2020 NEC 710.15(A) Supply Output

[Pressure]

If you read this code then you can assume that If you have a PV System tied to a BESS, then the combined output rating need only be as large as the single largest load posed in the system to be backed up in a stand-alone situation.

I am faced with designing a system that has an EV charger as its largest load. A 60A 2P that is likely pulling the full 48A continuously. Now if I use the rating of the two BESS units then I have 41.66A of continuous output, and have not met the requirements of 710.15(A). But if I add my PV of 39.15A, then I have a total stand-alone system size of 80.81A, and have met the requirements of 710.15(A). My personal opinion is that I should not count the PV. Even though the NEC says I can. There could be multiple times when the system is in stand-alone operation at night, when PV is not available, and the true output is 41.66A. At that point the charging of the vehicle would kill the system.

Has anyone faced this challenge in system design? Are there any clarifications to this rule? I want to have a discussion about how this rule could be abused to make poor designs for Stand-Alone Systems.

 

[jaggedben]

What code cycle?

In the 2020 NEC they added an informational note.


Informational notes are not officially considered binding, but in this case I think it gives pretty concrete guidance to an AHJ as to how to interpret the requirements.

So for example if you have some kind of smart control that ensures the EV charger is not connected when the sun doesn't shine, maybe you're okay.

I have to question the wisdom of having a 48A EV charger connected to this system without some kind of smart control to limit use and/or amperage in a stand-alone mode. Even if code compliant, I would not do it.

 

[Pressure]

2017 is the first year we got Article 710 for stand-alone systems. You are correct in that 2020 the Informational Note was added.

I find that there are potential abuses with this rule. In the majority of my training for BESS, direct from multiple manufacturers, they site this rule to justify system sizing. But I run into situations where, like this one, it just isn't practical to design around this rule without taking into account the variable sources. Or the lack of the variable source during stand-alone operation.

So, it is really up to the designer to just "know better" when making the system?

I would not consider an EV a specialty load that is meant to be charged from a "variable source". Most EV's get charged at night as peak demand goes off. EV's are mostly too large to be on a stand alone system. But this rule, when we have the option to add the variable source, allows for it. It is definitely unwise in my opinion as well.

 

[jaggedben]

...

So, it is really up to the designer to just "know better" when making the system?

...

I mean, yeah, the code is not a design manual.

To some degree I don't see why the code should have these capacity requirements at all, although I suppose it could impact safety to the extent that if the equipment isn't of adequate capacity then people might do unsafe things to the equipment in an attempt to work around that.

 

[Pressure]

Greatly appreciate your response. It did help.

 

[jaggedben]

...

I find that there are potential abuses with this rule. In the majority of my training for BESS, direct from multiple manufacturers, they site this rule to justify system sizing. ...

Do they tell you that you can use the combined PV and ESS output? Because in my experience they just say to use the BESS rating. FWIW.

Also fwiw, there has been some debate on this forum as to whether these systems must also comply with Artilice 702. IIRC, at least one person mentioned an AHJ requring that. Knock on wood, but I've yet to have an AHJ check compliance with either articles requirements. I still use prudence because I care about the result for the customer, both for their sake and because I don't want callbacks.

 

[Pressure]

My friend and mentor just mentioned that he was facing the same 702 issue with an AHJ. They wanted him to comply with both. 702 power sources do not interact with the primary/utility power source. But Energy Storage System/706 do.

There needs to be clarification in a future code cycle.

I will go back through my manufacturers resources to be sure I didn't gloss over the information they were trying to convey. Sometimes I find that they contradict themselves in some of the tech briefs vs. the installation manual, vs. the training courses. The updates come at us so fast and frequently.