Transformer for EV Charging Station: Sizing & Selection Guide

An EV charging station transformer is a key part of the electrical infrastructure that connects the utility grid to EV charging equipment. As charging power increases-from conventional AC charging to 150 kW, 350 kW, or higher DC fast charging-the transformer becomes especially important for voltage conversion, capacity, power quality, and reliable operation.

For a typical charging site, the transformer steps medium-voltage utility power down to the low-voltage level required by the EV charging system. DC charging equipment then converts the AC power into controlled DC power for the vehicle battery. IEC 61851-23:2023 covers DC EV supply equipment with input-side voltages up to 1,000 V AC or 1,500 V DC and output-side voltage up to 1,500 V DC.

A typical power path looks like this:

Utility Grid → Medium-Voltage Switchgear → EV Charging Transformer → Low-Voltage Switchgear → EV Chargers → Electric Vehicles

For example, a site receiving 11 kV or 13.8 kV utility power might use a transformer to provide 400 V, 415 V, 480 V, or another required low-voltage supply to the charging equipment.

The exact voltage depends on the local utility network and charger manufacturer's requirements.

Transformer sizing should not simply equal the sum of all charger nameplate ratings. The design should consider charger simultaneity, efficiency, power factor, auxiliary loads, harmonics, and planned expansion. A useful preliminary relationship is:

Transformer kVA ≈ Total charger kW × Simultaneity ÷ (Efficiency × Power Factor) + Auxiliary Load + Expansion Margin

For example, consider an EV charging site with eight 150 kW DC chargers:

Total connected charging power: 1,200 kW

If all chargers can operate simultaneously, the electrical load can approach 1.2 MW

After considering efficiency, power factor, auxiliary equipment, and future capacity, a transformer in the 1,500–2,000 kVA range might be considered for preliminary design

The final rating must be confirmed through an electrical load study and the utility's interconnection requirements.

EV Charging Application	Typical Connected Load	Possible Transformer Range
Small AC charging site	100–300 kW	150–500 kVA
4–6 DC fast chargers	400–900 kW	630–1,250 kVA
6–10 high-power DC chargers	1–2 MW	1,500–2,500 kVA
Large fast-charging hub	2–5+ MW	2,500–6,300+ kVA

These are preliminary ranges rather than universal specifications. Actual transformer selection depends heavily on charger ratings and the site's load profile.

For EV charging infrastructure, several transformer configurations can work well.

Oil-immersed transformers are a common choice for outdoor high-capacity charging hubs. They offer good thermal performance and can be supplied as pad-mounted or other outdoor distribution configurations.

Dry-type transformers can be attractive for indoor charging facilities, parking garages, commercial buildings, and locations where fire and environmental considerations favor a dry design.

For high-power charging stations, transformer selection should also consider:

High continuous loading

Harmonic currents from power electronics

Voltage regulation

Transformer impedance

Cooling method

Short-circuit withstand capability

Noise

Outdoor environmental conditions

Future charging capacity

Utility protection requirements

Harmonics deserve particular attention because EV chargers are power-electronic loads. Depending on the charger design, harmonic performance and transformer heating should be evaluated rather than assuming a conventional commercial-load profile.

A charging station can have a surprisingly demanding load profile. Several vehicles may begin charging at approximately the same time, creating rapid changes in electrical demand.

An undersized transformer can lead to excessive temperature rise, voltage drop, protection trips, reduced charger output, and limited expansion capability. On the other hand, substantially oversizing the transformer increases capital cost and can increase unnecessary no-load losses.

For this reason, a good EV charging transformer design balances capacity, efficiency, power quality, reliability, and future expansion.

When specifying a transformer for an EV charging project, the procurement specification should normally include:

Specification	Typical Consideration
Transformer type	Oil-immersed or dry-type
Rated capacity	Based on calculated maximum demand
Primary voltage	Utility-specific, e.g. 11 kV, 13.8 kV
Secondary voltage	Charger-specific, e.g. 400 V, 415 V, 480 V
Frequency	50 Hz or 60 Hz
Phase	Three-phase for most commercial charging hubs
Cooling	ONAN, ONAF, or appropriate dry-type cooling
Impedance	Selected according to system fault level and voltage regulation
Harmonics	Evaluate charger-generated harmonic currents
Tap changer	Depending on utility voltage variation
Enclosure	Indoor, outdoor, pad-mounted, etc.
Standards	Applicable IEC, IEEE, utility, and local requirements

The EV charging equipment itself is governed by the IEC 61851 series; IEC 60364-7-722 specifically addresses electrical installations supplying EVs and circuits allowing electricity to flow back from EVs.

For a manufacturer such as Jiangsu Yawei Transformer, the opportunity is not simply to supply a transformer with a larger kVA rating. The transformer should be designed around the complete charging application-charger capacity, utility voltage, site environment, expected simultaneous charging, harmonic characteristics, and future expansion.

For high-power EV charging stations, a three-phase oil-immersed transformer, pad-mounted transformer, or dry-type transformer can be engineered according to the project's requirements.

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Q: How soon can you delivery the transformer?

A: It depends on the quantity and capacity of the transformer, normally within one month since the date drawing confirmed by buyer.

Q: How long can you provide the quality warranty?

A: 24 months since the date transformer operated.

Q: What payment method do you accept?

A: T/T (wire transfer) preferred, L/C both accepted.