An on-site transformer is electrical equipment installed at a facility (charging hub, depot, commercial building, or substation area) that converts electricity from a higher voltage level to a lower voltage level suitable for local distribution—most commonly from medium voltage (MV) to low voltage (LV) for EV chargers and other site loads. It is a key component in providing sufficient power capacity and stable voltage for larger EV charging installations.
What an on-site transformer does
An on-site transformer typically:
– Steps down MV (e.g., 10–20 kV, market-dependent) to LV (e.g., 400/230 V in many regions)
– Provides electrical isolation and stable voltage reference for the site
– Defines the maximum continuous capacity available to the site (rated in kVA)
– Supports protection coordination with switchgear and distribution boards
Why on-site transformers matter for EV charging
Transformers often become the bottleneck (or enabler) for scaling charging:
– High-density AC sites and DC hubs may exceed the capacity of existing building transformers
– Larger transformer capacity can support more simultaneous charging sessions
– Voltage stability improves charging performance and reduces nuisance faults
– Proper sizing supports future expansion and avoids repeated upgrade cycles
– Transformer selection impacts losses, heat, and long-term operating costs
Typical use cases
– Fleet depots adding multiple 22 kW AC chargers or DC chargers
– Public hubs where simultaneous charging demand is high
– Mixed-use or office sites expanding EV-ready parking at scale
– Logistics and industrial sites electrifying vans, trucks, or yard equipment
– Sites where the existing transformer is fully utilized by building loads
Key design considerations
– Capacity sizing (kVA) based on peak demand, diversity, and expansion plan
– Load management strategy to reduce required transformer size and cap peak demand
– Location and footprint (transformer bay, kiosk, substation room, fire safety distances)
– Noise considerations (transformer hum) in residential or hospitality areas
– Cooling, ventilation, and environmental rating for outdoor installations
– Earthing arrangement (TN-S, TN-C-S, TT) and protection system design
– Metering and utility interface requirements (ownership boundary, seals, access)
Transformer upgrades and expansion planning
On-site transformer upgrades are often driven by:
– New charging capacity requests and grid connection offers
– Demand charge optimization and peak load planning
– Future-proofing for staged rollouts (reserve capacity for additional bays)
– Integration with on-site storage or PV systems where export/import limits apply
Common challenges
– Long lead times for utility approvals, transformer delivery, and commissioning
– Space constraints at existing sites (especially retrofits)
– Cost of civils, MV cabling, switchgear, and protection studies
– Noise and visual impact constraints in sensitive urban locations
– Coordination complexity between utility, EPC, site owner, and operator
Related glossary terms
Transformer capacity
Transformer upgrades
Site capacity assessment
Grid connection agreement
Main LV panels
LV / MV / HV grid levels
Load management
Maximum site demand limit
Power quality monitoring
Noise reduction