How to Select Transformers for EV Charging Infrastructure and Fleets

The most common mistake in sizing a transformer for an EV charging site is adding up the nameplate power of all the chargers and ordering a transformer to match. A site with ten 120kW DC fast chargers adds up to 1.2MW. But whether you need a 1.25MVA or a 1.6MVA transformer comes down to one critical variable: the simultaneity factor.

Simultaneity for a public fast-charging hub looks completely different from a fleet depot. At a public site, peak hours might see every stall pulling full power, so you use a factor of 0.8 to 1.0. At a fleet depot doing overnight charging, vehicles trickle in throughout the evening, so the factor drops to 0.4–0.7. If you blindly apply one universal coefficient, you either end up with a transformer that is constantly overloaded, or you waste half your capacity.

Here is what actually matters when sizing, and how to set the capacity for different scenarios.

AC Level 2 chargers are basically just smart switches with protection. The actual AC/DC conversion happens inside the vehicle's onboard charger. Since the onboard charger must meet strict harmonic standards, the impact on the transformer is relatively controlled. A standard distribution transformer with normal margin usually does the job.

DC fast charging is a different animal. The charger has an internal AC/DC rectifier, and the input current total harmonic distortion (THDi) can hit 12% to 25% depending on the rectifier topology. Power factor fluctuates during the charging session instead of staying constant. Every time a session starts or stops, the transformer takes a power step. Some rectifier designs even inject DC components into the AC side. Your standard clamp meter won't read it, but the cumulative effect will saturate the core.

DC fast-charging transformers must be designed for non-linear loads. A K-factor rating is non-negotiable. K9 is acceptable for light mixed-use sites, but for pure DC fast-charging hubs, K13 or higher is the safe bet. If the site has a lot of single-phase AC chargers, the neutral current will be massive, and the K-factor transformer needs to handle neutral overheating.

Do not just guess the simultaneity factor. If you have load profile data from a similar site, use it. If not, go conservative. A 0.8 factor is the floor for a public fast-charging hub; 1.0 is the conservative design. For fleet depots with a smart charging management system, you can push the factor down to 0.5 or lower, but only if the system is reliable. Otherwise, the transformer will choke when all the vans plug in at once.

A practical rule of thumb for site sizing: Total Load = Total Charger Output Power ÷ Average Efficiency ÷ Line Loss Factor × Simultaneity Factor. Typically, efficiency is 0.9, line loss is 0.9, and the simultaneity factor is 0.85. But that 0.85 assumes a mixed site. For a pure DC fast hub, it's not enough.

DC fast-charging harmonics aren't a "minor detail"-they demand specific design. A THDi of 12% to 25% means eddy current losses in the transformer windings spike, and localized hot spots can run more than ten degrees hotter than a sinusoidal load of the same capacity. For every 8 to 10K increase in temperature, insulation life is cut in half.

K-factor transformers are built for this. The winding transposition, conductor cross-section, and core flux density are all optimized for the harmonic spectrum. K13 means the transformer can handle that specific harmonic profile without overheating. If the rectifier topology is unknown or harmonic data is missing, K13 is the safe starting point.

Harmonic Mitigating Transformers (HMT) are worth considering for mixed sites. HMTs cancel 3rd harmonics and reduce neutral current, which is great for sites with lots of AC chargers. But pure DC fast hubs mostly deal with 5th and 7th harmonics, where an HMT isn't as direct as a K-factor transformer.

One often-overlooked trick: if you have multiple DC fast chargers, split them across two transformers with a 30° phase shift (e.g., Dyn11 and Dyn1). This creates a 12-pulse effect and cancels the 5th and 7th harmonics on the upstream grid. It requires an extra transformer, but it cuts the cost of upstream harmonic mitigation. For large hubs, it's worth running the numbers.

EV charging transformers usually sit outdoors or in semi-enclosed spaces, right next to the chargers. Summer heat, plus the transformer's own heat, can push the ambient temperature 10 to 15°C higher than the weather report. Standard transformers are rated at a specific ambient temperature; in high heat, they must be derated.

If installed outdoors without climate control, specify a low-temperature-rise design. A 115°C or 80°C rise transformer costs more than a standard 150°C rise unit, but it has more thermal mass and handles cyclic stress better. For overnight fleet depots, the transformer is idle during the day and fully loaded at night-a massive temperature swing. The value of a low-temp-rise design is reducing the accumulation of thermal stress.

Look at the protection rating too. Public charging stations have mixed traffic. The transformer needs bollards, a locked enclosure, and protection against unauthorized operation. If it's a dusty logistics park, IP54 is the absolute minimum.

Public DC fast hubs (highway service areas, city hubs): K13 or higher, oil-immersed or dry-type. Size at nameplate total × 0.9, with a simultaneity factor of 0.8–1.0. If harmonic data is missing, K13 is the fallback. For multiple fast chargers, use phase-shifted connections. Oil-immersed units handle overload better for peaky public sites; dry-type is for indoor or high fire-safety locations.

Fleet overnight depots (logistics, transit, taxi): Dry-type preferred. K9 or standard distribution with load management. Simultaneity factor of 0.5–0.7, assuming the charging schedule system is reliable. A low-temp-rise design extends life under continuous night loads. Don't size based on "one charger per vehicle" total power. Size based on how many vehicles actually charge at the same time.

Mall/hotel destination charging: Standard distribution transformer with load management. Simultaneity factor of 0.3–0.5. No K-factor needed. The key here is not to over-size. Destination charging utilization is low, the transformer will sit at light load, and if you buy it too big, you are just burning money on no-load losses.

Solar + Storage + EV integration: Transformer capacity can be reduced because the battery can shave the peak. But the transformer must support bidirectional power flow (power reverses when the battery discharges), and the OLTC voltage logic needs to be re-verified. Borrow the sizing logic from BESS transformers and watch the cyclic load capacity.

We build transformers and supply core materials. If you are evaluating transformer specs for an EV charging site-K-factor, capacity, harmonic profile, or core grade-send us the details. Charger mix, site type, simultaneous demand, voltage level, timeline. You will get a straight answer.

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[1] Transformer sizing for EV charging infrastructure, Elinta Charge, 2026.

[2] Transformer Considerations for EV Charging Infrastructure, Rex Power Magnetics, 2026.

[3] How to Choose the Capacity of the Box Transformer for Charging Stations, Henan Electrical Industry Association, 2026.

[4] IEEE P3765, Standard for the Technical Requirements for Voltage-Regulating Power Electronic Converter Embedded Transformers in Consumer-Side Energy Systems, IEEE Standards Association, 2025.