Transformer sizing is the process of selecting a transformer capacity (kVA) and configuration that can safely and reliably supply an EV charging site’s expected electrical demand. In charging infrastructure, transformer sizing applies to new installations (dedicated site transformers) and upgrades (when existing transformers may not support additional EV load).
Correct sizing ensures chargers can operate without nuisance trips, overheating, or excessive voltage drop, while avoiding unnecessary overinvestment.
Why Transformer Sizing Matters in EV Charging
Transformer sizing directly affects site performance, cost, and expansion:
– Determines how many chargers can operate simultaneously at full power
– Impacts voltage stability and charging reliability (especially on long feeders)
– Influences CAPEX (transformer + civil works + grid connection scope)
– Affects operating limits and the need for load management
– Enables future expansion when designed with spare capacity
– Reduces the risk of overheating, accelerated aging, and losses
For fleet depots and public hubs, transformer constraints often become the real limit—not charger count.
Key Inputs for Transformer Sizing
Transformer sizing is typically based on:
Connected Load vs Diversified Load
– Connected load: sum of nameplate charger ratings (worst-case)
– Diversified load: expected simultaneous demand considering utilization, duty cycles, and control strategies
EV charging can be a long-duration load, so diversity assumptions should be evidence-based.
Charger Mix and Power Levels
– AC chargers (e.g., 11 kW / 22 kW)
– DC chargers (higher demand and more “peaky” depending on usage)
– Other site loads (building HVAC, lighting, process loads)
Load Profile and Peak Demand
– Expected peak demand periods and seasonality
– ToU incentives and time-of-use optimization
– Fleet departure schedules and batch charging behavior
Electrical Parameters
– Supply voltage (LV/MV level), phase configuration, and earthing arrangement
– Power factor and harmonics (driven by equipment characteristics)
– Allowed voltage drop and network operator limits
– Ambient temperature and installation method affecting transformer rating
How Transformer Sizing Is Commonly Done
A typical sizing approach includes:
– Estimate peak charging demand under realistic scenarios
– Add existing site load peak and any planned growth
– Apply diversity factors based on use case (public vs fleet vs workplace)
– Decide the control approach: unmanaged vs managed charging with a cap
– Select transformer rating with margin for:
– Future expansion
– Thermal conditions and losses
– Short-term overload capability (if applicable)
– Verify voltage regulation and short-circuit performance with the network
Many sites combine transformer sizing with a maximum site demand limit and load control to avoid oversized transformers.
Practical Rules and Trade-offs
– Oversizing increases CAPEX and losses at low load, but improves headroom
– Undersizing can force frequent power derating, trips, or expansion rework
– Load management can reduce required transformer size by capping demand
– Poor phase balance can cause overheating and neutral issues even if kVA looks adequate
– High utilization sites may need less diversity and more firm capacity than “destination” sites
Signs an Existing Transformer May Be Undersized
– Repeated overload alarms or thermal protection operations
– Voltage drop complaints or chargers failing to start under load
– High transformer temperatures during predictable charging peaks
– Protective device trips upstream during simultaneous charging events
– Restrictions imposed by the DSO / network operator
Best Practices
– Use measured load profiles where possible (not only nameplate assumptions)
– Size for realistic peak + growth, not just average utilization
– Implement load management from day one to control peaks and enable scalability
– Plan spare capacity and physical space for expansion (switchgear ways, ducting)
– Validate assumptions with power studies and network operator requirements
– Monitor after commissioning and adjust caps based on real data
Related Glossary Terms
Substation Capacity
Substation Upgrades
Maximum Site Demand Limit
Load Management
Load Balancing
Power Quality
Phase Balancing
Three-phase Power
Power Derating
Time-of-use Optimization