Commercial PV Design Questions

[wwhitney]

Aside: Sometimes you can use a three phase panel to combine single phase inverters to interconnect them to a three phase service. The determining factor is the total of the ratings of the breakers connected to a given bus, not simply the sum of all the two pole breaker ratings. If the split is equal (balanced) and all the breakers are the same, the minimum bus size is 2/3 the sum of the breaker ratings, since only 2/3 of the breakers in the panel connect to each bus.

Do we get to construe "sum" as vector sum so that we can apply a factor of sqrt(3) when adding the A bus contribution from A-B connected inverters to the contribution of B-C connected inverters? That makes it 1/sqrt(3), rather than 2/3. : - )

Cheers, Wayne

 

[wwhitney]

Per other posts, I disagree with the interpretation that the PV AC combiner panel is subject to 705.12 due to the language of 705.12 seeming to indicate that it is referring to where the power source output circuit is connected to the a piece of distribution equipment.

But the "power source output circuit" is all the premises wiring from the inverter to the service. So at every intervening panel, the power source output circuit is connected to a piece of distribution equipment. Your description covers every intervening panel, not just the last one.

You also never responded to my last paragraph in post #32.

Cheers, Wayne

 

[jaggedben]

I'm confused...why is 705.12(B)(3)(3) being applied to the combiner panel sizing? Wouldn't that fall under 690, specifically 690.8(A)(1)(e)?

...

That strikes me as real stretch.

Suppose instead of 6 solar inverter breakers you have 4 solar inverters and 2 battery energy storage inverters. How does 690 still encompass that?

Now flipping back again... why should the 6 solar inverters be subject to a different busbar rule than the combo of solar and battery inverters?

Note further that the combiner panel is a point of interconnection in the sense that it is where utility and solar sources meet, without further overcurrent protection between them. The combiner is energized by utility power.

Beyond that, there's the history of the code, how 705 used to be language duplicated in 690 and 694 and was put into its own article precisely to be agnostic of what the source of the inverter (or other technology) output is. I believe that was in 2011. And then in 2014 705.12(B)(3)(3) was put in with the intention (if not the wording) of legitimizing combiner panels without having to use the 120% rule. The code making panel clearly has seen 705 as applying here.

 

[jaggedben]

For the record, I didn't see the last two pages of discussion before I made the last post.

@pvgreeze
It seems to me your thinking relies on the idea that a group of inverters connected to a combiner panel constitutes a single 'source'. My first question to you would be: where is that clearly defined? Aren't the inverters all sources capable of operating independently? Suppose I have multiple solar systems at different points of connection around the premises, how would that be one 'source' with respect to 705? And if that's multiple sources, why would multiple connections at the same location not be multiple sources? Also you have repeatedly mentioned one side or another of the 'point of interconnection' but that is not a phrase that is actually used in the code. In other words, it's not clear that there is a single point where 690 ends and 705 takes over. 690 and 705 arguably overlap in all inverter output circuits. Compare 705.28 to the section of 690 you quoted earlier, and tell me whether at least subsection (C) applies all the way to the inverters.

If you look at the 2023 NEC, 705.11 even more clearly defines that a supply-side connection is to be considered a service disconnect. Which means that 705.12's references to 'load side of the service disconnect' all the more apply to distribution equipment that includes panelboards solely used to combine PV sources. In my opinion (not necessarily popular, but corroborated by the recent evolution of the code), unless you have only one OCPD in your system that connects directly to service conductors, then you always have a supply-side connection in series with a load side connection.

While I see that there are some gaps in the code as far as making crystal clear what we are saying, I agree with Wayne and ggunn that your interpretation is unusual, and you'd be lucky to win the argument with an AHJ (unless the AHJ is just generally ignorant about the relevant sections of code).

 

[ggunn]

While I see that there are some gaps in the code as far as making crystal clear what we are saying, I agree with Wayne and ggunn that your interpretation is unusual, and you'd be lucky to win the argument with an AHJ (unless the AHJ is just generally ignorant about the relevant sections of code).

Unusual?

I would be curious to know if the poster is actually involved in designing commercial PV projects and getting them approved and successfully inspected.

 

[mddorogi]

Unusual?

I would be curious to know if the poster is actually involved in designing commercial PV projects and getting them approved and successfully inspected.

As OP, I am .

We are still on NEC 2017. It does not appear that there are any real differences between 2017 and 2020 in these regards.

Based on these discussions, I am seeing if I can upsize the MLO combiner panel bus from 800A to either 850A, 900A, or 1000A. This combiner panel is already costing me $12K with the 800A bus, so, ouch.

 

[wwhitney]

Based on these discussions, I am seeing if I can upsize the MLO combiner panel bus from 800A to either 850A, 900A, or 1000A.

How does that compare in cost to using (6) fused disconnects or enclosed circuit breakers (6 separate enclosures): an 800A disconnect with 5-fold lugs on the load side, and 5 feeder taps to the individual disconnects sized to each inverter? Feeder taps would be based on a supply of 1566A, for better or worse.

Cheers, Wayne

 

[ggunn]

As OP, I am .

We are still on NEC 2017. It does not appear that there are any real differences between 2017 and 2020 in these regards.

Based on these discussions, I am seeing if I can upsize the MLO combiner panel bus from 800A to either 850A, 900A, or 1000A. This combiner panel is already costing me $12K with the 800A bus, so, ouch.

Sorry; I was referring to the poster who was trying to (incorrectly) argue that 705.12(B)(3) only applies to the panel at the interconnection point and not to an inverter combiner panel. You are correct that the 2017 and 2020 codes are the same or nearly so in this.

 

[mddorogi]

How does that compare in cost to using (6) fused disconnects or enclosed circuit breakers (6 separate enclosures): an 800A disconnect with 5-fold lugs on the load side, and 5 feeder taps to the individual disconnects sized to each inverter? Feeder taps would be based on a supply of 1566A, for better or worse.

Cheers, Wayne

I'm sure it would be a lot cheaper.

It is clear that the 800A bus is non-compliant with 705.12(b)(3)(3). From what everyone has posted, this seems to be a code issue, with no bearing on actual function or safety. I am wondering if that is true. Is there is a technical reason why bus bar ampacity (sum of OCPD) in this section is treated differently than wire ampacity (sum of max continuous output currents * 1.25)? I would think that bus bars would heat less than wires, since they are not completely enclosed in insulation. Wires are also in a conduit which would seem to have less space for heat dissipation than a panel board enclosure.

Sorry; I was referring to the poster who was trying to (incorrectly) argue that 705.12(B)(3) only applies to the panel at the interconnection point and not to an inverter combiner panel. You are correct that the 2017 and 2020 codes are the same or nearly so in this.

The odd thing about these situations is that you can have highly qualified, experienced people come to different interpretations of the same code in the same application. That is what confuses me, as I am not as experienced. Consider the other point that I mentioned: whether the combiner panel needs a main breaker or not. I don't think anyone here said that code requires a main breaker. Yet the most experienced electrical PE I know, who does not work for me but for a competing firm, strongly argues that 705.30 requires a main breaker in the combiner panel. A second PE agreed. A third PE did not agree. I know for a fact that local inspectors do not require one, as we've done many without them.


My reading of this is:
a) Power source output circuit conductor means the wire between the inverter and the combiner panel; the breakers in the combiner panel provide the overcurrent protection for these;
b) "Circuits connected to more than one electrical source" means the combiner panel (I think) and all the wires and equipment between the combiner panel and the fused switch in the switchgear; all of those circuits are protected by either the fuses in that switch (from the grid power source) and the breakers in the combiner panel (from the inverter(s) power source).

Does my interpretation seem correct, or do you read 705.30 as requiring that main breaker?

 

[ggunn]

I'm sure it would be a lot cheaper.

It is clear that the 800A bus is non-compliant with 705.12(b)(3)(3). From what everyone has posted, this seems to be a code issue, with no bearing on actual function or safety. I am wondering if that is true. Is there is a technical reason why bus bar ampacity (sum of OCPD) in this section is treated differently than wire ampacity (sum of max continuous output currents * 1.25)? I would think that bus bars would heat less than wires, since they are not completely enclosed in insulation. Wires are also in a conduit which would seem to have less space for heat dissipation than a panel board enclosure.


The odd thing about these situations is that you can have highly qualified, experienced people come to different interpretations of the same code in the same application. That is what confuses me, as I am not as experienced. Consider the other point that I mentioned: whether the combiner panel needs a main breaker or not. I don't think anyone here said that code requires a main breaker. Yet the most experienced electrical PE I know, who does not work for me but for a competing firm, strongly argues that 705.30 requires a main breaker in the combiner panel. A second PE agreed. A third PE did not agree. I know for a fact that local inspectors do not require one, as we've done many without them.

View attachment 2565461
My reading of this is:
a) Power source output circuit conductor means the wire between the inverter and the combiner panel; the breakers in the combiner panel provide the overcurrent protection for these;
b) "Circuits connected to more than one electrical source" means the combiner panel (I think) and all the wires and equipment between the combiner panel and the fused switch in the switchgear; all of those circuits are protected by either the fuses in that switch (from the grid power source) and the breakers in the combiner panel (from the inverter(s) power source).

Does my interpretation seem correct, or do you read 705.30 as requiring that main breaker?

Conductors do not need protection from the inverters; they are sized to withstand 125% of the maximum current that the inverters are capable of sourcing. All the OCPDs on the AC side are to protect the conductors from fault current coming from the utility; that is why they are on the other end of the conductors from the inverters. My guess that the two PEs who think a main breaker is necessary on an inverter combiner panel are not very experienced in dealing with PV.

 

[mddorogi]

Conductors do not need protection from the inverters; they are sized to withstand 125% of the maximum current that the inverter(s) are capable of sourcing. All the OCP on the AC side is to protect the conductors from fault current coming from the utility; that is why it is on the other end. My guess that the two PEs who think a main breaker is necessary on an inverter combiner panel are not very experienced in dealing with PV.

Thanks, all very helpful.

 

[wwhitney]

It is clear that the 800A bus is non-compliant with 705.12(b)(3)(3). From what everyone has posted, this seems to be a code issue, with no bearing on actual function or safety. I am wondering if that is true. Is there is a technical reason why bus bar ampacity (sum of OCPD) in this section is treated differently than wire ampacity (sum of max continuous output currents * 1.25)?

There's no physics reasons, just a code reason: ease of enforcement and simple field verification.

Cheers, Wayne

 

[jaggedben]

....

It is clear that the 800A bus is non-compliant with 705.12(b)(3)(3). From what everyone has posted, this seems to be a code issue, with no bearing on actual function or safety. I am wondering if that is true.

Yes, that's true. I think the way you've stated it is exactly right. You're getting killed by the arbitrariness of breaker sizes in this case. That's why I think maybe asking the AHJ if they'll accept an EE's stamp that it is safe, maybe you can ask them to approve it per 90.4. Might be appropriate in this case.

Is there is a technical reason why bus bar ampacity (sum of OCPD) in this section is treated differently than wire ampacity (sum of max continuous output currents * 1.25)?

No, not really. The problem is that the 705.12(b)(3)(3)is written to apply to multiple situations, including those that involve loads. For example suppose you had an 800a busbar with five 200A loads and one 200A inverter breaker, as well as an 800A main OCPD. The total of the sources and loads can conceivably overload the busbar by delivering 900A. Remove two of the loads, and this is no longer possible, as max draw is now 600A.

Wayne has pointed out that per Kirchoff's law you could actually ignore one OCPD that is smallest. (In the above example, remove just one 200A load, instead of two, and the max draw is 800A.) However the CMP evidently thinks that's too complicated for a code rule. They can't write this rule to refer only to 125% of inverter output, because that doesn't account for how loads might behave. There's also the question of whether we can rely on Kirchoff's law or if we need testing of busbars at full current in such a scenario.

So, we are stuck with what is most likely an overly conservative code rule.

 

[wwhitney]

They can't write this rule to refer only to 125% of inverter output, because that doesn't account for how loads might behave.

They could, e.g. "Exception: any breaker connected solely to inverter outputs may be counted at 125% of the maximum connected inverter output current." But if language like that is to be incorporated into the required warning sign, that would make the warning sign more complicated.

Cheers, Wayne

 

[jaggedben]

... But if language like that is to be incorporated into the required warning sign, that would make the warning sign more complicated.

Yeah, and that particular label is already incomprehensible to most people I know.

 

[ggunn]

However the CMP evidently thinks that's too complicated for a code rule.

That made me laugh.

 

[pvgreeze]

Unusual?

I would be curious to know if the poster is actually involved in designing commercial PV projects and getting them approved and successfully inspected.

I have plenty of commercial designs built and operating. Also, an arrangement in which the circuit breakers on the panels sum above the likely standard MLO rating is rare anyway. It is particularly uncommon given the SE120KUS inverter has such a bizarre output current that doesn't directly relate to a standard OCPD size.

I'm reading the code and interpreting it as such, the 2020 NEC, which is the one I am most familiar with. This kind of professional disregard for reading the code is pretty appalling. I figured this group was more supportive, especially considering we were just discussing various scenarios and applications. It is an extremely specific example that I'm not even designing.

I've also found most AHJs are pretty willing to have discussions on code interpretations. I've done this before with the need for expansion fittings and other smaller code applications that are relevant in niche cases.

I found the discussion interesting regardless of your thoughts on my interpretation and the unnecessary questioning here. Hope others did as well.

*Lastly; I have not had an inspector question the combiner panel rating versus the connected OCPD ratings, but if that is the consensus interpretation from others who have worked in the field for longer than I have, then that is extremely useful to know and I will keep that in mind on the projects I am currently working on. I've only ever designed projects with a single dedicated combiner panel (no loads), but I can absolutely see where adding loads/BESS onto the combiner panel could cause issues, so I was just looking at the combiner panel through that particular lens as well.

 

[electrofelon]

That made me laugh.

I was going to say the same thing, that's just about the most hilarious CMP response I've ever heard

 

[wwhitney]

Wayne has pointed out that per Kirchoff's law you could actually ignore one OCPD that is smallest. . . . However the CMP evidently thinks that's too complicated for a code rule.

For the record, the actual CMP response to PI 92-NFPA 70-2019 was "Adding additional features to this very simple rule creates unnecessary confusion for the enforcement and marking requirements." Which is basically the same.

Cheers, Wayne

 

[ggunn]

I have plenty of commercial designs built and operating. Also, an arrangement in which the circuit breakers on the panels sum above the likely standard MLO rating is rare anyway. It is particularly uncommon given the SE120KUS inverter has such a bizarre output current that doesn't directly relate to a standard OCPD size.

The SE120K-US has 144.3A maximum rated output current, which takes a minimum 180.4A OCPD, which rounds up to 200A. There is nothing "bizarre" about that; that it does not land directly on a standard OCPD size is the rule rather than the exception; most inverters don't and there is no reason why they should.

I have a hundred or more commercial PV systems under my belt in many different AHJ territories and every single one of them evaluates my PV AC combiner panels for compliance with 705.12(B)(3)(3). Maybe I could claim compliance with 705.12(B)(3)(5), but I don't choose to go down that road. YMMV.