NEC 2017 710.15

[BayElectric]

Does anyone understand the requirement of largest utilization equipment when it comes to PV system?
Personally, it doesn't make much sense to me as other, smaller loads consuming appliances can pose a problem to the supply of the largest utilization equipment, let alone that some installers I've talked with don't understand the requirement completely.
Anyone familiar with the subject and can clarify it?

 

[pv_n00b]

The NEC is not a design manual so as far as the NEC is concerned the stand-alone system does not need to operate well, it just needs to be safe. So it requires that the supply only be able to supply the largest single load in the system. As long as it can supply the largest load it does not matter to the NEC that if you turn on other loads it overloads the supply. That's a design issue that is the responsibility of the designer to get right.
This is actually intentional. Otherwise, an AHJ could require that the supply be large enough to run all the loads at the same time even though all the loads will never be on at the same time. That would lead to an oversized supply that is more expensive than it needs to be. So the NEC putting in this basic minimum of supplying the largest single load is a good thing.

 

[BayElectric]

I appreciate your response, but if this is the reasoning for it, it's one of the worst I've ever heard. If the NEC can require load management for generator operated ATS, it can do the same for PV operated ATS.
Not sure how safe it is when A/C unit keeps trying to kick in but doesn't have enough power to do so, or even your fregirator if it doesn't have enough power to function properly.

 

[pv_n00b]

The NEC does not care if an electrical system performs, only that it is electrically safe. It's right in 90.2(A). That's just how the NEC works. I'm not providing a reason, I'm just pointing it out.

 

[jaggedben]

I appreciate your response, but if this is the reasoning for it, it's one of the worst I've ever heard. If the NEC can require load management for generator operated ATS, it can do the same for PV operated ATS.
Not sure how safe it is when A/C unit keeps trying to kick in but doesn't have enough power to do so, or even your fregirator if it doesn't have enough power to function properly.

I would agree that 710 could make an exception for systems that instead comply with article 702, Optional Standby Systems. The purpose of article 710 was originally only to cover systems that are completely off-grid. You'd say the requirement makes sense then, right? (It's a much more lenient requirement for system power than we have for utility services.) When they expanded the scope of article 710 to include islanded systems they created confusion. They should make either 702 or 710 an option for islanded systems. One reason to keep 710 in the mix is that some of these systems could really become a regular power source with the utility as backup, rather than the other way around.

That said, I've yet to have any discussions or friction with any AHJs over these issues. I think most inspectors have never heard of Article 710, and if you wanted to comply with 702 instead I bet only the most unreasonable would object.

 

[don_resqcapt19]

Does anyone understand the requirement of largest utilization equipment when it comes to PV system?
Personally, it doesn't make much sense to me as other, smaller loads consuming appliances can pose a problem to the supply of the largest utilization equipment, let alone that some installers I've talked with don't understand the requirement completely.
Anyone familiar with the subject and can clarify it?

Are you really working on a system that is within the scope of Article 710?

710.1 Scope.
This article covers electric power production systems that operate in island mode not connected to an electric utility or other electric power production and distribution network.

Even if you are, the NEC only covers electrical safety. You are free to design the stand alone system to operate in the manner you think it should.

If you really seen a need for a change in the requirements for these systems, you can submit a Public Input to make that change.

www.nfpa.org/70next

The closing date for PIs to make a change for the 2026 code is 9/7/2023

 

[wwhitney]

When they expanded the scope of article 710 to include islanded systems they created confusion.

That's apparently a 2020 NEC only problem. Don quoted the 2023 Scope. The 2020 scope is:

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.

The deletion of the two words "and installations" in 2023 makes the scope match your expectations. Article 710 is now about installations that have both features, not all installations with either feature alone.

Cheers, Wayne

 

[jaggedben]

Actually now that I look back I think the scope of 710 has always included both types of systems, in as much as it hasn't been vague on the point, which it is. Ever since they moved it from 690.10 in the 2017 cycle, it has always included the word 'mode' which implies that it covers systems that can operate in other modes. I can't make heads or tails of why the extremely slight changes in wording in the last two cycles are considered to make a clear difference. (Unless it was thought that the 2017 wording inadvertently left out systems that always operate in stand alone mode, but that's what the section in 690 originally clearly covered.) I haven't looked back at the PIs to see what the given reasoning for the word changes was.

 

[pv_n00b]

Back in the day, some AHJs were requiring that a stand-alone PV system be sized for the total sum of the site loads. Even if in operation all those loads were never on at the same time or if in stand-alone mode some loads were never turned on. This was a hugely expensive problem and lead to the requirement that the system only has to be sized for the largest single load. It was specifically to stop AHJs from doing this and allow PV designers flexibility in designing the system. Same with the allowance to supply 120V single phase only to 120/240V panels that met specific requirements. It's moved around from 690 to 710 but the purpose has been the same.