Fill ’er Up—With Electricity: The ins and outs of residential EV chargers

Shutterstock / Dragon Claws
Shutterstock / Dragon Claws
Published On
Nov 15, 2022

Many of us have vehicles that are either totally or partially (hybrid) electric—cars, trucks, motorcycles, golf carts and more—and need charging systems for them. How much power is needed, and how long is the charging session for each vehicle? That will vary with the type and model of EV and whether it is totally or partially electric.

A hybrid vehicle supplements its electrical drive with a gasoline engine. Some high-end vehicles have electric motors at each drive axle. They rely on the electric motors for fast startup with almost instantaneous speed, but have an internal combustion engine (ICE) for longer distances. These hybrid vehicles have an external connection for charging the car batteries when parked, but rely on an internal charging system from the alternator to the battery and are powered by the ICE when driving on the road.

My wife has driven a hybrid for many years and just upgraded to a 2020 model. We plug it into a 20A, 120V receptacle outlet in the garage. It takes about five hours to charge the battery for approximately 25 miles of gasoline-free driving. Understand that this is a slow charge at 120V.

The car can be charged with a 240V supply. Since the voltage is doubled, that would more than likely substantially decrease the charging time. I could get a 240V supply from my electrical service, but it would be difficult and expensive because the garage is on the opposite side of my house from the electrical service.

Your mileage may vary

Do not take the example of our hybrid as the average, since each EV model has a different range. Looking at just mileage versus charging time is very misleading, since other factors must be considered when analyzing the overall real-world highway and street driving range. You have to do this same analysis to determine the range of a gasoline-only powered car.

The temperature, wind load, slope of the land and stop-and-go traffic all affect battery range (or fuel economy in gas-powered vehicles). Additionally, what accessories need to be used when driving? Does the driver need to operate the heater or air conditioning for their own comfort and their passengers’? What is the vehicle’s speed, and is it constant or stop-and-go?

Also, consider battery charging. What is the charging time for the vehicle’s battery, and does it require a full or partial charge? How much charge is needed for the next trip? Is a partial charge acceptable, and how much battery energy does the user need to travel one or both ways? Are there charging stations along the way, and how much time is required at the charging station before travel can continue?

These are all questions a consumer must know before buying and using an EV. I certainly don’t have enough space in this article to answer all of them.

The EC has the answers

As an electrician or electrical contractor being asked to provide power to these EV charging systems, you are more interested in knowing the following:

  • What is the kilowatt-hour battery size (usually lithium-ion batteries) of the vehicle?
  • What is the charging rate?
  • How many EVs will be charged at that single location?
  • Is single-phase or three-phase voltage available?
  • What size is the existing service?

Common sizes of residential AC wall-mounted EV chargers are 7 kilowatt (kW) and 9.6 kW at 120V or 240V, single-phase, and various sizes are available up to 22 kW at three-phase. Most homes do not have three-phase services, so the smaller size single-phase chargers are more realistic. For single-family dwellings, the minimum service disconnecting means rating is 100A, with many homes exceeding that with either 150A or 200A, 120/240V, single-phase services.

At 120V or 240V single-phase, the charging time can take a few hours up to 12 or more, depending on the battery level at the time of charging. The batteries usually charge much faster at the beginning, especially when the battery level is very low, and then slow down toward the battery’s peak voltage at full charge.

Since the subject of EVs and the size of an electrical service required for new and existing homes is fairly complex, I will continue explaining the calculation requirements, as well as the requirements in Article 625 for EVs, in next month’s column.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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