Electric Vehicles and Extension Cords

Manufacturer guidance is clear: Do not use extension cords to charge the car! I’ll tell you what you need to know, then you make your own decision.

Charging with an extension cord (through the mailbox)

The Difference Between a Extension Cord and a Socket in the Wall

What is the difference of plugging into an outlet right next to your main panel using a 100′ extension cord vs plugging into an outlet that is 100′ away from your main panel without an extension cord?

  • The far away outlet is connected by wires installed inside a wall, installed by a professional. The extension cord is laid by a lay person. You need to make sure you pick an extension cord that matches what would be in the wall, and what you want to power. Thousands of home fire are started by (often cheap) electric extension cords in the US every year. Be careful.
  • Extension cords have many disadvantages even if properly sized. They are unsightly tripping hazards and they break over extended use. But sometimes adding another socket is just not an option – e.g. if you rent your home. Be careful.

Electric Vehicles draw a lot of electricity when charging compared to anything else average consumers use, increasing the risk of fire. If you were a car manufacturer, you wouldn’t recommend the use of extension cords either. So proceed at your own risk!

Picking the Right Size

A regular household outlet in the US provides electricity at 110V (Volt). It can provide up to 15A (Ampere) without danger to the wiring. For our extension cord, we need to make sure our cord can handle 15A at 115V. Many extension cords cannot!

The two main considerations are

  • Length
    We will limit ourselves to extension cords less than 50 feet for simplicity. Longer cords just need to be thicker.
  • Thickness (aka Gauge)
    Thickness of extension cords is listed using American Wire Gauge (AWG) or just Gauge, and the smaller the number, the thicker the cable.
    We lookup the required thickness at the ESFI (Electrical Safety Foundation International) which has more detailed information, including thickness for cords up to 150ft.

To extend a common household socket (NEMA 5-15) by 50ft, you need at least a 14 Gauge cord.

However it may be worth purchasing a 12 Gauge cord for extra safety and to have less power loss in the cable. The thicker the cable, the more electricity goes into your car, instead of heating up the cable. The price difference is not material.

Other Considerations when Selecting

  • Buy an outdoor rated cable even if you don’t plan to use it outdoors. You never know, and the garage floor could get wet, too.
  • Buy a cable with a little indicator light telling you if there is power. It helps greatly to check if you actually have power.
  • Buy one with three-prong plugs.
  • Buy one with only one receptacle (not a splitter). You should not plug in anything other than the car.
  • Buy only cords approved by an independent testing laboratory, such as Underwriters Laboratories (UL), Intertek (ETL) or Canadian Standards Association (CSA).

Other Considerations When Using

  • Do not daisy chain, i.e. do not connect one cord to another. Buy a longer cord if you have to (and thicker if you need more than 50 feet)
  • Do not run through walls. If the cord is covered the heat cannot escape. Hot cables are fire hazards.
  • Do not use an extension cord for more than one appliance.
  • Never use a cord that is getting hot.
  • Check for damage regularly. Don’t use a damaged cable.

Extending Dryer Outlets / NEMA 5-20

Extending a Dryer Outlet works similarly, you just need a thicker cable. A NEMA 5-20 (dryer outlet) can provide up to 20A at 110V. According to the same source at ESFI, you will need a Gauge 10 cable for up to 50 ft.

A NEMA 5-20 has a different receptacle, one of the two flat prongs is actually a T. If you want a smart EV (e.g. a Tesla) to know that it’s a 20A rated circuit, you need to buy the NEMA 5-20 adapter (the process may vary for other manufacturers). That adapter will not fit in a regular extension cord, so you need an extension cord that has NEMA 5-20 receptacle, and it will have a NEMA 5-20 plug. That plug will not fit into a regular household outlet, only into a NEMA 5-20.

Extending RV Outlets / NEMA 14-50

A NEMA 14-50 RV outlet provides some serious charging power (Level 2). They can be found in pretty much any RV campground in North America, and even in some regular campgrounds. Reserve an electric site.

There are extension cables available, but they are very heavy and expensive. It’s not something you buy or carry around just in case. You also don’t really need it, as they are typically designed to park a huge RV right next to them, so your car easily fits. You will need an adapter for your charger.

Tesla at Sunset in Rancho Jurupa, Riverside, CA
Charging overnight in Rancho Jurupa, Riverside, CA
The Tesla Mobile Connector is long enough.

Fine Print

The household outlet specification is for 15A peak load. For sustained load – like charging an EV for hours – we only want to use 12A. The Tesla will do that automatically. If it would use 15A, the fuse or circuit breaker may allow it, but the wires may heat up in the walls, and possibly cause electric fires. We still size our cable to 15A, because we may plug in something else that will do 15A later. Also, we want to make sure the cable doesn’t get warm over extended charging times. Similarly, if we charge from at NEMA 5-20, we would only use 16A, and 40A on a 14-50 RV receptable.

Last Updated on July 30, 2021.