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Pushing the limits.

OK my question is about pushing the limits of the 80% rule. First let me say the #1 reason I like the OpenEVSE board is that I have access to it. It is not some manufactures board that is totally locked down and I can’t adjust things or tune them.


That said let’s get the NEC out of the way as I know every time, I post things like this someone will quote me the NEC. So, to put that matter to rest let me summarize the NEC for EVSE.


80% vs. 100%-rated Circuit Breakers


The NEC code is massive. But, 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?


How does load management help to install more charging stations at one location for EV fleet charging load management vs no load management


If you must 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 it’s 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.


OK so we got past the NEC and we are to question.


Let’s take the Advanced kit with the standard 50A relay. We are going to assume it’s equipped with a power cord and charging cord that can handle a continues 50A. It is also connected to a NEMA 14-50 outlet that is installed with the proper gauge wire and on a 50A breaker.


If we follow the 80% rule, we will limit this charging station to 40A. But remember the 80% rule only applies to “continuous loads” of 3 hours or more. Now according to the NEC we can over subscribe if we limit the load withing the NEC rules. Now yes, they are talking about throttling the oversubscribed load to 80%. I am going to read into it just a bit and here is my proposal.


First, I use TeslaFI to track all my cars data, so I have years of data for my charging habits. Looking back year and years there are only a handful of times I needed to charge my car for over 3 hours. So that would mean that in most cases there is no reason that I could not ramp up my charging current past the 40A closer to the 50A limit. Now before someone says anything I would not go to the full 50A or even 48A I am thinking more like 44A. This would be more in alignment with a Tesla HPWC that can be wired on 6AWG @ 48A.


So, to my idea to make this meet code and be fully safe. I see no reason that we cannot set a timer on the OpenEVSE to 3 hours. Example for the above proposed EVSE charge current limit set to 44A. However, if charging session is equal to or longer then 3 hours reduce current to 40A. In this case it will ensure that the load limit is never exceeded at any level. This ensures that the 80% and continuous load rule is always followed. Further the OpenEVSE has high temp limits and the whole reason for the continuous load rule of 3 hours is the temp rise over time.


Looking for some feedback as I would like to implement this on my charging stations.


Yes, I know 40A 44A so close does it really make a difference. Most of the time I just like to do things because I can or to play, not because I need. If you need an argument, it’s over time the charging is more efficient the higher the current. This is shown in years of data in TeslaFI the higher the charge currents the more efficient the charging session. So maybe I am just saving the planet a little bit one amp at a time.


Thoughts anyone?

EV loads do not have the 3 hour qualifier, they are considered continuous immediately and subject to the 80% limitation regardless of time. Technically, on a cool day you probably could get away with higher current and a ramp down without tripping the breaker.

If you charge sessions are mostly under 3 hours, you may want to reduce your charge current not increase it. It is better to have your vehicle finish charging as close to the time of departure as possible so the battery is still warm and has sat with a higher SOC as short of time as possible. Lower current also reduces stress, heat and provides greater safety margin on all the hardware.

Thanks for the reply.

Shoot I was hoping to avoid the NEC debate and just talk about the feature. Maybe since this is not for everyone I should have posted under programming.

The only reason the NEC considers an EV as a continuous load is that it can exceed 3 hours that is clear from the code. In fact any load that last longer than 3 hours is considered continuous. But you know the NEC can burn your house down if you follow it to the letter. Example I have a 240v space heater I used in my garage. The NEC does not consider a space heater as a continuous load because it has a thermostat and normally does not run for 3 hours continuous. But my heater draws 28A on a 30A circuit this is by the manufactures UL approved design. But in the winter when it is really cold this heater runs 24/7 to keep my garage in the 60's. The wires get crazy hot but hey no issue this meets NEC code it is not a continuous load right so it won't burn down my home. Since I know the NEC is a guide and can't account for everything I use my common sense to not burn down my home. I installed a 14-50 outlet on 6 AWG and use an adaptor right at the outlet to convert the 14-50 to the 6-30 this keeps everything cool and nothing gets overloaded. See the NEC has to be taken with a grain of salt it is not all inclusive.

I really just want to talk about how to implement this feature as it is very much like the load balance that the Tesla HPWC uses but based on time.

Again I don't want to debate the NEC and go down a rabbit hole I just want to take about implementing this on an open product.

I like to play with things keeps me from going crazy.

Maybe this is not the right place for this topic?

It is hard not to talk about NEC for a Feature to circumvent NEC technicalities.

OpenEVSE can be fully controlled by MQTT. Setup a MQTT broker and Node Red, you can then create whatever logic you wish to control your station.

This seems to be what I was looking for.


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