NC has an NEC interpretation problem that will affect all ESS systems

[solarmn]

A state electrical inspector is causing issues and I wanted to share his email with you all to see if it makes to you and if any other state has experienced this:

Good evening, all,

This email is a brief review of the discussions that have been had. A more formal response will be provided at a later date.

To clear any misunderstandings, I at no point interpreted that an ESS used with an Article 705” Interconnected Electric Power Production Source” system would be permitted to be installed per Article 710 or a “Stand Alone System”. It is my understanding that Article 710 deals only with systems that at no time are connected to a utility power source. This appears to be the purpose of creating Article 710 for the 2017 version of the NEC. To my knowledge I have been consistent with this fact. Article 702 is the appropriate standard to use when sizing an ESS used for an optional system and must meet the requirements of 702.4 & 702.5.

I am unable to draw a connection directly from Section 705.40 to Article 710 or Section 710.15. The presence of the term “ island mode” or “ stand alone system” do not immediately send a user to Article 710. The text does not send the user to either Article 710 or Section 710.15. Neither Articles 705 nor 710 are exclusive to ESSs. Both of these articles could also apply to wind, prime movers, or other power production sources. A building or structure that is not connected to a serving utility but has only a generator as its power source may fall under Article 710, and a generator that is interconnected and is in parallel with the serving utility would then fall under Article 705. If the latter is able to operate separately from the utility in “island or stand alone” mode, this generator, if not used as an emergency or legally required system, when then be required to meet the applicable requirements of Article 702.

As I stated above, a more extensive and thorough interpretation is being authored and will be released in the near future as soon as it is vetted.

This interpretation will include all three versions of the State Electrical Code currently posted on our website so that it will be consistent through all that would apply.

Feel free to reach out to me with any further questions or concerns.

 

[don_resqcapt19]

What is the issue? That is the intent of the rules in the NEC.

 

[wwhitney]

What is the issue? That is the intent of the rules in the NEC.

The issue, I believe, is whether 702.4(B)(2) on system capacity with an automatic transfer switch applies to an ESS with an MID. Or alternatively, when the MID is in island mode, the controlling requirement is 710.15(A).

Which raises the question, if you have an engine generator and ATS, why do we have 702.4(B)(2)? Will an automatic transfer into an overload of the generator cause damage to the generator, rather than it shutting down gracefully? To my knowledge, ESS will just shutdown gracefully if overloaded.

My argument that 702.4(B)(2) does not apply to an MID is that an MID is not transfer equipment. If you read the definition of "transfer equipment" it basically says that it acts as a double throw switch, transferring power from one source to another. The standby source is disconnected when the grid is up.

In contrast, an MID acts as a single throw switch. The ESS is always connected, it's not on standby. All that happens is that the grid is disconnected, there is no transfer. Therefore an MID does not create a system subject to 702.4(B)(2).

Cheers, Wayne

 

[Joe.B]

710.1 Scope. This article covers electric power production systems that operate in island mode and installations not connected to an electric power production and distribution network.
Informational Note: These systems are capable of operating in island mode, independent from an electric power production and distribution network, and include isolated microgrid systems or interactive with other power sources. Stand-alone systems often include a single or a compatible interconnection of sources such as engine generators, solar PV, wind, ESS, or batteries.

 

[don_resqcapt19]

The issue, I believe, is whether 702.4(B)(2) on system capacity with an automatic transfer switch applies to an ESS with an MID. Or alternatively, when the MID is in island mode, the controlling requirement is 710.15(A).

Which raises the question, if you have an engine generator and ATS, why do we have 702.4(B)(2)? Will an automatic transfer into an overload of the generator cause damage to the generator, rather than it shutting down gracefully? To my knowledge, ESS will just shutdown gracefully if overloaded.

My argument that 702.4(B)(2) does not apply to an MID is that an MID is not transfer equipment. If you read the definition of "transfer equipment" it basically says that it acts as a double throw switch, transferring power from one source to another. The standby source is disconnected when the grid is up.

In contrast, an MID acts as a single throw switch. The ESS is always connected, it's not on standby. All that happens is that the grid is disconnected, there is no transfer. Therefore an MID does not create a system subject to 702.4(B)(2).

Cheers, Wayne

That system is not a stand alone system and nothing in that article would apply. It is acting as an optional standby system and the rules in 702 apply.

 

[wwhitney]

That system is not a stand alone system and nothing in that article would apply.

Per the scope in 710.1, Article 710 doesn't just cover "installations not connected to an electric power production and distribution network," but it also covers "electric power production systems that operate in island mode." That precisely describes an ESS when the MID is open and the system is operating in island mode.

So Article 705 applies when the MID is closed and the system is in grid-interactive mode; Article 710 applies when the MID is open and operating in island mode.

It is acting as an optional standby system and the rules in 702 apply.

Again, it's not on standby, ESS often are charging and discharging when the grid is up for demand reduction, TOU arbitrage, time shifting PV production etc. Rules in 702 may apply (some further thought is required for that), but 702.4(B)(2) does not, as there is no automated transfer equipment.

Cheers, Wayne

 

[Dennis Alwon]

I wish you guys would explain what all these initial stand for so I don't have to keep looking it up. lol
TOU- Time of Use?
MID- Measuring Instrument Directives?
ESS- I assume energy storage system

 

[wwhitney]

I wish you guys would explain what all these initial stand for so I don't have to keep looking it up. lol
TOU- Time of Use?
MID- Measuring Instrument Directives?
ESS- I assume energy storage system

Yes on TOU and ESS. MID is Microgrid Interconnect Device, it's an NEC defined term.

Cheers, Wayne

 

[LarryFine]

I wish you guys would explain what all these initial stand for so I don't have to keep looking it up. lol
TOU- Time of Use?
MID- Measuring Instrument Directives?
ESS- I assume energy storage system

"Excuse me, sir. Seeing as how the V.P. is such a V.I.P., shouldn't we keep the P.C. on the Q.T.? 'Cause if it leaks to the V.C. he could end up M.I.A., and then we'd all be put on K.P." ~ Robin Williams as Adrian Cronauer in Good Morning, Vietnam

 

[Dennis Alwon]

FWIW, I updated the abbreviations that are listed on the forum here on post 2,556.801 haha

 

[don_resqcapt19]

Per the scope in 710.1, Article 710 doesn't just cover "installations not connected to an electric power production and distribution network," but it also covers "electric power production systems that operate in island mode." That precisely describes an ESS when the MID is open and the system is operating in island mode.

So Article 705 applies when the MID is closed and the system is in grid-interactive mode; Article 710 applies when the MID is open and operating in island mode.


Again, it's not on standby, ESS often are charging and discharging when the grid is up for demand reduction, TOU arbitrage, time shifting PV production etc. Rules in 702 may apply (some further thought is required for that), but 702.4(B)(2) does not, as there is no automated transfer equipment.

Cheers, Wayne

That is very simply a mistake that will be corrected in the 2026. The correction in the scope is based on the definition of stand-alone system in Article 100. The term "island mode" should have never been in the scope of 710. The language change in the scope made for the 2020 did not have any technical substantiation and was not even suggested by the PI that resulted in the FR.
The NC interpretation matches the intent of the code.

 

[wwhitney]

That is very simply a mistake that will be corrected in the 2026. The correction in the scope is based on the definition of stand-alone system in Article 100. The term "island mode" should have never been in the scope of 710. The language change in the scope made for the 2020 did not have any technical substantiation and was not even suggested by the PI that resulted in the FR.
The NC interpretation matches the intent of the code.

OK, the 2017 NEC 710.1 Scope was:

"This article covers electric power production sources operating in stand-alone mode."

Certainly the inclusion of the word "mode" suggest that power production sources may operate in different modes, and Article 710 covers the case that they are operating in stand-alone mode. Which describes how an ESS plus MID operates when the MID is open and the grid is disconnected. The inverter in many (most?) ESS has two different operating modes, grid-following and grid-forming, and when the MID opens, it is in grid-forming mode. A stand-alone mode.

The 2017 NEC definition of "stand-alone system" is "A system that supplies power independently of an electrical production and distribution network." Which describes the operation of a grid forming inverter on a microgrid disconnected from the grid.

And looking at the 2026 NEC First Draft, nothing changes. 2026FD NEC 710.1 is "This article covers stand-alone systems." While the definition of "stand-alone system" is "A system that is not connected to an electric power production and distribution network." Which again describes the premises wiring microgrid and power sources when the MID is open and the system is disconnected from the grid.

So an ESS in island mode is definitely subject to Article 710. That, of course, doesn't mean it isn't also subject to other articles, like 702. So far you haven't commented on 702.4(B)(2), either on why it's in the NEC, or whether an MID constitutes transfer equipment. I would be interested in your thoughts on that.

Cheers, Wayne

 

[wwhitney]

FWIW, the 2023 NEC definition of "stand-alone system" is " A system that is not connected to an electric power production and distribution network." This definition comes from FR 9350-NFPA 70-2021, which has the accompanying committee statement "This revision clarifies that a stand-alone system is one that is not connected to a utility. The definition is modified to complement the definition of a microgrid system."

For comparison, the 2023 NEC of "microgrid" is "An electric power system capable of operating in island mode and capable of being interconnected to an electric power production and distribution network or other primary source while operating in interactive mode, which includes the ability to disconnect from and reconnect to a primary source and operate in island mode."

So an ESS with MID is certainly a microgrid. However the committee statement for FR 9350-NFPA 70-2021 suggests that the definition of "stand-alone system" was intended to exclude all microgrids. If so, they have clearly failed in crafting the definition. The use of the simple present tense "is" in the definition means that whether a system is stand-alone depends on the current state of any controlled connections to the grid. So for a microgrid with MID, when the MID is open (disconnected), you have a stand-alone system. While when the MID is closed (connected), you no longer have a stand-alone system, obviously.

If the intention is for "stand-alone systems" to exclude all "microgrids" then the definition needs to be something like "An electric power system without any provision for connection to an electric production and distribution network." Or perhaps that should be "without any automatic provision".

Until that definition is changed, any microgrid in island mode is a "stand-alone system."

Cheers, Wayne

 

[wwhitney]

Also, in re MIDs and 702.4(B)(2), it is worth noting that UL 1008 "Transfer Switch Equipment" covers ATSs. While UL 1008B "Source Interconnection Switches" covers MID and specifically excludes UL 1008 Transfer Switches (Scope 1.4). So an MID is not transfer equipment.

Cheers, Wayne

 

[LarryFine]

FWIW, I updated the abbreviations that are listed on the forum here on post 2,556.801 haha

I have another one for you: TLDR: Too Long, Didn't Read.

 

[jaggedben]

That is very simply a mistake that will be corrected in the 2026. The correction in the scope is based on the definition of stand-alone system in Article 100. The term "island mode" should have never been in the scope of 710. The language change in the scope made for the 2020 did not have any technical substantiation and was not even suggested by the PI that resulted in the FR.
The NC interpretation matches the intent of the code.

Well, the code still says what it says and many ESS installers have acted under the understanding that 710 applies to such systems. We should also ask ourselves why the rules should be so different between 710 and 702.

 

[Elect117]

NEC 2020 handbook's Enhanced Content:

"Article 710, which covers stand-alone systems, and Article 702, which covers optional standby systems, are not mutually exclusive. In respect to how the two articles interact, a stand-alone system operating in island mode (such as a generator or a solar PV system) serves as the alternate electric power production source for the optional standby system loads covered by Article 702. The connection of the stand-alone system to the optional standby system loads can be done automatically or manually using transfer equipment or multimode inverters. The capacity of the stand-alone source is affected by whether the optional standby system loads are connected automatically or manually as specified in 702.4(B)(1) and (B)(2)."

I think they are pretty clear on it. You are allowed demand factor reductions and use of 220.87 by 702.4(B)(2). Isn't that enough?

** Not in NEC 2017, that addition came in 2020 **

 

[tortuga]

So far you haven't commented on 702.4(B)(2), either on why it's in the NEC, or whether an MID constitutes transfer equipment. I would be interested in your thoughts on that.

Yes some if not most generators can be damaged by overload, there should be an exception for devices that can shutdown gracefully under overload.
The interesting thing about the 2023 wording is also you need a listed article 750 Energy Management System (EMS) which the major generator brands ATS load management systems are not listed as. So if you have a Generac ATS with a load shed module that would not comply with the 2023 702.4, for example.

 

[jaggedben]

Yes some if not most generators can be damaged by overload, there should be an exception for devices that can shutdown gracefully under overload.
The interesting thing about the 2023 wording is also you need a listed article 750 Energy Management System (EMS) which the major generator brands ATS load management systems are not listed as. So if you have a Generac ATS with a load shed module that would not comply with the 2023 702.4, for example.

Oh fer cryin' out loud.
At least 702 does not actually say 'listed' and 750.6(3) has some options.

 

[wwhitney]

NEC 2020 handbook's Enhanced Content:

". . .The connection of the stand-alone system to the optional standby system loads can be done automatically or manually using transfer equipment or multimode inverters. The capacity of the stand-alone source is affected by whether the optional standby system loads are connected automatically or manually as specified in 702.4(B)(1) and (B)(2)."

FWIW, that's not quite what 702.4(B)(1) and (2) say. They refer to transfer equipment, automatic or manual. An ESS with MID has no transfer equipment.

Cheers, Wayne