Understanding the Differences in Electrical Currents and the Role of Extension Cords in Electric Cars

Electric cars require specific types of electrical currents to charge their batteries safely and efficiently. This article explains the differences between the electrical current from a wall outlet and an extension cord, and how electric cars recognize these differences. Additionally, it highlights the importance of using the correct type of extension cord to prevent damage and ensure safe operation.

Understanding Electrical Currents in Electric Cars

When it comes to charging an electric car, the electrical current from a standard wall outlet or an extension cord is fundamentally different from the current provided by a direct connection to a high-capacity charger. The electrical current from an extension cord is limited by the thickness of the wire. Typically, many extension cords are made with 18-gauge wire, which is much thinner than the 14-gauge wire commonly found in wall outlets.

In the United States, the thinnest wire that is usually hidden in the wall and allowed to power outlets is 14 gauge. However, the car does not 'know' that it is pulling 12 amps continuously from a cord intended for a maximum of 10 amps. If a thin extension cord is used, it will become quite warm, and under certain conditions such as bundling or coiling, it may get so hot that it melts. This can pose a significant risk of electrical fires.

Adequate Extension Cord Length and Gauge

Using a correctly-sized extension cord can mitigate these risks. For example, a short 10-foot or less 16-gauge cord rated at 13 amps should be safe. A somewhat longer 25-foot or less 14-gauge cord can also be used safely. For heavy-duty applications, a 12-gauge cord should work fine even at 50 feet. Bottom line: car manufacturers do not want their customers to misuse extension cords and therefore advise against using them, as customers may not thoroughly understand the limitations.

Charging via Regular Electric Outlets

An electric car plugged directly into a regular 110V outlet or through an extension cord charges at approximately the same rate. However, very long extension cords can cause a voltage drop, leading to slower charging. While the extension cord is not the primary issue, it is important to note that cars at home are often connected to two-phase sockets that provide 8.8 kW. Standard single-phase sockets typically provide only 1.8 kW. Therefore, even with a regular extension cord, you start with 1.8 kW and see a reduction as the cable becomes very long.

On-Board Chargers vs. Outboard Chargers

Electric cars have two main charging methods: outboard chargers and on-board chargers. Outboard chargers are wall- or pedestal-mounted and connected to high-power three-phase power supplies, such as 240V and 40A. These chargers convert standard grid power to whatever output is needed based on the car and charge port type.

On the other hand, most electric cars come with an on-board charger, which is a lower-performance device. On-board chargers convert 120V single-phase "house current" from a standard power cord into the high-voltage DC required to charge the batteries. These chargers are limited to a maximum of 120V 15A, and most extension cords are not rated for this power. This fact is often a reason for car manufacturers to issue a clear warning against using extension cords.

The Importance of Proper Extension Cords

When most people think of an extension cord, they imagine something like a common indoor extension cord, which is not rated for such powerful applications. Instead, you would need an extension cord made from thicker, sturdier wire, such as 12 gauge copper wire. Even with such thicker cables, closely monitoring the cord’s temperature is crucial since running high power through the cord can cause it to heat up significantly.

The primary issue with extension cords in electric cars is that they are designed for devices pulling relatively insignificant amounts of power, such as 100W light bulbs or 250W leaf blowers. Hooking up something that requires all available power can melt the cord and cause an electrical fire. Product liability concerns have led to manufacturers issuing strict guidelines against using extension cords, to minimize the risk of accidents and property damage.