What are electric vehicle service equipment design considerations?

Learn electrical design options for electric vehicle (EV) charging stations

By Sam Cipkar, PE and Francesca Price, PE April 4, 2024
Charger installation per Americans With Disabilities Act requirements. Courtesy: SmithGroup

EV insights:

  • The increasing popularity of electric vehicles (EVs) in recent years, driven by technological advancements and government incentives, has underscored the critical need for a robust electric vehicle supply equipment (EVSE) network.
  • Engineers play a crucial role in the design process, considering factors such as the type of EVSE, code requirements, site assessment, safety concerns and planning.
  • The choice of EV charger type depends on user requirements, with three main categories differing in voltage levels and charging capacities and the overall EVSE system comprises a power source and distribution system, a charging station unit and a communication and control system.

This article has been peer-reviewed.

Over the past several years, the use of electric vehicles (EVs) in lieu of internal combustion engine (ICE) vehicles has been gaining popularity. This has been fueled by technological innovation as well as significant government incentives. As a result, the importance of a robust network of electric vehicle supply equipment (EVSE) to charge vehicles has dramatically increased.

The role of the engineer in the design process cannot be understated as there are many factors that must be considered.

These factors include:

  • Choosing the type of EVSE.

  • Understanding code requirements.

  • Doing a site assessment and electrical load calculations.

  • Understanding safety and accessibility concerns.

  • Designing for the future.

Types of electric vehicle chargers

When it comes to choosing the right type of EV charger, it is important to understand the requirements of the user.

The key questions to ask as the designer are:

  • What kind of vehicles are being charged? Small sedans or large pickup trucks and SUVs?

  • How fast is a full charge desired? Can the vehicle charge overnight? Does it need to be charged in less than an hour?

Once the client use case is understood, then a proper decision can be made about which charger should be specified and what the infrastructure design will look like. There are three types of EV chargers. They mainly differ in voltage level and charging capacity (see Table 1).

Table 1: Electric vehicle (EV) charger types are outlined. Courtesy: SmithGroup

Table 1: Electric vehicle (EV) charger types are outlined. Courtesy: SmithGroup

The entire EVSE system comprises three main components:

  • Power source and distribution system.

  • Charging station unit.

  • Communication and control system.

The actual charging unit itself is designed by an equipment manufacturer. The specification of the unit will be based mainly on the needs of the client and the capacity of the power source being designed. The communication system will be based on the charging station design, and many EV chargers are now made with integral cellular gateways and require no special data connections.

Other units, however, rely on a local remote cellular gateway or Wi-Fi signal. In these cases, understanding data equipment and cabling requirements is very important.

EV code requirements

When it comes to code requirements, the 2023 edition of NFPA 70: National Electrical Code (NEC) lays out the electrical requirements for designing the power source and distribution system for EV chargers.

NEC Article 625: EV Power Transfer Systems is the starting point for understanding EVSE specific requirements. The scope of the article covers “electrical conductors and equipment connecting an EV to premises wiring for the purposes of charging, power export or bidirectional current flow.”

The article breaks down the following topics and others in more detail:

  • Specifics for designing charging stations including connectors and cables.

  • Specifics for installation of chargers including branch circuit sizing, load calculations and requirements for means of disconnecting chargers.

  • Requirements for bi-directional charging.

  • Ventilation requirements for chargers installed indoors.

  • Requirements for wireless vehicle charging.

Other important NEC articles that apply to the design of chargers are:

  • Articles 210, 220: Branch circuits.

  • Article 230: Services.

  • Article 240: Overcurrent Protection.

Article 250 Grounding and Bonding.

It is important to refer to local ordinances and building codes as they will often include some requirements or metrics for the number of EV chargers required for new construction projects. Some building certifications like U.S. Green Building Council LEED also require EV chargers.

Safety and accessibility concerns

Human safety is always the primary consideration when engineering any system. EV system design is no different. Bringing EV chargers into an electrical design poses unique safety implications. EVs can burn more than three times hotter and are much more difficult to extinguish than a standard ICE vehicle fire. This is due to thermal runaway that can occur when lithium-ion battery cells become damaged. A chemical reaction can occur that spreads to other cells as it burns.

As EVs become more prevalent, codes and standards are beginning to react to this new technology. Until the industry settles on a standard approach, local authorities having jurisdiction (AHJ) may have their own policies to minimize the added danger of an EV fire. These could include providing “fire gaps,” which involves intentionally placing ICE parking spaces or drive aisles in between groups of EV charging spaces.

Other potential safety items include EV fire extinguishers, bollards and other barriers, emergency stops and restricting the location of EV chargers within parking structures (discussed further below). It is important to discuss any requirements with the AHJ early in the design process.

It is also important to note site requirements from the Americans With Disabilities Act (ADA), which requires a specific number of chargers to be designed as accessible. Requirements include clearances around equipment, mounting heights of operable components and minimal sloping of the grade.

The U.S. Access Board has published a standard for ADA EV parking spaces, which offers a comprehensive list of recommendations. These requirements can add significant challenges and costs. This is especially true for projects at existing sites and parking lots where existing conditions do not meet current ADA requirements. Some jurisdictions may also require that the ADA EV spots have an accessible path to the building and public way depending on its use. Consult with the local AHJ for specific requirements (see Figure 1).

Figure 1: Charger installation per Americans With Disabilities Act requirements. Courtesy: SmithGroup

Figure 1: Charger installation per Americans With Disabilities Act requirements. Courtesy: SmithGroup

EV site assessment

In addition to the safety and accessibility concerns, sites should also be evaluated for electrical design considerations. Before any design of EV charging infrastructure is pursued, it is critical to carry out a detailed site assessment and perform electrical load calculations. The site layout will have significant impacts on installation costs, the placement of chargers and the source of power.

It is very important to consider how far the power source is from the EV charger load. Voltage drop can quickly become a problem and results in upsizing wiring. Unfortunately, this exponentially increases material costs. Local step-down transformer(s) and distribution near the EV chargers will limit voltage drop for both the feeders and branch circuits. However, when locating distribution equipment outside, it is important to specify equipment and foundations that are rated to withstand weather conditions.

If EV chargers are being placed in existing parking structures, make-ready installation costs are typically lower than in existing parking lots because all the equipment can be surface mounted. Additionally, excavation is typically not required.

However, it is crucial to understand how the parking deck is constructed. Cast-in-place structures will have fewer obstructions and predictable rebar patterns. Post-tensioned decks will have tensioned cables running throughout the slab that can easily be damaged during coring and drilling. Damaging a tensioned member can have serious implications that may be costly to repair. All decks should be scanned with ground penetrating radar before the commencement of any coring or drilling.

It is also important to note if the chargers are being installed in an underground level. The AHJ may require special systems to be installed to control fire and smoke in an enclosed area like underground levels of parking structures. AHJs may also require that EVs be located away from egress stairs, contain “fire gaps,” contain emergency stops, etc.

Another item to note is that EVs typically weigh more than a standard ICE vehicle. When curb weights are compared for similar vehicles, the EV typically weighs about 30% more than its ICE counterpart. Consult with a structural engineer to determine if any structural reinforcement is required.

It is also important to consider communications when conducting a site assessment. If the EV charger requires a hardline data connection for communications, a pathway back to a network switch would be required. If the specified EV chargers have a cellular gateway, is there an adequate cellular signal in that location? Chargers located inside parking structures or adjacent to tall buildings may suffer from weak or unstable signals. In these cases, it might be required to install a cellular signal boosting system (see Figure 2). Because different manufactures have various communication standards or systems for their chargers, careful coordination is required.

Figure 2: Post-tensioned cables in a parking structure located using ground penetrating radar. Courtesy: SmithGroup

Figure 2: Post-tensioned cables in a parking structure located using ground penetrating radar. Courtesy: SmithGroup

Electrical load calculations

When running load calculations, it is important to be aware of NEC code requirements as well as any other local codes and ordinances. Adding EV chargers to an existing power system requires understanding the existing loads. Historical utility bills or meters are the ideal source for determining how much capacity a power system has. If existing billing or metering does not provide enough information, then panelboards and distribution boards should be metered for at least 30 days in accordance with NEC 220.78. This article also requires a 125% demand factor to be added to loading information that was derived from metering or billing.

When calculating the EV loads, it is important to note the 2023 NEC requirement that states the charger load used must be the larger of 7,200 volt-amps (VA) or the nameplate rating of the charger. Chargers are typically rated based on amperes and the Level 2 chargers are typically dual rated for 208 and 240 V. When calculating the load, the voltage of the system must be known. An example of how a 50 A Level 2 charger load can vary significantly based on voltage can be seen below:

208 V × 50 A = 10,400 VA

240 V × 50 A = 12,000 VA

The difference is about 15%, which adds up significantly as the number of chargers increases. Currently the NEC does not allow for demand factors to be applied to EV chargers.

If the desired number of EV parking spaces results in a load calculation that exceeds the available capacity of the power system being used, a power system upgrade is needed. However, a more economical approach is to use shared or sequential charging. In some applications, where it is expected that vehicles will be parked for extended periods of time, chargers can be programmed to charge sequentially. Sequential charging means chargers will charge at full power one after the other but not at the same time.

Another option would be to program chargers to “power-share” or “shared charging.” This is where chargers pool a set maximum power capacity and scale their output so that the combined power does not exceed a set amount. It is important to ensure the charger being specified has these capabilities. Some AHJs may require documentation to be supplied to show the programming capabilities before accepting demand calculations based on modified charging schemes (see Figure 3).

Figure 3: Diagram of shared charging schemes. Courtesy: SmithGroup

Figure 3: Diagram of shared charging schemes. Courtesy: SmithGroup

Electrical design considerations

When designing the infrastructure for EVSE equipment, there are some things that should be considered.

Voltage drop is a problem that almost always shows up on electrical site projects. It is especially exacerbated by high loads such as EV chargers. It is important to determine early on what the feeder and branch circuit lengths will be. The cost of a couple transformers to step up/down the voltage for a single feeder can quickly become less than installing very large duct banks to transfer power over long distances at a lower voltage. This is especially noticed with high power Level 2 and Level 3 chargers.

Another thing to consider is the K-factor ratings of transformers. Because EV chargers inherently convert alternating current (ac) power to direct current (dc) through power electronic inverters, it is plausible to assume that there would be significant harmonic distortion.

However, it has been found that the current Level 2 chargers are surprisingly clean and high K-factor rated transformers are not always necessary. Transformers rated K-4 are typically sufficient for Level 2 EV charging loads. For high-power Level 2 chargers and large capacity Level 3 chargers, it is recommended to consult with manufacturers for test data regarding harmonic distortion.

There are a few typical electrical system configurations that are usually seen for providing power to chargers. The configuration depends on the various factors discussed already such as circuit lengths, type of charger and quantity of chargers desired.

Some common configurations are:

  • 208 V branch circuits from indoor panelboard out to Level 2 charging stations.

  • 208 V feeder to local panelboard near several Level 2 charging stations.

  • 480 V feeder to local 480-208 V transformer and 208 V panelboard for Level 2 charging stations.

  • 480 V feeder to local 480 V distribution board to feed Level 3 charging stations.

  • New dedicated electrical service with medium voltage service transformer and 480 V or 208 V distribution board.

Remember that any electrical equipment outdoors should be in a minimum National Electrical Manufacturers Association 3R rated enclosure.

From time to time, EV chargers may need maintenance; however, they are often installed in remote locations. It is good practice to locate convenience receptacles near chargers. If the EV chargers are being served with 208 V, this is a simple issue resolved with a Y-connected service; 120 V is readily available.

However, if the EV chargers are served with 240 V, this is not so easily accomplished, because 3-phase 240 V systems are always delta connected. A high leg delta transformer would be required to allow for 120 V receptacles to be installed near the chargers (see Figure 4).

Figure 4: Electrical equipment near electric vehicle chargers with convenience receptacle and wireless gateway. Courtesy: SmithGroup

Figure 4: Electrical equipment near electric vehicle chargers with convenience receptacle and wireless gateway. Courtesy: SmithGroup

NEC Article 625 does not explicitly list any special grounding requirements for wired EV chargers above and beyond an equipment ground. However, EV charger manufacturers may recommend installing a dedicated ground rod at each charger. Consult with the EV charger manufacturer for any specific requirements during design.

Planning for the future

When designing a new building, it is important to verify if the project is under any building codes or local ordinances that require a minimum quantity of EV chargers, “EV ready” space or “EV capable” spaces. Even if there are no codes requiring it, it is still best practice to design the system with some ability to serve future EV chargers.

This can be done by reserving capacity during the load calculations for future chargers and installing equipment or reserving space for breakers and panels that can be used for EV chargers in the future. It is recommended that EV chargers be placed on their own dedicated feeders and panelboards. If future EV spaces are being pursued, then all the service equipment will need to be installed during construction. Spare conduits will need to be routed to the site and capped so that wires can be pulled in the future. It is wise to design the panelboard with a set number of appropriately sized breakers for the parking spots. This is to help ensure that the chargers being purchased and installed in the future will not overload the system.

Dedicated panelboards with surge protection or providing isolation transformers might also be considered to protect the facility distribution system from ground faults or surges. This is especially important if the future EV chargers and any integral surge protection are unknown at the time of construction.

If a large quantity of EV chargers is anticipated in the future, increasing the distribution and service size will require a high upfront cost. An alternative solution is to provide spare conduits for a future dedicated EV electrical service.

The need for EV charging is becoming more prevalent across the globe. Understanding project needs and charging requirements can help determine which type of charger makes the most sense for each application. Once the basics are understood, code requirements, site constraints and safety considerations must also be considered. Whether designing for a handful of EV chargers or several hundred, these principles can assist with any EV design.

Author Bio: Sam Cipkar, PE, is an Electrical Engineer with SmithGroup. He has four years of experience and currently focuses on workplace and technology projects. Francesca Price, PE, is an Associate and Electrical Engineer with SmithGroup. She has more than 10 years of experience, currently focusing on cultural and higher education projects.