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SAE J3400 and the game-changing advances in AC charging

Posted: December 19, 2023
Guest Post by Rodney McGee, Ph.D., P.E., University of Delaware.

SAE International's committee responsible for passenger electric vehicles has recently published SAE J3400, covering a coupler system used by the majority of electric vehicles on North American roads today. This marks a significant advancement for the industry, steering us away from proprietary/closed systems and towards industry-standard approaches for mass electrification. While much attention has been given to the switch from SAE J1772 to SAE J3400 (the North American Charging Standard, or NACS) for DC fast charging, this article will focus on AC charging because the majority of Americans with electric vehicles rely on it for their day-to-day commutes.

Since SAE first introduced SAE J1772 in 1996, the industry has matured significantly. At that time, vehicle original equipment manufacturers (OEMs) were not involved in deploying nationwide networks of charging stations. Now, many vehicle OEMs have energy business units that directly provide charging services and equipment off-board the vehicle, signifying a major shift in perspective.

Consider a vehicle OEM optimizing costs strictly around the electric vehicle. Under this paradigm, cost optimizations made on the vehicle could lead to a many-fold increase in the installation and ongoing operational costs of off-board infrastructure, including increased energy usage. This would result in higher total system costs, slowing the adoption of electric vehicles. Vehicle OEM energy units would then be burdened by higher capital costs and a smaller customer base.

The key to electrifying transportation lies in architecting electric vehicles and supply equipment together as a single system. SAE J3400, with the support of a supermajority of OEMs, has made sound engineering decisions that will pay dividends for decades by lowering barriers to electrification in North America. While SAE J1772 provided good AC charging solutions for single- and two-family homes, SAE J3400 better addresses the broader market in several ways.

It supports AC voltages commonly available in commercial areas, streets, parking garages, and dense housing locations, specifically 480/277-V three-phase power, one of the most common configurations provided by utilities in the United States. Retaining the support for 277-V charging, already present in existing NACS (Tesla) electric vehicles, demonstrates that OEMs are aligning their decisions with mass electrification by widening the cost-optimization bounty to include supply infrastructure.

Many utilities require 480/277 VAC above a certain service size; with EV support for 277 VAC, the site avoids paying for secondary (customer-owned) transformers to create a lower voltage. Additionally, most DC fast chargers utilize 480/277-VAC three-phase power; with EV support for 277 VAC, a parking lot with AC and DC chargers can share the same panel board and power feed for each type of station.

These sites mentioned above avoid an additional panel board, eliminate space for transformers, reduce wire sizes, reduce conduit sizes, eliminate additional overcurrent protection devices, and may reduce site engineering design work. Some sites with limited space, like street charging locations, can significantly reduce the footprint of the installation by eliminating those bulky components. Overall charging efficiency can be increased by 2 to 8%, depending on the load factor at the sites without secondary transformers. Additionally, because 277 V reduces the current by 33% for the same power when compared to 208 V, copper losses in the wires are reduced by more than 50%.

Electric vehicle supply equipment (EVSE) with detachable (carry-along) cable assemblies offers several advantages over stations with permanently attached cables. This approach, widely deployed in the rest of the world, is introduced to North America through SAE J3400 and SAE J3068. Referred to as the “universal AC socket-outlet” in SAE documents, this approach has the following advantages:


  • Reduces maintenance costs and makes the cable user-replaceable rather than requiring a technician. This commoditization reduces system costs, enabling more places to charge your EV.
  • Reduces the risk of damage to the coupler from being dropped on the ground or run over with a vehicle, facilitating street cleaning and snowplows to operate near stations while also reducing the risk of petty theft and vandalism.
  • Eliminates adapters for AC by having the cable assembly fitted with a universal plug on the infrastructure side and your connector type on the vehicle side. There are no interposers (adapters) in this configuration, which creates mating points that neither the EVSE nor EV can monitor.
  • Allows EVSE to optionally supply three-phase AC power (via SAE J3068). This is important for the electrification of medium- and heavy-duty vehicles, which often charge at locations with three-phase power; it allows these vehicles to reach AC charging power levels just above 50 kW at 480/277 VAC, all while permitting single-phase vehicles to charge at the same charging station.

It’s crucial to consider how the fast-charging use case, where users fill their battery and immediately unplug, may conflict with the objectives of vehicle-grid integration. Ideally, most vehicles should be plugged in at locations where they spend the majority of their parked time. This approach allows for flexible/managed energy transfer and increased energy storage capacity, enabling higher penetration of renewables while maintaining grid stability. The lower cost and other advantages of EVSEs with detachable cables make charging more accessible and affordable for vehicles that don’t park in a home garage, aligning with the broader objectives of vehicle-grid integration and mass electrification.

Many consumers are well aware of the ongoing transition among vehicle OEMs to adopt the NACS vehicle inlet. As a result, there is a reluctance to invest in the last-model year of vehicles equipped with the legacy SAE J1772 electric vehicle inlet. Regulators need to move decisively to support the configurations supported by SAE J3400.

In the short term, it is reasonable for certain location types to have a percentage of EVSE that supports legacy SAE J1772 Level 2 AC charging, but long term, this will constrain electrification due to the reasons mentioned above. Similar to how CHAdeMO DC stations serve their share of legacy vehicles, the existing and in-progress installations of SAE J1772 infrastructure will support those legacy vehicles throughout their operational lifespan. Electric vehicle infrastructure investments should prioritize electrifying the 95% of the market that still relies on combustion-based vehicles.

Finally, the broad environmental implications of SAE J3400 cannot be overlooked. By streamlining infrastructure with a universal AC connection and enhancing efficiency with support for 277/480 VAC, it accelerates the broader adoption of electric vehicles by lowering costs, improving access, and enhancing charging performance.


About the author: Dr. Rodney McGee is a research engineer at the Transportation Electrification Center at the University of Delaware. In this role, McGee leads a team of engineers in designing, testing, and productizing advanced bidirectional EVSE and EV systems while working closely with OEMs and suppliers to foster cutting-edge technology development. In addition to his work at the University of Delaware, McGee chairs SAE's Medium- and Heavy-Duty Conductive Power Transfer Task Force ( J3068) and the NACS (SAE J3400) Task Force. Through these leadership roles, he actively contributes to advancing industry standards in the electric vehicle sector.