Commercial PV Design Questions

[pvgreeze]

690.8(A)(1)(e) applies to the circuit from the AC-side of a single inverter to the first OCPD. 705.12 applies to everything from that OCPD to the utility.

Is that specific definition defined anywhere? That 705.12 applies to everything beyond the first OCPD? Is that the same for instances where 705.11 is invoked? Looking at 705.11(A) specifically, it is clear that it defines Output Rating as "the sum of the power source continuous current output ratings" which we defined using 690.8(A)(1)(e). 705.12(A) even specifies that a Dedicated Overcurrent and Disconnect are needed (the 800A fused DS) at the point of interconnection and that this device can service "one or more power sources." So why are we disregarding that device in the language of 705.12(B)(3)(3), which states that it is "the busbar," not "any busbar" and includes very specific language for a warning label regarding the calculations of the ampacity of devices on that "equipment," not any equipment associated with the "one or more power sources".

Even staying in 705, 705.28(A) defines the maximum current to be the continuous output current rating of the power production equipment (4*144.3 + 36.25 = 577.2A) and assuming we can invoke 705.30(B) that the OCPD should be 800A, hence the 800A fused DS in the switchboard. The "other than generator systems" line in 705.30(B) is unclear though...

I would agree that 705.12(B)(3)(3) would be invoked if that 800A fused DS did not exist for some reason, which would be in violation of 705.12(A), but that is the device that is being installed in the switchboard. If there is clear language that "690.8(A)(1)(e) applies to the circuit from the AC-side of a single inverter to the OCPD. 705.12 applies from everything from that OCPD to the utility" then I understand that interpretation, but I'm not sure where that specific definition is coming from.

 

[ggunn]

Is that specific definition defined anywhere? That 705.12 applies to everything beyond the first OCPD? Is that the same for instances where 705.11 is invoked? Looking at 705.11(A) specifically, it is clear that it defines Output Rating as "the sum of the power source continuous current output ratings" which we defined using 690.8(A)(1)(e). 705.12(A) even specifies that a Dedicated Overcurrent and Disconnect are needed (the 800A fused DS) at the point of interconnection and that this device can service "one or more power sources." So why are we disregarding that device in the language of 705.12(B)(3)(3), which states that it is "the busbar," not "any busbar" and includes very specific language for a warning label regarding the calculations of the ampacity of devices on that "equipment," not any equipment associated with the "one or more power sources".

Even staying in 705, 705.28(A) defines the maximum current to be the continuous output current rating of the power production equipment (4*144.3 + 36.25 = 577.2A) and assuming we can invoke 705.30(B) that the OCPD should be 800A, hence the 800A fused DS in the switchboard. The "other than generator systems" line in 705.30(B) is unclear though...

I would agree that 705.12(B)(3)(3) would be invoked if that 800A fused DS did not exist for some reason, which would be in violation of 705.12(A), but that is the device that is being installed in the switchboard. If there is clear language that "690.8(A)(1)(e) applies to the circuit from the AC-side of a single inverter to the OCPD. 705.12 applies from everything from that OCPD to the utility" then I understand that interpretation, but I'm not sure where that specific definition is coming from.

Be that as it may, 705.12(B)(3)(3) is the code section that AHJs (the ones I work with, anyway) refer to in order to verify compliance to code for the busbars in a PV AC combiner panel. As someone else (wwhitney?) said, enabling these panels to be compliant is the reason that section of the NEC was written.

 

[pvgreeze]

Be that as it may, 705.12(B)(3)(3) is the code section that AHJs (the ones I work with, anyway) refer to in order to verify compliance to code for the busbars in a PV AC combiner panel. As someone else (wwhitney?) said, enabling these panels to be compliant is the reason that section of the NEC was written.

Where I'm having trouble is following where that is defined. As a newer PV designer, that implication is not clear at all. In fact, it seems more clear that it is only concerning the point of interconnection. Consider 705.12(B)(3) where is says "one of the following methods shall be used to determine the ratings of busbars" and instead of using 705.12(B)(3)(3) to justify interconnection, you used 705.12(B)(3)(2), the 120% rule. Does that mean that the AC combiner panel interpretation is moot, because as the heading of that section says, "one of the following methods...," implying we do not need to conform to more than one section there?

If this was a 4000A service, with a 4000A main protecting a 4000A breaker, and you interconnected the 800A fused DS at the opposite end of the busbar (800A sized for 125% of the "the power-source(s) output circuit current" as defined by the code) using the 120% rule, then the MLO concerns for the combiner panel would be not relevant considering 705.12(B)(3)(3) would not be invoked because 705.12(B)(3) says I only need to use one of the following methods, and I used 705.12(B)(3)(2) instead of 705.12(B)(3)(3)?

I apologize if I'm coming across as obtuse or combative, I am just trying to understand the reasoning, especially as a newer PV designer that wasn't around for previous code iterations.

 

[mddorogi]

This is an excellent discussion, thanks everyone. This will give me a lot to digest.

One detail I'm not aware of is the rating of the main service disconnect switch. I assume it is 3000A, since that's what's listed on the switchgear sections, but don't know what for sure. Someone will check that out tomorrow.

The other question is about load. We have yearly one-hour load from the utility. The maximum one hour power draw over the past 12 months is 440 kW, or 529A actual current. It is pretty far from 3000A, and there are only a couple hours over the entire year that are above 400 kW. The are all at mid-day in the summer. I am seeing if 15 minute data is available. I don' really think there are high peaks as this is a machine tool shop for automotive/aerospace/defense and they turn on equipment in the morning and leave it on to the end of the day.

 

[wwhitney]

Where I'm having trouble is following where that is defined.

Short answer: 705.12 first sentence permits interconnecting the electric power source at any distribution equipment on the premises. The second sentence then defines which equipment is subject to 705.12(A) through (E). It says any equipment "fed simultaneously by a primary source of electricity and one or more other power source." That certainly covers the combiner panel, as it is fed by the utility and the inverters. So the MLO combiner panel must comply with 705.12(A) through (E).

See also the 705 definition of Power Source Output Circuit, which covers all conductors between the inverter and the service, as an example of the intended scope of Article 705. Likewise, read 690.1 and 705.1 for the scopes.

Cheers, Wayne

 

[pvgreeze]

Short answer: 705.12 first sentence permits interconnecting the electric power source at any distribution equipment on the premises. The second sentence then defines which equipment is subject to 705.12(A) through (E). It says any equipment "fed simultaneously by a primary source of electricity and one or more other power source." That certainly covers the combiner panel, as it is fed by the utility and the inverters. So the MLO combiner panel must comply with 705.12(A) through (E).

See also the 705 definition of Power Source Output Circuit, which covers all conductors between the inverter and the service, as an example of the intended scope of Article 705. Likewise, read 690.1 and 705.1 for the scopes.

Cheers, Wayne

I appreciate your time and the discussion.

Again, I would just say that, the sentence before your quotation reads "The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises. Where distribution equipment or feeders are fed simultaneously..."

Now, I would argue that the definition and scope of what distribution equipment does not change between those two sentences to include the PV combiner panels when the phrase distribution equipment is meant in the prior sentence to discuss the equipment being interconnected to, but I can understand the interpretation that it applies to all equipment in support of the PV system. Regardless, as mentioned previously, it is all regarding whatever the AHJ interprets anyway. Thanks!

 

[wwhitney]

But each PV inverter is an electric power source. So as soon as you connect it to the combiner panel, you are interconnecting it. You can't treat the 800A feeder from the service to the MLO combiner panel as a single electric power source you are interconnecting at the service. There's nothing the scope of Article 690 (690.1) that would suggest 690 covers the combiner panel.

Now if a manufacturer chose to sell you a 5 enclosure 480 kW PV inverter, which some specific wiring to field connect it all, and under the hood it was just (4) 120kW PV inverters plus a common controller that includes a bus and OCPD for each 120kw PV inverter, then I could see treating that as just one electric power source. But a field assembled and installed MLO combiner panel is different.

Sorry I haven't read your later 705 references to respond to them, as the above seems pretty clear to me.

Cheers, Wayne

 

[pvgreeze]

But each PV inverter is an electric power source. So as soon as you connect it to the combiner panel, you are interconnecting it. You can't treat the 800A feeder from the service to the MLO combiner panel as a single electric power source you are interconnecting at the service. There's nothing the scope of Article 690 (690.1) that would suggest 690 covers the combiner panel.

If each inverter is to be viewed as an its own interconnected power source, then why is there language in sections like 705.12(B)(3)(1) that specifically mention taking the sum of the power source(s) output circuit currents for certain applications of 705.12(B)?

Also, it seems illogical to me that none of this would apply if this was a service side interconnection via 705.11, where the output rating is also defined as the sum of the power source continuous output ratings in 705.11(A). If the initial proposed design was for a supply-side connection with an 800A Fused DS next to the POI and an 800A MLO, then the MLO concerns raised in 705.12(B)(3)(3) wouldn't apply to that combiner panel?

 

[wwhitney]

If each inverter is to be viewed as an its own interconnected power source, then why is there language in sections like 705.12(B)(3)(1) that specifically mention taking the sum of the power source(s) output circuit currents for certain applications of 705.12(B)?

Because when a busbar has more than two sources of supply (e.g. there's more than just one inverter and the utility), you need to add them all up? I don't understand what you see as a conflict.

Suppose instead of one combiner panel, you had cascaded combiner panels. Each combiner panel would use the sum of the inverter currents that run through its busbar.

Also, it seems illogical to me that none of this would apply if this was a service side interconnection via 705.11, where the output rating is also defined as the sum of the power source continuous output ratings in 705.11(A). If the initial proposed design was for a supply-side connection with an 800A Fused DS next to the POI and an 800A MLO, then the MLO concerns raised in 705.12(B)(3)(3) wouldn't apply to that combiner panel?

No, they still would. A 705.11 supply side connection works fine for a connection that goes: single inverter - OCPD/new service disconnect - service conductors on the utility side of any other service disconnects.

But as soon as you have multiple inverters and a combiner panel, that portion of the system essentially becomes a load side connection on a new dedicated service disconnect. 2023 NEC 705.11 cleared this up quite a bit, so as someone new to the field and forward looking, I suggest sticking to that interpretation.

Cheers, Wayne

 

[ggunn]

Where I'm having trouble is following where that is defined. As a newer PV designer, that implication is not clear at all. In fact, it seems more clear that it is only concerning the point of interconnection. Consider 705.12(B)(3) where is says "one of the following methods shall be used to determine the ratings of busbars" and instead of using 705.12(B)(3)(3) to justify interconnection, you used 705.12(B)(3)(2), the 120% rule. Does that mean that the AC combiner panel interpretation is moot, because as the heading of that section says, "one of the following methods...," implying we do not need to conform to more than one section there?

If this was a 4000A service, with a 4000A main protecting a 4000A breaker, and you interconnected the 800A fused DS at the opposite end of the busbar (800A sized for 125% of the "the power-source(s) output circuit current" as defined by the code) using the 120% rule, then the MLO concerns for the combiner panel would be not relevant considering 705.12(B)(3)(3) would not be invoked because 705.12(B)(3) says I only need to use one of the following methods, and I used 705.12(B)(3)(2) instead of 705.12(B)(3)(3)?

I apologize if I'm coming across as obtuse or combative, I am just trying to understand the reasoning, especially as a newer PV designer that wasn't around for previous code iterations.

I don't see it as combative or obtuse, only incorrect. 705.12(B)(3)(2) is the code that normally governs bus qualification for a PV AC combiner panel; you could try another choice but it will be worse. Whatever subsection you are using for the interconnection itself is a different issue. 705.12 rules need to be applied separately to every panel in the entire installation between the inverter(s) and the interconnection.

 

[pvgreeze]

No, they still would. A 705.11 supply side connection works fine for a connection that goes: single inverter - OCPD/new service disconnect - service conductors on the utility side of any other service disconnects.

But as soon as you have multiple inverters and a combiner panel, that portion of the system essentially becomes a load side connection on a new dedicated service disconnect. 2023 NEC 705.11 cleared this up quite a bit, so as someone new to the field and forward looking, I suggest sticking to that interpretation.

Okay, so in regards to the 2020 NEC (which is widely adopted by most places I work) and based on this interpretation and definition, more than one inverter makes it a Load Side Interconnection (705.12) via a new dedicated service disconnect even though 705.11 has language referencing taking the sum of power source continuous output ratings, implying more than one power source output rating. Am I following that correctly?

Also, I agree re the combiner panel using the sum of the inverter currents and not the sum of the OCPDs as 705.12(B)(3)(3) implies, that is what I've been saying this whole time.

 

[wwhitney]

Okay, so in regards to the 2020 NEC (which is widely adopted by most places I work) and based on this interpretation and definition, more than one inverter makes it a Load Side Interconnection (705.12) via a new dedicated service disconnect even though 705.11 has language referencing taking the sum of power source continuous output ratings, implying more than one power source output rating. Am I following that correctly?

I would say that interpretation is consistent with the 2020 NEC, but that the 2020 NEC permits interpretations of the OCPD on the 705.11 interconnection as other than a new service disconnect.

Again, that language about the sum of power source continuous output ratings would be needed for connections like "multiple microinverters - #12 Cu conductors - 20A OCPD - #6 Cu "tap" conductors -- service conductors."

I guess going back to your original question today, if you see the combiner panel as not subject to 705.12, then what section tells us what bus size the panel should have? 690.8(A)(1)(e) only tells us the current for an individual inverter, there should be a section that tells us to add those up for sizing the combiner panel bus.

Cheers, Wayne

 

[ggunn]

Okay, so in regards to the 2020 NEC (which is widely adopted by most places I work) and based on this interpretation and definition, more than one inverter makes it a Load Side Interconnection (705.12) via a new dedicated service disconnect even though 705.11 has language referencing taking the sum of power source continuous output ratings, implying more than one power source output rating. Am I following that correctly?

Also, I agree re the combiner panel using the sum of the inverter currents and not the sum of the OCPDs as 705.12(B)(3)(3) implies, that is what I've been saying this whole time.

No, 705.12(B)(3)(3) is clear; it's the sum of the breaker ratings, not the sum of 125% of the inverter output currents, that determines the minimum bus rating. 705.12(B)(3)(3) is the only selection that allows you to ignore the rating of the OCPD protecting the bus.

Multiple inverters do not change the interconnection from 705.11 to 705.12, but with multiple inverters connected on the line side both have to be complied with - 705.11 at the point of interconnection and 705.12 where the inverters combine.

 

[wwhitney]

No, 705.12 (B)(3)(3) is clear; it's the sum of the breaker ratings, not the sum of 125% of the inverter output currents that determines the minimum bus size.

Which, of course, is a flaw for the case that every breaker is connected only a single inverter. Has anyone tried to submit a PI to adjust 705.12(B)(3)(3) accordingly?

Cheers, Wayne

 

[pvgreeze]

No, 705.12 (B)(3)(3) is clear; it's the sum of the breaker ratings, not the sum of 125% of the inverter output currents that determines the minimum bus size.

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. I also have only ever worked with dedicated PV AC combiner panels. 705.28(A) indicates that the maximum current for the circuit is the continuous output rating of the power production equipment, and 705.30(B) dictates that OCPDs need only be sized to 125% of the calculated current in 705.28(A). For each individual inverter, that is 125% of 144.3A (interpretations of 220.5(B) not withstanding), 240.4 & 240.6 tell us 200A for each OCPD. For the overall OCPD, 125% of 613.45A, which is where the 800A fused DS comes from. This 800A fused DS protects 'power source output circuit conductors and equipment (the 800A MLO)' per 705.30(A), and not 705.12(B)(3)(3) which is specific to a load side interconnection as spelled out in the section.

 

[ggunn]

Which, of course, is a flaw for the case that every breaker is connected only a single inverter. Has anyone tried to submit a PI to adjust 705.12(B)(3)(3) accordingly?

Cheers, Wayne

Ours is not to reason why; ours is just to code comply.

 

[ggunn]

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. I also have only ever worked with dedicated PV AC combiner panels. 705.28(A) indicates that the maximum current for the circuit is the continuous output rating of the power production equipment, and 705.30(B) dictates that OCPDs need only be sized to 125% of the calculated current in 705.28(A). For each individual inverter, that is 125% of 144.3A (interpretations of 220.5(B) not withstanding), 240.4 & 240.6 tell us 200A for each OCPD. For the overall OCPD, 125% of 613.45A, which is where the 800A fused DS comes from. This 800A fused DS protects 'power source output circuit conductors and equipment (the 800A MLO)' per 705.30(A), and not 705.12(B)(3)(3) which is specific to a load side interconnection as spelled out in the section.

I don't mean to be cavalier about it, but your disagreeing with it will not make you pass inspections. 705.11 or 705.12 must be complied with in every panel between the inverters and the interconnection. It's not just me; it's every AHJ I have ever dealt with. Obviously, 705.11 could only be used at the POI.

You could conceivably have a PV system connected to a panel more than one cascaded subpanel away from the MDP; 705.12(B)(3) would apply to every one of them.

 

[pvgreeze]

I don't mean to be cavalier about it, but your disagreeing with it will not make you pass inspections. 705.11 or 705.12 must be complied with in every panel between the inverters and the interconnection. It's not just me; it's every AHJ I have ever dealt with.

And I'm not trying to be difficult, I'm just reading the code as it is written.

If you acknowledge that it is either/or 705.11 vs 705.12, then would an AC combiner panel on a supply side interconnection not need to comply with 705.12?

 

[ggunn]

No, 705.12(B)(3)(3) is clear; it's the sum of the breaker ratings, not the sum of 125% of the inverter output currents, that determines the minimum bus rating.

And I'm not trying to be difficult, I'm just reading the code as it is written.

If you acknowledge that it is either/or 705.11 vs 705.12, then would an AC combiner panel on a supply side interconnection not need to comply with 705.12?

I don't know what you are driving at. For multiple inverters connected on the line side of an MDP, 705.11 applies in the MDP and 705.12 applies in the PV AC combiner panel. I didn't mean that you could choose which one to use; the one you use is determined by the circuit. You can insist all you want that 705.12 should not apply to a PV inverter combiner panel, but it won't make a difference to an inspector or a plan reviewer.

 

[ggunn]

No, 705.12(B)(3)(3) is clear; it's the sum of the breaker ratings, not the sum of 125% of the inverter output currents, that determines the minimum bus rating. 705.12(B)(3)(3) is the only selection that allows you to ignore the rating of the OCPD protecting the bus.

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.