Last Update: August 21, 2021
Charging an electric car at home is the most convenient refueling experience possible. But most homes were not designed with this in mind, and most of our collective experience does not involve refueling vehicles at home. In most cases supporting EV charging is a simple retrofit, involving a 240 volt circuit and power outlet near where your car is parked. Fortunately installing an electrical circuit for a 240 volt single phase AC can be performed by any electrician, and for most this will be easy and cheap. In this article we'll talk about getting to the goal of home-based EV charging, while contemplating several long-term considerations.
For most of human experience with automobiles we haven't been required to install refueling equipment at home. Instead we've relied on public refueling at gasoline stations. Some may have cognitive dissonance over having to install a home charging station. If we followed the gasoline car approach with electric cars, we'd be relying on public charging stations. But, electricity is everywhere, making it easy to refuel EV anywhere that electricity exists, such as at home.
Electric vehicle owners have a big advantage over gasoline/diesel vehicle owners. We are not required to go out of our way to find a refueling station. Instead we have the luxury of charging at home, which has major cost and convenience advantages.
At the high level, charging station installation is little different from installing other appliances requiring high-power, like a washer/dryer combination. The project itself is conceptually very simple, but there are several considerations we'll go over.
Overly simplified, the steps involved are:
- run wires from the service panel to a place where your charging station will be installed
- add power outlet to those wires
- install the charging station
But of course there are considerations at each step.
The cost to install a charging station at home can run anywhere from free, to a couple hundred dollars, to thousands of dollars. The free route involves using the charging station which was sold with the car, powered by a power outlet that might be fortuitously present. It's best to avoid using an extension cord along with the charging station, but we have a guide covering what to do if you must: Safely use Extension Cords when charging an electric car or electric motorcycle. Otherwise you're looking at buying a charging station, and installing some wiring to bring power near the parking space. Usually the cost is minimal, but it can run into thousands of dollars depending on the circumstances.
Because this is electrical work, it's usually best to hire an electrician. It's possible to do a DIY charging station installation project, if you're handy with that sort of thing. We've attached a few videos to the bottom of the page demonstrating parts of the process, should you feel adventurous.
It's normal to wonder this: "Why can't I just plug the car directly into a regular power outlet?" What happened is that the car industry determined that safely charging an electric car required several safety interlocks. The bottom line is that a charging station is required between the normal power outlet and the charging inlet on the car.
It's preferable to plug the charging station into a power outlet. Some charging stations are designed to be hard wired to a junction box connected to the electrical service panel. This is thought to be safer than connecting it to a power outlet. Other charging stations have plugs, allowing them to be unplugged when required. It's best to avoid routinely plugging and unplugging the charging station, especially the ones using heavy duty power plugs like the NEMA 14-50, to avoid wear and tear on the outlet.
While you might be focused on installing one charging station for one electric car, it is useful to contemplate the future. In a few years will your household own some more EV's? Will you then need more than one charging station? We'll go over the parameters of those decisions later.
This is a more complete outline of the planning process:
- Inspect the service panel to determine whether there is spare capacity in the service panel.
- Determine where the car(s) to be charged are parked
- Determine the likely route and distance for any electrical wiring
- Determine how many electric cars are in the household today, and how many are likely for the future
- From that determine how many of those electric cars will be charging simultaneously, and the charging rate
- Plan for the electrical service capacity to handle charging those electric cars
- Consult or hire an electrician to assist or to do the wiring (after getting necessary permits from the city)
- Install charging station(s) attaching them to the newly installed wiring
How much it costs to install dedicated 240 volt wiring for electric car charging requires some thought and analysis. For a single circuit the cost is fairly low, but you could possibly be repeating this process in a few years. While you could wait until that day to ponder how to add a second charging station, a little pre-planning today could be useful.
Learning about total electricity capacity at the home
The beginning of planning electric vehicle charging, is to start with the capacity of the electricity service to the house.
Somewhere attached to your house is a Service Panel, containing circuit breakers controlling the electricity supply to each power outlet in the house. Attached to the service panel is the Electricity Meter. Attached to the meter is the wires leading to the electricity grid.
This equipment controls the total electricity consumption available in the building. Because you're thinking about adding an electric vehicle charging station to your home, you must know whether that will raise electricity consumption above the limits imposed by this equipment. Even if it's "just one charging station", conditions in your building may dictate that even that one EVSE will necessitate an upgrade to your electrical service.
There are three limits to be aware of:
- The size (and Ampacity) of the wires connecting your house to the grid
- The capacity of the electricity meter -- there should be a label on the front of the meter
- The capacity of the main circuit breaker in the service panel -- This will be clearly marked on the circuit breaker as the number of amps.
In the next section we'll talk more about calculating the currently assigned capacity, versus the maximum allowed in your building.
Calculating service panel capacity to prepare for charging station installation
We've already pointed you to the service panel of your building. This, and the other components just described, dictate the maximum electricity consumption in this building. What we need to determine is the electrical capacity assigned to existing circuits, and therefore how much capacity remains to be assigned.
The assigned electrical capacity in a building is determined by existing electric circuits. The remaining capacity is the difference between the total capacity and the assigned capacity.
available capacity = total capacity - assigned
The total capacity is determined by the size of the main circuit breaker (and the capacity of the service meter, and of the service wiring). This circuit breaker will be the biggest one in the service panel, and is typically at the top. Turn this off and the entire house will be without electricity. It will have a number on it like 100 or 150. Whatever number is on the circuit breaker is the absolute maximum number of amps your building can consume at a time.
It is very important to understand that this is a Peak Rating, meaning it is a maximum Momentary consumption. The electrical code has a concept called Continuous Load, which is continuing electricity consumption for more than an hour. According to the code, the consumption of a continuous load must be less than 80% of the peak rating. Since electric car charging can take several hours, it is a continuous load, as would be leaving an electric heater turned on for several hours.
We determine assigned capacity while looking at the service panel. Write down the numbers on all the other circuit breakers. Each circuit breaker controls one circuit in the building. Like with the main circuit breaker, there will be a number written on each circuit breaker giving its capacity. Typically this will 20 or 40 amps. Each circuit is where some of the total electrical capacity of the house is assigned. Simply add together these numbers to derive the assigned capacity.
To determine the available capacity we simply subtract the assigned capacity from the total.
It is a code violation for this number to exceed the rating of the main circuit breaker.
For example -- if your house has five circuits of 20 amps apiece, that is 100 amps assigned capacity. A main circuit breaker rated for 150 amps means your house has 50 amps of remaining capacity. It's that simple. Just add up the numbers to learn the available capacity.
You might be noticing that electrical capacity is measured in Amps.
Think about this for a moment. It is required that you can turn on every device in the house at the same time, and to remain within both the maximum capacity of every circuit, and the total capacity of the house. The circuit breakers are there to ensure the limits aren't exceeded.
An electrician can assess service panel capacity for you. But it's a simple exercise to calculate your current electrical capacity, since the mathematics is what we learned in grade school. It will expand your mind to learn about this, and it will prepare you for understanding the technical jargon the electrician will tell you.
Determine parking location for the electric car(s) in the household
This may seem like a mundane unimportant detail, but it is extremely important. The cost of installing the wiring is dependent on how much wire must be run from the service panel to the charging station. Buying 100 feet of wire is obviously more expensive than buying 50 feet of wire. For longer wire runs, the wire must also be thicker in order to avoid voltage drop.
This means the ideal location to park the car is next to the service panel. But, of course, that won't always be the case.
Something to produce in this step is a drawing showing where the service panel is in relation to the parking place, a rough estimate of how to get wire from point A to point B, and a rough estimate of the distance.
If you hire a professional electrician they will produce a better plan. But if you've done your homework, you'll be better prepared for the electrician.
How many electric cars today, and in the future
A household getting one electric car often buys another sometime later. There may be kids growing to maturity who "need" a car, too. You might be planning today to install one charging station for one electric car. But in two years, will you be planning another charging station because your household now has three electric cars? Therefore, it's good to think ahead about supporting multiple electric cars.
How many electric cars will be charging simultaneously?
An electric car charging station can charge one electric car at a time. A household with more than one electric car will either have to work out how to share one charging station, or have multiple charging stations. Given the difficulty households have had with sharing phone lines, leading to the teenage daughter often getting her own phone line, we might confidently predict difficulty sharing charging stations. Those of you who grew up after wired telephones became a thing of the past might not understand the analogy.
For the youngsters here's a little remedial education: In the distant past of the 1990's and before, telephones were connected to wires connected to the telephone network. To have another telephone meant running another wire. To answer or use your telephone, you had to be at home, and it was impossible to use the one telephone line to make more than one call at a time.
With telephones we solved that problem with wireless mobile telephones. But we cannot do that with electric car charging. There is no such thing as long distance transmission of electric power at the level required for electric car charging. Instead, electric car charging involves wires and circuit breakers in the service panel.
The bottom line is that your household could require more than one charging station in the future. To do so usually requires assigning multiple electric circuits. There are some options that we'll discuss later to mitigate that need.
At this moment you're most likely considering one charging station. What we're asking you to do is pre-plan what to do in a few years if you need more charging capacity at home.
Electrical service capacity required to charge the electric car(s)
We're starting to have enough data to estimate the electrical capacity required to charge the electric car (or cars) in our fleet. Specifically, how many charging stations will you need, and how much current will each station require?
Once you can answer those questions, the calculation is simple:
service required = number of charging stations x amps per charging station
In the previous two sections we considered how many cars you'll need, and how many charging stations you'll require.
If each charging station supports 32 Amp charging, then each charging station requires a 40 Amp circuit. That's because 80% of 40 Amps is 32 Amps. Notice that the higher the charging rate, the more quickly your electrical capacity is consumed.
Two 40 Amp circuits, for 2x 32 Amp charging stations, requires 80 Amps of service. It wasn't that long ago that 80 Amps was sufficient for an entire house, and now we're considering to dedicate that much to electric car charging.
The 32 amp 240 volt charge rate equates to 6.6 kiloWatts. The rule of thumb is 20-25 miles range gained per hour of charging. That's enough to completely recharge a 200+ mile range electric car in an overnight 10 hour charging session.
Most folks do not need to completely recharge that much range per day. For most of us, a lower power charging station is sufficient. A 3 kiloWatt charge rate (10-11 miles range per hour of charging) supplies over 100 miles range in an overnight charging session. See What electric car charging rate do we need at home, at the office, on road trips, at airports, or elsewhere?
What are your real needs? It's tempting to install the highest charging rate available, to ensure your car is always quickly recharged. But do you truly need this? And is it worth the cost if it ends up requiring a electrical service upgrade?
There's a slippery slope of wanting faster charging. It's worth some time to ponder our real needs.
How much electrical service capacity do we really need for electric car charging?
Logically speaking whether one NEEDS high power charging at home depends on ones true driving needs. To measure that it's useful to keep a diary for a couple months of your driving patterns. Your real driving needs will emerge from the data.
Most of us drive 40 miles or less per day. Recharging 40 miles of range overnight can be done with a simple 120 volt outlet supplying 4-5 miles range per hour of charging. A 240 volt 16 amp charging station supplies 11-12 miles range per hour of charging, meaning it supplies 40 miles in about 4 hours, or over 100 miles range in an overnight charging session. That means a low-end 3 kiloWatt charging station is more than adequate for most people.
But, some of us truly drive hundreds of miles per day. We might be a real estate agent, or an independent contractor for a delivery company like Instacart. Some of us truly have large needs. The fuel cost savings of using electricity as a fuel, charged at home, can be a tremendous incentive to buy an electric car for those who drive a lot. These people will frequently want to recharge their car as quickly as possible, and therefore will have a strong need for higher power charging.
Applying the 80% rule to charging rates, we get these circuit capacity requirements:
| Charge rate | Range Gained | Circuit capacity needed |
|---|---|---|
| 240 volt 16 amp | 12 miles / hour charging | 20 amp |
| 240 volt 24 amp | 18 miles / hour charging | 30 amp |
| 240 volt 32 amp | 23 miles / hour charging | 40 amp |
| 240 volt 40 amp | 29 miles / hour charging | 50 amp |
| 240 volt 80 amp | 50+ miles / hour charging | 100 amp |
This is the tradeoff mentioned earlier between charging rate and the cost to install electrical capacity.
See Installing cheap/inexpensive electric car charging at home
Minimizing the electrical service capacity for electric car charging
Suppose the previous sections convinced you to keep the costs low. Can we do so while being able to charge multiple cars at home?
Here's a few ways to assign power to three charging stations:
| Count | Circuits | Assigned power |
|---|---|---|
| 3x 16 amps stations | 3x 20 amp circuits | 60 amps assigned |
| 2x 16 amps, 1x 32 amps | 2x 40 amp circuits | 80 amps assigned |
| 1x 16 amps, 2x 32 amps | 1x 20 amp, 2x 40 amp circuits | 100 amps assigned |
| 3x 32 amps | 3x 40 amp circuits | 120 amps assigned |
Wisely choosing the power levels for your charging stations can make a big cost difference.
Choosing electric car charging stations for charging electric cars at home
By now you will have decided how many charging stations of what capacity each. It is now time to go shopping having determined your real needs.
We have a full catalog of charging stations and advice to peruse: The best electric car charging stations for home charging or on trips
Another product category to consider is what kind, if any, extension cords to get. A household with multiple electric cars might find a J1772 extension cord necessary. See Safely use Extension Cords when charging an electric car or electric motorcycle
Electric car charging station installation - wiring
This particular charging station install was at a commercial location - but it demonstrates how simple it is.
It starts by bolting the charging station to the wall. Sometimes a mounting bracket will be used, and other cases there are mounting holes. You then drill screws into the wall to firmly attach the EVSE.
Once the station is secured, there is a short wire - "Pigtail" is the technical term - to connect to electricity. In this case it is attached to a junction box. These have dedicated wiring to the service panel, and implements what's called a hardwired charging station installation. Any electrician can handle running a circuit from a service panel to a junction box, and then wiring a device into the circuit at that junction box.
An alternate is to instead use a power outlet. On the pigtail will be a plug matching the power outlet. Deinstalling the charging station is then as simple as unplugging it from the outlet, and unbolting it from the wall.
Very simplified wiring diagram borrowed from a video embedded below. From the circuit breaker there are two "hot" lines - these are the red and black wires - and there is a ground line - this is the green wire. These directly correspond to the three connectors in every AC power outlet in use. The actual implementation means the circuit breaker is in a service panel, and the three wires are run from the circuit breaker to a junction box or power outlet. The video embedded below is a personal tale of a very simple EV charging station installation.
Safety
Go to your favorite news search engine and type in "electrical fire". You'll be greeted with story after story of homes destroyed because substandard wiring or otherwise overloaded wiring caused an electrical fire. This problem existed before electric cars. Don't let naysayers portray electric cars as dangerous because of electrical fires.
Electrical fires can, and already have, happened with electric car charging equipment. Most of the time the problem is substandard wiring. We bought our electric cars for good reasons, and the last thing we want is to be on the evening news for causing an electrical fire. Therefore it's necessary to avoid that problem by charging safely.
It comes down to ensuring the wiring is up to the job. This isn't a mystery. The electrical code was designed based on experience with electrical fires. An electric car charging station is little different from any other electrical device. The primary difference is that an EVSE running at 6 kiloWatts is a bigger load than the typical house, and it runs for quite a lot longer than usual. Perhaps that means the risk is a little higher, but any electrician worth their salt should be able to calculate the wire thickness required to handle the current.
What you should NOT do is plug into that dodgy old outlet in the garage. That's asking for trouble, so hire an electrician to replace it with a good quality outlet.
Refer to Electric car charging within electrical code and power outlet limits for an overview of electrical safety requirements.
Automatically sharing a 240 volt outlet with a clothes dryer and an EV charging station
Devices going by the name Dryer Buddy or Splitvolt let you share a power outlet to two devices. Typically this will be used with a clothes dryer outlet. In use, you connect the outlet splitter to the power outlet currently used by the clothes dryer, then you connect both the clothes dryer and the EV charging station to the power outlets on the device. Once set up, the device automatically switches power between one or the other depending on which is powered up.
There are cables called power splitters which have one power plug, and two power sockets. Hence, the power is split between two devices. But, these cables do not switch between devices, instead both devices are actively powered at the same time. It's up to you to ensure your electricity consumption doesn't overload the circuit.
We recommend against using the passive power splitters. Instead, we recommend an active power splitter like the Dryer Buddy or Splitvolt. The active devices make sure that only one device is active at a time, limiting the chance of overloading the circuit.
There are two scenarios where these devices are perfect:
- Your service panel is full, and you want to avoid paying for a service panel upgrade.
- You want to avoid paying an electrician to install your charging station. With these devices you're simply plugging things into power sockets.
You might be tempted to avoid even buying this device, and unplug then plug-in the dryer or EV charger as desired. The problem is that the power outlet will physically wear out due to over-use. It's better, especially with high power outlets, to plug something in and leave it plugged in.
In the videos shown below is a demonstration of the "Dryer Buddy" which automatically switches the power back and forth between two outlets.
| Splitter | Power | Input plug | Output plug | |||||
|---|---|---|---|---|---|---|---|---|
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24 amps @ 240 volts | NEMA 10-30 | NEMA 10-30 | |||||
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This device lets you share a single power outlet between two uses, such as powering a clothes dryer along with an electric car charging station. It connects to a NEMA 10-30 power outlet, and provides two NEMA 10-30 outlets. When two appliances are connected to these outlets, power is automatically switched between them based on which is turned on and drawing power. Where this helps is to reduce the number of electrical circuits required for EV charging. The idea is to plug a clothes dryer into the outlet marked Dryer, and an EV charging station into the outlet marked EV. If only one of the two is powered up, then that appliance will receive power. But as soon as both are powered up, the device kicks in to select which gets power, giving preference to the clothes dryer. Normally, each device has its own electrical circuit connecting to the service panel. An unexpected cost of adding EV charging is that sometimes you'll need to upgrade the service panel, because you've run out of capacity. By buying this SplitVolt device you can avoid that cost. This device also lets you avoid the expense of hiring an electrician. It's simply a matter of attaching this to the wall, then plugging things together. The input side of this device uses either the NEMA 10-30 or NEMA 14-30 plugs. This means the device is limited to 30 Amps peak, or 24 Amps continuous. For the EV output socket, you can buy it with NEMA 10-30, NEMA 14-30, or NEMA 14-50. If your power outlets or EV charging station does not have matching plugs, then you'll require an adapter cord. |
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Sharing power between multiple charging stations
Some charging stations have built-in computers offering advanced features, as we discuss later. One such feature comes into play when you have multiple charging stations. Some stations can form a network connection between them, and the stations communicate with each other to limit total consumption below the capacity of an electric circuit.
The result is you could have two 40 amp charging stations sharing the same 50 amp circuit. Instead of installing two 50 amp circuits, you have one circuit, shared between two charging stations. If both stations are operating simultaneous, each will lower their charge rate to 20 amps.
Handling a recharge-from-empty event in 200+ mile range cars
Maybe you decided lower power 16 amp charging is enough. It's good to save some money. You might have a 200+ mile range car, and a full recharge at 3 kiloWatts takes almost a full day. But you typically only recharge enough for the daily commute and 3 kiloWatts is more than enough.
But what happens after you arrive home from a long road trip? Not only are you likely to be exhausted, but your car might be fully discharged. And, you might need to go to work in the morning or for some other reason need to fully recharge the car overnight.
What do you do?
You could have splurged on the higher power charging system. But, as we said earlier that might not fit your budget.
For these exceptional cases, when you're facing "OMG I MUST CHARGE FULLY NOW", it's possible to rely on a fast charging station in the neighborhood.
In a real-world application, someone might have level 1 charging (120 volt 1.2 kiloWatts) at home and be perfectly happy. But a weekend comes up requiring that on Friday evening, on Saturday and on Sunday, that you drive from home to a location 100+ miles away, then return home, and repeat the next day. This required using fast charging at the destination, then using fast charging at a location near home. On each end, level 2 or level 1 charging was used to top up to 100%.
Smart (networked) charging stations versus non-networked
The current version of "modern times" says that tiny computers can (and should) be embedded into every device you see. There are completely computerized ovens that are connected to the Internet, that even has a webcam allowing you to make movies of your food cooking, and all kinds of other Jetsons-like features you'd expect by going over the top with computerized integration of an oven. The cost was outrageous ($1500) but for those who seek the ultimate in technology wouldn't this be "cool"?
The fact is the capabilities of tiny-sized computation devices are growing by leaps and bounds. In a charging station, a built-in computer could handle these functions:
- Scheduling the charging time
- Recording data about charging sessions
- Coordinating the charging time or rate with electricity price signals
- Remote monitoring of charging sessions
- Notification when charging ends
On the flip side, the computerized charging stations are more expensive than the simpler non-computerized EVSE's. Your budget may prefer the lower cost units.
V2G - Powering your house from your car
When the power goes out might it be tempting to use the electricity in your car to keep the fridge running. Some have done that and there is much research going into implementing this safely.
For now this is a matter of research, and it is still "years" before we'll see Vehicle-to-Grid (V2G) become more than just a dream. When V2G does arrive for the masses it'll do more than just handle power outages.
A big issue is that current electric car warranties prohibit using the battery for any purpose other than driving the vehicle. The reason is, perhaps, that the battery will die earlier than would be expected by the miles driven. Manufacturer warranties are based on the miles driven.
Wireless charging
Wireless charging stations are also on the horizon, and can be purchased today as an aftermarket add-on. They still connect to a regular circuit in the service panel, and therefore do not require any changes to the house wiring. Current wireless charging systems have power limitations and won't impose much demand on the power capacity of the house.
Resource guides
San Diego Gas and Electric published
Electric Vehicle Charging Station Installation Best Practices: A Guide for San Diego Region Local Governments and Contractors (PDF) giving excellent advice. Importantly it names the relevant building codes and electrical codes.
National Electrical Contractors Association (NECA) NECA 413-2012, Standard for Installing and Maintaining Electric Vehicle Supply Equipment is the official guidance document for electrical contractors.
Raleigh, North Carolina, has a web page describing the process, and conveniently including a link to download the permit application form:
raleighnc.gov/PlanDev/Homeowner
The US Dept of Energy has a sample permit application form:
afdc.energy.gov/EV_charging_template.pdf
Pacific Gas and Electric has several web pages discussing not only the installation process, but the advantages of electric vehicles:
pge.com/residential/solar-and-vehicles/options/clean-vehicles
Service panel installation videos
