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July 16, 2022
Paul Gipe

Charging an EV with an Extension Cord from an Outlet?


I got this question recently.

“Any thoughts on extension cord charging off a regular outlet? I’ve done a little research and the reviews are mixed.”

My knee-jerk reaction was the old admonition, “Don’t be pennywise, pound foolish.” I don’t know whether my response was appropriate or not as I don’t know their motivation for asking the question.

I don’t know if they asked that question because they were concerned about a temporary arrangement. Or if they were thinking about using the extension cord and a 120-volt outlet as a way around investing in a suitable EVSE (Electrical Vehicle Supply Equipment) what’s otherwise known as a home charge station.

Temporary Use May be OK

When we first got our Leaf (long ago in a land far away), we charged for a few weeks off an outdoor receptacle while we waited for the electrician to install our EVSE.

This was temporary. The extension cord was short. The outdoor outlet was new and it was rarely used.

It maybe surprising, but outlets (designated R or Receptacle in the trade) do wear out. When they do, they should be replaced as worn outlets can create a fire hazard.

Worn outlets can be problematic in older homes or commercial buildings. Frequent use of outlets can lead to worn contacts or loose wires that make them unsafe. When in doubt, hire an electrician to inspect the outlet.

EVs are Not Toasters

Outlets, especially those in a kitchen, are designed to handle the current drawn by a toaster at 120-volts. Toasters draw about 10 amps or 1.2 kW (10 A x 120 V = 1,200 W) from the outlet when their heating elements are powered.

However, an EV plugged into the same outlet could draw as much as 12 amps, 1.44 kW, continuously. If there are any loose connections they can overheat the outlet and start a fire.

For this reason, the default charge setting on GM’s Chevy Bolt at 120-volts is only 8 amps or 0.96 kW. This is to compensate for older outlets. For the Chevy Bolt, charging at this level would require 66 hours—nearly three full days--to fill the battery from empty to full.

If the Bolt driver is confident that the outlet is safe, they can manually raise the setting to 12 amps and charge at 1.44 kW. Still, this would require nearly two days to fully charge a 66 kWh Bolt.

Extension Cords

We haven’t even gotten to extension cords yet. Here, the same concerns apply. Will the connections overheat? Are the connections and the cable rated for the current that will flow through it continuously?

Electrical ratings are confusing to the uninitiated. In North America, electrical devices are rated on what current they can handle intermittently. When charging an EV, where current is drawn through the conductors continuously, the device is down rated 20%. So a 15-amp receptacle can only be used at 12 amps to charge an EV (15 A x 80% = 12 A).

As an extension cord length is increased, its current-carrying capacity is decreased. Thicker cables can carry more current than thin cables. Without wading into the gauge or thickness of the wires in the cable, let’s just say to charge an EV you would need a very short and very thick cable.

What you would need as an extension cable, for example, would be something thicker than the cable from the portable EVSE that came with the car. Those cables are so thick because they have to carry high currents continuously.

So yes, charging an EV from an outlet with an extension cord can be done, has been done, and is being done. For some drivers, that’s all they’ve ever used. However, the key is to do it safely and most people don’t have the skills or knowledge to do so safely.

I don’t recommend charging an EV from an outlet at 120-volts, or using an extension cord. Temporarily, yes, if you’re careful and you’re confident you know what you’re doing.

Maximizing Your EVs Utility

For us, there was never a question of using the extension cord or the 120-volt outlet to charge our car permanently. To get the most out of our EV, to make it as useful as possible, we always planned to install a dedicated EVSE that would charge at 240-volts. It was just part of the cost of moving to an EV and we had budgeted for it.

Installing an EVSE by a licensed electrician is like building your own gas station. It’s where you charge 90% of the time. It’s still cheaper than going to a gas station even when you include the cost of having the EVSE professionally installed.

When it’s done right you can sleep peacefully and not worry about the safety of your family and you’ll wake up in the morning with a fully charged car.

I recommend at least a 40-amp EVSE. That means it can charge your car at 32-amps at 240-volts or more than 7 kW. For a Chevy Bolt, a 40-amp EVSE will charge the car overnight.

What this means in practice is that every day you come home and plug your car in, it will be ready in the morning with a full battery. This is like having a gasoline tanker truck driving to your house and filling your tank every night while you sleep.

Installing a high power 240-volt EVSE means you will always have a car that is ready to do what you want whenever you want. This is the best way maximize the utility of driving an EV.

Some Ways to Economize on the EVSE

You don’t always have to wire a new circuit just for your EV. Some homes already have a 240-volt circuit that’s not being used. You may have an abandoned 240-volt circuit in the garage for an electric dryer or a welder left by the previous owner. If that’s the case, you can mount the EVSE nearby and plug it into the 240-volt outlet, assuming that the charge cable will reach your car.

Another way to save a few dollars is to buy an EVSE that is rated at less than 40 amps. For me, the savings were not worth it.

Many new EVs today come with a mobile or emergency charge cable that is rated at 120 volts or 240 volts. If this is the case, you could use the car’s included mobile charge cable to charge your car at 240 volts. Keep in mind that how you mount the EVSE is important to how it will operate safely. For example, in most jurisdictions you can not mount this EVSE outside as it has to be hard wired. You can use it in a garage, but there should be a way to mount the EVSE so it doesn’t hang on the plug connector.

Unless your EV is a Tesla, most mobile charge cables won’t give you any more than 3 kW. That’s enough for some, but it wasn’t enough for us. 

To summarize, yes you can use a short, heavy-duty extension cord to charge an EV from a wall outlet temporarily if you’re confident the outlet is safe. However, I don’t recommend it. It’s much safer if you have a licensed electrician install an EVSE—or charge station--on a dedicated 240-volt circuit. This will enable you to get the most out of your EV.

 


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