We’re going to get a little bit technical here, but before we get started, this article is intended to be an overview. Always consult a certified electrician and applicable building codes when dealing with electrical installations. We have a network of electricians that can help with that.

Pop Quiz!

Question: Can you safely charge your EV using a 40 Amp EV charger on a 40 Amp circuit?
Answer: No!
Why Not? Read on!

Safety First

The NEC (National Electric Code) defines safety electrical standards for homes, and state electric codes often build on the NEC standards. Even before electric vehicle (EV) home charging was commonplace, the concept of “continuous use” of a circuit was defined by NEC as any electric load where the maximum current is expected to continue for three hours or more. Think of electric water heaters, electric baseboard heaters and snow melting cables that can be on for hours at a time.

The NEC 80% rule stipulates that electrical circuits should not be continuously loaded (three hours or more) to more than 80% of their maximum rated capacity.

Why It Matters

EV charging can be expected to continue at its peak level for more than three hours so it’s important that the EV charging activity does not overload the circuit’s rating. This means that an EV charger rated for 40 Amps should only be connected to a dedicated branch circuit of 50 Amps or more, otherwise it could stress and overheat your wiring. Similarly, a 32 Amp charger requires a 40 Amp or greater circuit rating.

Understanding EV Charging with the 80% Rule

Most people may not be aware of this, and reasonably (but mistakenly) assume that they may charge at 48 Amps on a 50 Amp circuit. However, according to the NEC, this can lead to repeated heating and cooling of the circuit components and put your wiring and home at risk of fire.

Circuit breakers are an integral safety feature in modern electrical systems. They work by automatically cutting off the flow of electricity when the current exceeds the breaker’s rating. Because a circuit breaker is not designed to detect continuous use below its rating, a circuit breaker likely will not trip under such a load when it’s below the circuit breaker’s rating.

What Should You Do?

For specifying and installing a home EV charging circuit, always work with a licensed electrician to perform the design and installation. They will make sure that all components in the circuit are rated for the appropriate load. Nonetheless, it’s good to know about the 80% rule so don’t hesitate to ask the installer questions.

Just be aware that most home chargers have a mechanism to reduce the charging Amps to a safe level to match the 80% rule for circuits. In the example shown, this home charger can be set to several Amp levels using internal switch settings based on the circuit’s maximum amperage.

80% Rule While Traveling

When visiting friends and family, you might have the opportunity to charge your vehicle from, say, an electric dryer circuit. Most electric dryer circuits are rated for 240 Volts and 30 Amps (but please check to verify) which means for EV charging, they should only be loaded to 80% of 30 Amps = 24 Amps.

Even if you’re just using a standard 120 Volt 15 Amp outlet, make sure that your charging current is set to a maximum of 12 Amps (80% of 15 Amps).

Many EVs (Tesla example shown) will let you reduce the charging amps to a safe level when using a mobile charging cable. Either on the car’s screen, the car’s smartphone app or sometimes on the charging cable itself, you can reduce the charging amps to a safe level.

Take Away

While most EV charging is already pre-configured for the right current level, it’s a good idea to understand the NEC 80% rule when configuring home charging for the first time, and for special situations like travel charging. Safety always comes first!

Need Help with a Home Charger Installation?

We can help no matter where you are in your EV journey. Find EV resources including licensed electricians in your area that can help with your install at: https://consumersenergy.com/ev.