There is no standard transformer size for an EV charging site. For a new site, the size is the output of a load statement: the building’s demand, plus every charger counted as a continuous load, plus whatever headroom the design agrees for later phases. The utility and the project’s electrical engineer turn that statement into an actual rating — the owner’s job is to make the statement complete and honest.
Key Takeaways
- There is no standard transformer size: the rating is the output of the site’s load statement.
- Count every charger as a continuous load (125% breaker rule) and state plainly how much runs at once.
- Include future phases in the statement now.
- The engineer calculates, the utility decides — get the agreed capacity in the written connection offer.
Start With the Load Statement
A transformer feeds everything downstream of it, so the statement has to count everything, in the order the engineer will check it:
- The building’s own demand. Existing or planned lighting, cooling, machinery and services — measured where records exist, estimated by the engineer where they do not.
- The charging load, at full rating. Each charger counts at its rated power, and because EV charging is a continuous load under the National Electrical Code, its circuit is sized at 125% of charger current: 32 A sits on a 40 A breaker, 40 A on 50 A, 48 A on 60 A.
- How much of it runs at once. Whether all chargers draw full power at once, or a site limit caps the total.
- The phases after this one. Chargers planned for next year belong in the statement now, so the service is built once.
The U.S. DOT bands show why the charging line dominates: Level 2 runs 7–19 kW per port, DC fast charging 50–350 kW. A handful of DC ports can outweigh the building they stand beside — which is why the statement, not the charger count, is the document to get right.
What Pushes the Size Up or Down
| Design decision | Pushes the size up when… | Pushes it down when… |
|---|---|---|
| Charger mix | DC fast ports are included — each is a large continuous load | The site is Level 2 only, matched to long dwell times |
| Simultaneity | Every port is assumed at full power at the same moment | A site power cap is designed in and the chargers share it |
| The building behind it | The chargers share a service with a heavily loaded building | Charging gets a service of its own, sized on its own statement |
| Future phases | Tomorrow’s ports are counted in today’s statement | Growth is left to a later upgrade — accepted knowingly, with the civils sized for it |
None of these are tricks to shrink a number. Each is a real operating decision — how the site will actually charge, and how it will grow — recorded before the utility designs the connection. Our breakdown of commercial charging installation costs shows where this work sits in the wider budget.
Who Actually Decides the Rating
Not the charger supplier, and not a rule of thumb. The project’s licensed electrician or electrical engineer produces the load calculation; the utility checks it against its network, chooses from its standard ratings and designs the connection, as the overview of how commercial charging stations work sets out. Get the agreed capacity in writing: the connection offer is the document the rest of the project is built against.
Hardware Choices That Shape the Statement
The load statement follows the hardware plan, so settle the plan early. A row of EV-AC03201T dual-connector AC stations — up to 43 kW shared per station, OCPP 1.6J, RFID, IP54 — produces a very different statement from a single EV-RHDC30K-L compact DC charger at 22 kW. Bring the hardware list and phase plan to our commercial charging team before the statement goes to the utility, so the number it designs to is the number the site actually needs.
Frequently Asked Questions
Can we just specify an oversized transformer to be safe?
Capacity has to be justified in the application — the utility designs to the statement you submit, in its standard rating steps. Headroom for named future phases is a normal design decision; unexplained oversizing is a question the utility will ask, and the engineer should answer it in the statement.
Is the transformer the same thing as the site’s panel?
No. The transformer steps the utility’s supply down and feeds the site’s service; the panel distributes that power inside the site. Different equipment, usually different owners, sized in the same exercise.
What if the site grows beyond the statement later?
Then the supply is revisited — a larger transformer or a second service, on the utility’s schedule. Designing the buried work (conduit, bases, switchboard space) for the final layout now keeps that later step cheap.
Related Reading
- How Much Does It Cost to Install a Commercial EV Charging Station?
- What Affects EV Charger Installation Cost? 8 Factors to Check
- How Do Commercial EV Charging Stations Work?
