With the rapid growth of electric vehicle (EV) charging infrastructure, one of the most common engineering questions is: what size transformer is needed to support multiple high-power charging piles?
For a typical case of 10 units of 120 kW DC fast chargers, the total installed capacity reaches a significant load level, and proper transformer sizing is critical for safe, stable, and economical operation.
In this article, we break down the calculation logic in a practical way and explain how to select a suitable transformer for this scenario.
1. Total Power Demand of the Charging Station
Each charging pile has a rated output of 120 kW.
So for 10 units:
Total installed load = 10 × 120 kW = 1200 kW
However, transformers are rated in kVA, not kW. Therefore, we must also consider the conversion between real power and apparent power, as well as system efficiency.
2. Transformer Efficiency and Load Factor
In real-world operation, transformers do not operate at 100% efficiency. A practical long-term operating efficiency range is typically:
- 60%–75% (conservative design range)
- Around 80% (commonly used in engineering practice for balanced cost and performance)
Assuming an 80% load factor, the required transformer capacity can be estimated as:
Required capacity = 1200 kW ÷ 0.8 = 1500 kVA
This means a transformer rated around 1500 kVA is theoretically required to support full simultaneous operation.
3. Selecting a Standard Transformer Size
Since transformer capacities follow standard ratings, the nearest practical choice would be:
👉 1600 kVA transformer or prefabricated substation (compact substation/box-type transformer)
This provides sufficient margin for:
- Peak load fluctuations
- Thermal stability
- Future expansion
- Electrical safety redundancy
4. Considering Simultaneity Factor (Important in Real Projects)
In actual charging station operation, it is rare for all 10 chargers to run at full power simultaneously.
For example, if we assume only 8 chargers operate at full load at the same time:
- Active load = 8 × 120 kW = 960 kW
- Adjusted capacity = 960 ÷ 0.8 = 1200 kVA
In this case, a:
👉 1250 kVA transformer
may be sufficient for areas with moderate traffic or lower utilization rates.
5. Practical Selection Guidelines
Based on engineering experience:
- Low to medium utilization sites (residential, small commercial areas):
✔ 1250 kVA transformer is usually adequate - High traffic or highway charging stations:
✔ 1600 kVA transformer is recommended
However, final selection should always consider:
- Local grid capacity
- Load diversity factor
- Future expansion plans
- Transformer type (oil-immersed or dry-type)
- Investment and energy efficiency requirements
6. Conclusion
For a 10 × 120 kW EV charging station, transformer sizing typically falls into two practical solutions:
- 1250 kVA transformer → when simultaneity is lower
- 1600 kVA transformer → when full-load operation is more likely or for safer design margin
Because every charging site has different operating patterns, it is strongly recommended to consult a qualified electrical engineer before finalizing the design.