Every charging site sits behind a transformer. Its job is to bring the medium-voltage grid, typically 10 kV to 20 kV in the Netherlands and up to 36 kV elsewhere in Europe, down to the 230/400 V that AC chargers need or to the input voltage a DC charger rectifies. The hard part is not the voltage ratio but the sizing: charging load is peaky, harmonic-rich and grows over the life of the site.
What does a charger actually draw?
AC charge points sit roughly between 3.7 kW single-phase and 22 kW three-phase. A DC fast charger draws far more, commonly 50 kW to 350 kW, and ultra-fast installations combine several dispensers behind one power cabinet. The AC side of a DC charger is a rectifier, not a motor or a heater, and that changes how the transformer sees the load.
For a home or workplace connection the distribution transformer usually already exists, and the question is whether the street cable and the 400 kVA or 630 kVA unit feeding it still have headroom once a share of the households charges in the evening peak. For a public fast-charging site the transformer is normally dedicated to the site and sized specifically for it.
Size for the simultaneous peak, not for the sum
Adding up nameplate ratings gives an answer that is almost always too large. Six 150 kW dispensers rarely deliver 900 kW at once: vehicles taper their charging curve as the battery fills, and arrivals are staggered. A realistic simultaneity factor, combined with a load management system that caps total site power, keeps the transformer within a sensible rating.
The opposite mistake is worse. A transformer that is too small runs hot, ages its paper insulation faster and forces the site to throttle exactly when it is busiest. Leaving one step of rated power in reserve, and specifying the foundation and cable duct for a larger future unit, is usually cheaper than a second grid connection later.
Harmonics, inrush and power quality
Rectifiers draw non-sinusoidal current. The resulting harmonic currents cause extra eddy-current losses in the windings and additional heating, so a transformer feeding a large rectifier load may need derating or a design with a higher tolerance for harmonic content. Modern active front-end chargers behave considerably better than older six-pulse designs, but the effect should still be checked against the connection conditions of the grid operator.
Inrush deserves the same attention. Energising a transformer draws a short current peak that can be several times rated current, which matters for protection settings and for coordination with the medium-voltage fuse or circuit breaker at the site.
Indoor, outdoor or underground?
Outdoors, an oil-filled unit in a sealed tank is robust and thermally forgiving, provided the enclosure meets the required IP rating and a bund or spill tray is in place. In a car park, under a building or in an enclosed retail environment, a cast-resin transformer avoids the liquid entirely and is easier to justify on fire-safety grounds. Where a liquid is preferred indoors, a biodegradable ester fluid with a high fire point is a middle road.
A practical rule: fix the grid connection capacity around the simultaneous peak you can actually justify, then let load management absorb the rest, and choose the transformer type from the location constraints rather than the other way around. Protrafo supplies both oil-filled and cast-resin units for charging infrastructure and can advise on the derating a specific rectifier load calls for.
