Randall Lamb Association

INsight: The Rise of Electric Cars

January 27, 2015

Electric Vehicle Technology: The Engineering Challenges

p011g992-2Due to unstable gas prices, an increasingly sustainable and green-minded public, improvements in battery technologies and financial incentives from the government, the use of electric vehicles (EV) is on the rise in the United States. California, in particular, has seen an increase in the number of electric vehicles on highways and city streets. EVs present some building infrastructure practical challenges that need to be addressed in order to make the transition to electrical vehicles viable.

This isn’t the first time electric vehicles have been popular in the U.S. In fact, in the early 20th century, 38 percent of automobiles in America were powered by electricity, compared to 22 percent powered by gasoline. The remaining 40 percent of automobiles were steam-powered until petrol engines became the power of choice. The first electric vehicles which required lead-acid batteries, were capable of reaching speeds up to 20 miles an hour, and were typically “recharged” by trading exchangeable batteries. In contrast, most of today’s electric vehicles have fixed lithium-ion batteries that can provide as much as 75 – 90 miles on a single charge. Charging an electric vehicle is as easy as plugging it into the wall and waiting, although designing the electrical system to support the vehicle charger is no easy task. With the demand for chargers continuing to increase, Randall Lamb has been successful integrating EV charging provisions on many projects, large or small.

Article 625 of the National Electric Code (NEC) addresses electric vehicle charging stations and has been evolving since 1995. It’s an important code section outlining different design considerations that should be taken into account when designing the power supply to an electric vehicle charging station – namely load characteristics, diversity considerations and ground fault provisions.

Load Characteristics

Typical buildings have a 480V service and the electrical engineer specifies transformers to step voltage down to 208V. Because most vehicle chargers operate at 208V, buildings with EV charging will require longer or dedicated transformers to supply the load. The continuous loading for EVs stipulation in the NEC can cause loads to escalate quickly. For example, installing ten level two chargers requires a 112.5kVA transformer (10 chargers x 1.25 cont. x 208V x 32A = 83.2 kVA). If the vehicle charging load were non-continuous, the 1.25 multiplier would not be applied and a 75kVA transformer would be required.

Article 625 defines different types of car chargers by slotting them into three different ‘levels’ as follows:
  • Level 1 – Permits plugging into a common grounded 120-V electrical receptacle. The maximum load on this receptacle is 12A and the minimum overcurrent rating for this connection is 15A on a 15A branch circuit, or 20A on a 20A branch circuit.
  • Level 2 – This is the primary and preferred method of EV charging. It requires special equipment and connection to an electric power supply dedicated to EV charging. The voltage of this connection is typically 208V and the maximum load is 32A. The minimum circuit and overcurrent rating for this connection is 40A.
  • Level 3 – This can be considered the EV equivalent of a commercial gas station. It is high-speed, high-power and is capable of recharging an electric vehicle in the same amount of time it takes to refuel a conventional vehicle. Power and load requirements for level 3 chargers are specified by the manufacturer.

It is important to remember the continuous load stipulation in the NEC when estimating where to plug in a charging station in an existing electrical distribution system. When large banks of charging stations are added to the project late in the game, the dramatic increase in calculated load can cause major distribution system redesign and cost impact. Early planning for charging stations will help mitigate these impacts.

Factoring In Demand, Or Not

In commercial buildings, the NEC does not allow for any diversity factors to be applied, other than for lighting and receptacle loads. Because workers typically begin and end their working days at roughly the same time, it is entirely possible for all of the chargers to be charging at any given time throughout the day (considering that the typical charging time can be 6 to 8 hours). Without any demand or diversity on the load for the vehicle chargers, the design engineer is forced to calculate the load at 125%. There are power management systems available that theoretically limit the total power draw for a multiple vehicle charging system, and therefore charging rates would be slower.

Ground Fault Considerations

Typically, electric vehicle chargers are installed outdoors or in garage-type locations. Branch circuits and feeders serving these locations are prime candidates for ground fault, because they are often very long conductors installed underground. However, electric vehicle chargers typically do not play well with ground fault devices installed upstream because charger manufacturers often provide ground fault protection in the charging unit. While the user of the system is safe from being zapped by a ground fault, there is no protection between the charger and the upstream distribution. Randall Lamb recommends providing ground fault protection upstream of electric vehicle charging stations, provided it coordinates well with the downstream equipment.

Posted in Clean Energy, Project Insights