This article discusses NEC 2023 or 2026. ZinaVee's calculators use NEC 2017.
Published: June 2026 Technical Level: Advanced Category: Power Systems Design
The electrification of vehicle fleets creates electrical loads that are concentrated in time and location in ways that challenge existing distribution transformer and service entrance capacity. A Level 2 EVSE charger draws 7.2 to 19.2 kW continuously during charging; a DC fast charger (DCFC) draws 50 to 350 kW. A parking facility adding 50 Level 2 chargers adds a potential peak demand of 360 to 960 kW, a load increment that can exceed the capacity of a typical commercial building's service entrance transformer by a factor of two or more. NEC 2023 Article 625 and the NFPA 70-aligned EVSE load calculation methods establish the basis for sizing the electrical service for EV charging infrastructure; IEEE Standard 2030.1.1-2021 addresses the communication and control requirements for managed charging. This paper develops the transformer and service entrance impact assessment methodology: the demand factor analysis that accounts for simultaneous charging behavior, the transformer thermal impact from the concentrated evening demand profile, and the managed charging strategies that can reduce the peak demand impact by 40 to 60 percent relative to uncontrolled charging.
The impact of EV charging on distribution infrastructure depends critically on the charging behavior, specifically, the fraction of vehicles that begin charging simultaneously and the duration of the charging events. Uncontrolled Level 2 charging in a residential neighborhood follows a strong evening peak as vehicles return home from commuting, with 60 to 80 percent of vehicles plugging in between 5 PM and 9 PM. In a commercial parking structure, the daytime charging profile depends on how long vehicles park and whether the EVSE is priced to encourage extended parking. A DC fast charging corridor has a flatter, demand-driven load profile with high peak-to-average ratios.
The transformer impact analysis must characterize the expected simultaneous demand fraction (the fraction of installed EVSE capacity that will be in active use simultaneously during the peak period) and apply it to the transformer's rated capacity and thermal model to determine whether the existing transformer can sustain the combined load within its rated temperature limits, or whether a larger transformer or active thermal management is required.
NEC 2023 Article 625.42 establishes the load calculation requirements for EVSE installations. For commercial applications with 5 or more EVSE on a single feeder, the demand factor applied to the total EVSE nameplate load is 100 percent for the first EVSE plus 100 percent of additional EVSE to fill the circuit, with no demand factor reduction for simultaneously operating chargers unless a load management system is installed per NEC Section 625.42(B). When a listed energy management system that controls simultaneous EVSE loading is installed, the calculated load may be reduced to the maximum controlled output of the EMS.
The practical implication is that NEC-compliant service entrance sizing for uncontrolled EV charging requires full-rated conductor and overcurrent protection for each EVSE circuit, with the total service entrance sized for the sum of all EVSE loads plus the existing facility load. For a 50-space parking structure with 50 × 7.2 kW Level 2 EVSE: the NEC load = 50 × 7,200 = 360 kW of EVSE load. With a 40 percent demand factor for EMS-controlled charging, the design load reduces to 144 kW, a difference of 216 kW in required service entrance capacity.
The transformer thermal model (IEEE C57.91) predicts the hotspot temperature from the load curve. An existing 500 kVA service entrance transformer serving 300 kW of daytime commercial load with an additional 200 kW of evening EV charging peak presents a loading profile where the transformer is at 60 percent load during business hours and at 100 percent during the evening charging peak. The hotspot temperature during the 100 percent loading period, assuming a 35°C ambient and standard transformer thermal parameters (top-oil rise 55°C, hotspot gradient 15°C at rated load): hotspot = 35 + 55 + 15 = 105°C, within the 110°C hotspot limit for 65°C average winding rise transformers. The transformer can sustain this loading profile without accelerated insulation aging if the thermal parameters are within the rated values.
For a transformer already loaded to 80 percent during daytime hours, adding an EV charging peak that drives the transformer to 120 percent loading produces a hotspot temperature of approximately 140°C, well above the rated limit, and accelerated insulation aging that reduces the transformer's remaining service life.
[1] NFPA 70, National Electrical Code, Article 625, 2023 edition, NFPA, 2023.
[2] IEEE Standard 2030.1.1-2021, Standard Technical Specifications of a DC Quick and Bi-directional Charger for Use with Electric Vehicles, IEEE, 2021.
[3] IEEE Standard C57.91-2011, Guide for Loading Mineral-Oil-Immersed Transformers, IEEE, 2012.
[4] SAE J1772, SAE Electric Vehicle and Plug-in Hybrid Electric Vehicle Conductive Charge Coupler, SAE, 2017.
[5] SAE J3068, Electric Vehicle Power Transfer System Using a Three-Phase Capable Coupler, SAE, 2018.
[6] EPRI, Electric Vehicle Supply Equipment (EVSE) Installation Guide, EPRI Technical Report, 2022.
[7] NREL, Charging Infrastructure Deployment and Grid Integration, NREL/TP-5400-74198, 2019.
[8] IEC 61851-1:2017, Electric Vehicle Conductive Charging System: General Requirements, IEC, 2017.