Clearing up Confusion over electric vehicles, smart charging, and the NEC 80% rule

Updated: Nov 13


Oversubscribe electric vehicles?

Let’s face it — for many people, when it comes to charging electric vehicles, there is a whole lot of confusion.


When it comes to installing electric vehicle charging equipment, there are many things to plan and consider. For instance, you’ll need charge points, branch circuits, circuit breakers, and more.


Then there’s the issue of responsibility. Who is actually in charge of electrical systems? Who is testing the hardware and setting standards for “behind-the-meter” installations?

The good news is — it doesn’t need to be confusing.


This article will help you cut through the information overload out there and answer the critical questions.


What is The National Electric Code (NEC)


In the US, the NEC provides useful guidance on electric vehicle charging.


The NEC started as a very broad description of how to design electrical systems. It didn’t venture into installation and operation guidance, especially not when it comes to electric vehicle charging.


Fast forward to today, and the NEC has been adopted in all 50 states of the US and has become one of the most important benchmarks for electrical design, installation, and inspection to protect people.


Why the NEC standards matters for your EV charging project


So, what has made the NEC a key part of the safe installation and operation of EV charging projects?


The NEC introduced a new section that specifically describes how to install charge points and Electric Vehicle Supply Equipment (EVSE).


“The electrical conductors and equipment, external to an electric vehicle, that connects an electric vehicle to a supply of electricity by conductive or inductive means, and the installation of equipment and devices related to electric vehicle charging.”

- NEC part 625 (Electric Vehicle Charging and Supply Equipment Systems), 2020 edition


Ok, so that’s a long sentence and may seem a bit confusing. But what the NEC is trying to say is that Part 625 of the code describes the standards on how to connect EVs and charging stations.


This is especially relevant to the design of a charging location, fleet depot, or any location to charge up your electric vehicles.


It doesn’t tell you which equipment to use. You’ll need to decide which equipment works together and strictly follow the NEC rules.


80% vs. 100%-rated Circuit Breakers


80% rule from NEC code

The NEC code is massive. Luckily, we’re not interested in the whole thing!


We’re going to focus on the sections that are relevant to EV charging stations, specifically sections 625.40 and 625.42, which set out rules on how to apply load management with EVs.


Section 625.40 states that each outlet used to charge EVs needs to be connected by an individual branch circuit.


A branch circuit is an electric circuit extending beyond the last circuit breaker. The branch circuit starts at the breaker box and runs to the EVSE (charging station).


Up until 2014, the NEC required that every charge point has a dedicated circuit rate of 100%. “Oversubscription” was not permitted.


But, as charging points don’t usually require full power, the rule was changed in 2014. This gave rise to the additional section 625.42 with revised guidance.


The 80% rule and why it matters for EV charging


Section 625.42 describes how to rate the load of EV charging. This is relevant if you plan to install more chargers than technically “possible.”


It states that

“Electric vehicle charging loads shall be considered to be continuous loads.”

Continuous load means that the maximum current is expected to continue for 3 hours or more. Therefore, non-continuous applies to everything else. In other words, anything less than 3 hours.


This is the tricky part of figuring out.


If all your EV loads are non-continuous, you wouldn’t need to worry about oversubscribing, and you can just size your breakers for 100% of your load.


But, if your EV loads are continuous, you need to follow the 80% rule. This states that continuous load must be 20% below the breaker capacity.


Why can you oversubscribe charging stations on breakers?



load management vs no load management

If you have to treat EV load as a continuous load, how can you install more EV capacity than 80% of your panel breaker? Wouldn’t that violate the rule?


No. Here the next paragraph is important.


Section 625.42 also states:

“Where an automatic load management system is used, the maximum equipment load on a service and feeder shall be the maximum load permitted by the automatic load management system.”

In other words, when you want to run charging stations above 80% of line capacity, you must have an intelligent load management system to maintain control of the loop/loads. This load management system sets limits on the charge point (EVSE) and thereby ensures the 80% rule. You can now oversubscribe breakers.


The good news is that power management software such as Ampcontrol can be set to intelligently split the power at a circuit, panel, site, or transformer level.


Software like Ampcontrol can also combine any of those elements, in which case it will ensure that the load limit is never exceeded at any level. This ensures that the “80% rule” is always followed.


To select your breaker for continuous loads, you need to determine the load on each branch circuit, then calculate the required ampere.


But, you can’t just take 100%. You always need to follow the “80% rule.”


The upshot of all this is that, with Ampcontrol, you can install an average of 40% more electric vehicles and EV charge points per location.


Summary


Hopefully, this article has cleared up some of the confusion surrounding EV charging. To wrap up, here are the main points again:

  • EV charging installation can be confusing, especially when it comes to an understanding of the standards, best practices, and responsibility for testing.

  • In the US, the NEC’s latest version includes a comprehensive section on EV charging installation and maintenance — section 625.

  • Sections 625.40 and 625.42 describe the conditions for installation for continuous and non-continuous loading.

  • The 80% rule is the one you should try to follow for EV charging.

  • When the load is managed by smart charging software such as Ampcontrol, you’ll be able to install 40% more EVs and charging points per location.

  • Load management like Ampcontrol allows you to oversubscribe charge points on breakers.

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