against current minimum efficiency requirements.
Overlooking amorphous-core transformers as an option for an installation known to run lightly loaded for most of its life, where their inherently lower no-load loss is most valuable.