Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

How to plan for and install 240 volt circuit to charge an electric car/vehicle?

Charging an electric car at home is the most convenient refueling experience possible. But most homes were not designed with this in mind. Who in the 1950's thought we would need electric vehicle charging stations at home? Therefore we must consider retrofitting our homes to support EV charging, and if buying a brand new home checking that this is pre-wired. Fortunately this just requires installing an electrical circuit for a 240 volt single phase AC, and wiring up an electric car charger. For most folks this will be easy and cheap. Let's talk about installing an electric car charging station at home, and future-proofing the installation.

At the high level this is a simple project - run wires from the service panel to a place where your charging station will be installed - install the charging station - done.

But to do this right we are going to talk about a complete plan and installation:

  • 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 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 parts cost is about $50-200 dollars for the electrical circuit, and another $200-1000 for the charging station. To fully prepare not just for today's electric car but for a future where your household has 2 or 3 or more electric cars requires a bit more thought.

Keeping the cost low while preparing for future electric car charging needs is a delicate balancing act.

Calculating service panel capacity

Typical home service panel

If you hire an electrician they'll do this planning for you, but it is useful to understand the parameters.

The service panel is the grey box containing all the circuit breakers -- or if your house is old enough, it'll contain real fuses. Most of the circuit breakers are for individual circuits to various places in the house. There might be labels detailing where everything goes, if you're lucky.

The biggest circuit breaker is for the whole house. Turn this off and the entire house will be without electricity.

Each circuit breaker has a capacity rated in Amps. There should be numbers on each circuit breaker for the amperage rating. The circuit breaker rating tells you how many amps can be used on a given circuit.

The 80% rule is important to discuss at this point. For a continuous load, and electric car charging is a continuous load, the current drawn by the load must be 80% of the rating on the circuit. For a 40 amp circuit that means a maximum 32 amps charging rate, or a 20 amp circuit supports 16 amps charging rate. Don't do as I originally thought and charge at 20 amps through a 20 amp circuit. You might be lucky and have that work, but you're risking popping the circuit breaker or worse.

That was before I took some electrical training and learned about the 80% rule. I thought the rating on the components meant that's what the circuit could handle. And, yes, for short term bursts of power, that is what the circuit is rated to handle. But for a long-term use of that electrical circuit, like electric car charging, the rules are to take it easy and only use 80% of the maximum rated capacity of the circuit.

Think of it this way - it's likely that every rule in the electrical code is derived from having diagnosed the root cause of an electrical fire. In other words there is probably a sound technical reason behind every rule, and those of us who haven't studied power electronics well enough should just thank those who have for codifying their wisdom in a set of rules for us to follow.

As I understand the rule -- the sum of each individual circuit capacity has to be less than the capacity of the main circuit breaker. That is, if there are 10 circuits each rated for 20 amps, then the main circuit breaker has to be at least 200 amps capacity.

This step is simple - write down the capacity for each circuit breaker, and add them up. Then subtract that number from the main circuit breaker. That's the spare capacity in your house. If you're lucky there is enough spare capacity to power an EV charger or three.

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 far the car will be from the service panel. It boils down to how much wire must be run from the service panel to the charging station.

The farther the circuit is run the thicker the wires must be to avoid a steep voltage drop.

The "voltage drop" phenomena has to do with the resistance of wires. We don't need to go into the calculations behind this. It means that for a long-enough wire run, the 240 volts at the service panel could become 200 volts at the charging station. Charging 16 amps 240 volts is 3,840 Watts, but 16 amps 200 volts is 3,200 Watts, or a significantly lower actual charging rate. With thicker wires voltage drop is minimized, but it's better to have a shorter wiring.

This means the ideal location to park the car is next to the service panel.

How many electric cars today, and in the future

A household getting one electric car often buys another sometime later, and there may be kids growing to maturity who "need" a car too. Therefore it's good to plan for multiple electric cars. And getting back to the previous section, where will those cars park, and how long must the wiring go?

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. Sigh.

Therefore, should a household plan to have more than one charging station in the future?

Electrical service capacity required to charge the electric car(s)

The simple answer to this question is this equation

service required = number of charging stations x amps per charging station

If you decide two charging stations will be sufficient, each supporting 32 amps charging rate, that means two 40 amp circuits for a total of 80 amps service required. Aside: It wasn't that long ago that 80 amps was enough for an entire house. Meaning, this 80 amps figure is quite a lot of electrical capacity.

What one actually needs is far more complex than noting the car supports 6 kiloWatt charging, and therefore getting a 6 kiloWatt home charging station. For most of us one could argue that 32 amp (6 kiloWatt) charging at home is overkill, see What electric car charging rate do we need at home, at the office, on road trips, at airports, or elsewhere?

The issue you'll wrestle with is the desire for faster charging at home might force you into an expensive service panel upgrade. A service panel upgrade is expensive that's best to avoid.

For a sense of what it takes to install a service panel, scroll to the bottom of the page where there are some videos.

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. As we've suggested elsewhere, keeping a diary for a couple months of where you drive is a great way to evaluate your real driving needs.

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 can supply over 100 miles range in an overnight charging session.

Will having a 200+ mile range electric car mean your driving habits change? It may because you'll be tempted, and should, take it on longer trips. But would your daily commute change? That's unlikely.

Therefore a simple 16 amp 3 kiloWatt charging circuit is more than adequate for most of us most of the time. That requires a 20 amp circuit. Two circuits means 40 amps, three means 60 amps, and isn't it obvious that supporting a 16 amp charging rate will fend off needing a service panel upgrade?

But some of you are anxiously saying "BUT WAIT, I drive 300 miles a day". Maybe you're a Real Estate agent, or you drive for Lyft, or you're a mobile agent of some other kind, and you really are driving all day long every day. Your needs are way beyond what most of us need. Those driving this much truly need higher power charging at home. Even 6 kiloWatt charging, which might supply 200-250 miles range overnight, might not be enough. Some electric cars support 40 amps charging (requiring a 50 amp circuit) or even 80 amps charging (requiring a 100 amp circuit).

See Installing cheap/inexpensive electric car charging at home

Reiterating - how much electrical service capacity will YOU need for electric car charging?

Electrical service capacity is expensive. Your budget may dictate minimizing this cost, and therefore you must go with lower power charging stations. Or you may decide the convenience for higher power charging is worth the cost.

Consider this combination:

  • In the future we'll need 3 electric car charging stations
  • Two can have 16 amps service
  • One can have 32 amps service

This means two 40 amp circuits. The lower-power charging stations has stretched our service panel capacity.

  • All three have 32 amps service

That means three 40 amp circuits, or 120 amps total. Remember, it wasn't that long ago 80 amps was sufficient for an entire house.

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.

What we see is a charging station with a short wire - "Pigtail" is the technical term - connected to a junction box. 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.

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.


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.

Sharing a 240 volt outlet with a clothes dryer or similar appliance

What if your service panel is full, and the garage has a close washer/dryer combination? In the USA clothes dryers are commonly 240 volts and their power outlet could be a useful source of 240 volts with which to charge an electric car. But it is not a good idea to continually switch back and forth between the electric car charging station and the clothes dryer, since that means constantly plugging-in-and-unplugging these plugs.

In the videos shown below is a demonstration of the "Dryer Buddy" which automatically switches the power back and forth between two outlets.

The idea is you plug in a clothes dryer in one outlet, and an EVSE in the other outlet. Normally the clothes dryer is "off" and the EVSE will be given power. Once someone turns on the clothes dryer, the outlet for the EVSE loses power because it is automatically switched to the clothes dryer outlet. When the dryer is done, power is automatically switched back to the EVSE outlet.

Handling a recharge-from-empty event in 200+ mile range cars

Maybe you decided lower power 16 amp charging is enough, even though a full recharge of your 200+ mile range electric car will require almost a full day. Typically you won't need to do a full recharge and the 100 miles range you can gain with overnight charging is sufficient.

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 and need to fully recharge the car overnight.

What do you do?

You could splurge on the higher power charging service, but as we said earlier that might not fit your budget. The 16 amp charging service might even be too much, and you're instead charging on 120 volts.

It'll be tempting to have a higher power charging station at home. A 60 kiloWatt-hour battery pack will require close to 20 hours for a full recharge at 240 volt 16 amps. That obviously will appear daunting especially in the scenario just described.

How often will you face a recharge-from-empty event? That's for you to determine.

Put logically - the simple solution is rather than installing a more expensive charging station at home, to rely on a neighborhood fast charging station for those few times when you're facing "OMG I MUST CHARGE FULLY NOW".

I faced this exact situation last year. We did three days of back-to-back driving the full range of the car, needing to get back on the road early each morning, and only having 1 kiloWatt charging at home. The fast charging station at the nearby Whole Foods made this possible, where we charged to 84% and topped off the car at home using the 120 volt outlet. See Example longer trips with electric cars - putting range confidence into practice

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. The other day I saw a review of a completely computerized oven that's 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. The same issue is playing out in every industry. What advantage can be derived by embedding an internet connected computation device in the product? For example instead of putting buttons and a display on the device, you instead provide a smart phone app that connects to the device using Bluetooth.

For electric car charging stations some useful features in a networked smart charging station include

  • 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

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 (energycenter.org) 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: (www.raleighnc.gov) raleighnc.gov/PlanDev/Homeowner

The US Dept of Energy has a sample permit application form: (www.afdc.energy.gov) 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: (www.pge.com) pge.com/residential/solar-and-vehicles/options/clean-vehicles

Service panel installation videos

Electric Car Level 2 (240 Volt) Charger Installation, Chevy Bolt EV

DIY 240 Volt Outlet/50 Amp Breaker in my Home Workshop- Easiest Install ever!

EV Home Charging Station Installation

Dryer Buddy Plus AUTO with kWh meter Demo for Level 2 EV charging stations - 240v outlet splitter

Range Confidence is Copyright © 2016-17 by David Herron

How to plan for and install 240 volt circuit to charge an electric car/vehicle?

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