2023 NEC 705.12(B)(6) (Engineering Supervision)

[jaggedben]

My example was "200A busbar, 200A source or load, 200A source only, 200A load only."

If we try to bump this up to "200A busbar, 200A source or load, >200A source only, 200A load only," then this can overload the busbar when the "source or load" is a load Or if we try to bump this up to "200A busbar, 200A source or load, 200A source only, >200A load only" then this can overload the busbar when the "source or load" is a source.

So both the "source only" and the "load only" need to be capped at the busbar ampacity. Well, unless the "source or load" plus one of the others are less than the busbar ampacity; then the 3rd one can be unlimited.

Okay I think we were mostly just talking past each other.

From my practical installers point of view, it's like this: If I start with a 408.36 compliant installation with loads only, and I add (or convert one existing load to) one 'source only' or one 'source and load' breaker, if the remaining 'load only' breakers do not exceed the busbar ampacity, I cannot overload the busbar.

 

[wwhitney]

From my practical installers point of view, it's like this: If I start with a 408.36 compliant installation with loads only

Well, it better have a "source only" as well, otherwise those loads won't get power. : - )

, and I add (or convert one existing load to) one 'source only' or one 'source and load' breaker, if the remaining 'load only' breakers do not exceed the busbar ampacity, I cannot overload the busbar.

So you end up with a configuration that has two "source or load" connections (the grid plus the new interconnected source, which we allow to be a load to cover the case that it includes an ESS with grid charging) plus some number of "load only" breakers. Then yes, the only restrictions required to ensure that the panelboard bus is not overloaded is that each "source or load" not exceed the busbar rating, and that the sum of the "load only" breakers (per pole) not exceed the busbar rating.

Once we mentally aggregate those "load only" breakers and treat them as a single connection, we fall into scenario (1) in my OP. I don't see any way that aggregation of "load only" breakers can introduce the possibility of bus overload. Of course, you can't in general aggregate "load or source" breakers.

Cheers, Wayne

 

[pv_n00b]

Of course, but we need to be aware of the risk we take on when we color outside the lines.

Being a PE means you know how to safely color without the lines and are licensed to take on the risk. If PEs only did NEC compliant work then why be a PE? I can hire an licensed electrician to follow the NEC for less money.

 

[ggunn]

Being a PE means you know how to safely color without the lines and are licensed to take on the risk. If PEs only did NEC compliant work then why be a PE? I can hire an licensed electrician to follow the NEC for less money.

No PE knows everything, and in many jurisdictions a PE seal is required for every commercial PV system design. Some even require one for residential systems.

 

[ggunn]

No PE knows everything, and in many jurisdictions a PE seal is required for every commercial PV system design. Some even require one for residential systems.

In my 15 years of designing commercial and residential PV systems I have only invoked "engineering supervision" a couple of times.

 

[tortuga]

Another reason I need to get my PE, I kept shooting for 50 but perhaps by the time I am 60.
The thing is a residential installer could have these same 4 installations come up again and again and each job would need the engineers plans, and then a potential permit delay for complex plan review. I am going to suggest Oregon add these to its state code amendments.

So those of you who are engineers [or who can get an engineer to sign off on the design under 2023 NEC 705.12(B)(6), aka 2020 NEC 705.12(B)(5)], can do any of the following:

1) Use a panelboard with at most 3 busbar connections, each of which is protected at the busbar ampacity. [Of course, you can do the same thing with a feeder interconnection with 705.12(A), so it's silly that to use a busbar instead of a wire-type feeder you need engineering supervision.]

2) Ignore one breaker of the smallest size present when applying 705.12(B)(3). So you can add one breaker for monitoring equipment or whatever to your combiner panels, as long as that breaker is not larger than any other breaker present. This is due to a counting argument--the extra breaker can be either a load or a source, but not both, so if you do the accounting "current in = current out" there's no way to get both sides to exceed the busbar rating due to one extra breaker not larger than any other breaker.

3) When you know what is connected to each breaker, label each breaker "source only," "load only," or "source or load." Then as long as the sum of all the sources, or the sum of all the loads, is no more than the busbar ampacity, you know the busbar can't be overloaded.

4) Take advantage of a 100% rated OCPD. E.g. if you have a PV-only service entrance with a 200A 100% rated service disconnect/OCPD, then you can connect that to a 200A MLO panel (all such are rated for 200A continuously) connected to (5) 50A breakers, each connected to an inverter with a maximum inverter output current of 40A.

 

[solarken]

The issues brought up in this thread reinforce the need for more inverter manufacturers to support 705.13 EMS/PCS. In the vast majority of cases, the busbar never gets close to loaded so the PCS would likely never kick in, but it would eliminate the risk.

 

[jaggedben]

The issues brought up in this thread reinforce the need for more inverter manufacturers to support 705.13 EMS/PCS. In the vast majority of cases, the busbar never gets close to loaded so the PCS would likely never kick in, but it would eliminate the risk.

Unfortunately I have yet to see a manufacturer support this concept of PCS. Not that I've been looking comprehensively, but still.

 

[solarken]

Unfortunately I have yet to see a manufacturer support this concept of PCS. Not that I've been looking comprehensively, but still.

SolarEdge says they do but as far as I can tell it is only for the Energy Hub residential. Enphase does as well, but no 480V commercial inverter. It's not hard, many inverter manufacturers already support grid export limiting. This is just a slight mod to limit to: grid import + inverter output < IbusRating, or in other words inverter output < IbusRating - grid import. A programming change really.

 

[jaggedben]

SolarEdge says they do but as far as I can tell it is only for the Energy Hub residential. Enphase does as well, but no 480V commercial inverter. It's not hard, many inverter manufacturers already support grid export limiting. This is just a slight mod to limit to: grid import + inverter output < IbusRating, or in other words inverter output < IbusRating - grid import. A programming change really.

Well it's a different sort of algorithm and from one conversation I had with someone in manufacturing, they feel it's a lot more complicated. Note that you cannot determine inverter output from [IbusRating - grid import] because inverter output determines grid import. So instead I think you maybe do something like [inverter output < .8(IbusRating)] *unless* (grid import+solar)>IbusRating, in which case you drop inverter output completely and let the load trip the grid side main breaker, probably also with some kind of timeout for when it doesn't trip. (The .8 is because of presumed continuous loading, and also provides a fudge factor to avoid tripping the main breaker with solar output to the grid.) So I don't know if UL has a problem with that idea or if it's just lack of imagination by manufacturers, or if it's confusion over what the code allows, or what.

BTW I hate what they did with 705.13 in the 2023 NEC, it just leaves it totally vague what you're actually permitted to do. Wish I'd noticed and gotten some PIs in for 2026.

 

[jaggedben]

BTW to my knowledge Enphase does not have anything like this yet, although something better is supposed to be coming from them soon.

 

[ggunn]

If you were to detune the inverter AC output would you possibly run afoul of the 150% DC:AC limit?

 

[solarken]

BTW to my knowledge Enphase does not have anything like this yet, although something better is supposed to be coming from them soon.

I have not installed an enphase system with this feature, but enphase University says you can enable it in an Envoy with a system that uses IQ7 and IQ8 inverters. Since they reference 2020 NEC, don't think it is brand new.

 

[jaggedben]

I have not installed an enphase system with this feature, but enphase University says you can enable it in an Envoy with a system that uses IQ7 and IQ8 inverters. Since they reference 2020 NEC, don't think it is brand new.
View attachment 2573327

Right, that's basically just derating, it allows a system with higher nameplate than otherwise allowed to meet the 120% rule or other limit. It doesn't work by monitoring the total current to the busbar like we talked about above.

 

[wwhitney]

No, not derating. It's also in the enphase config guide. It monitors the grid point in real time to limit the battery and/or PV microinverter output.

While the diagram shows monitoring the grid input to the premises wiring, the text you quoted does not discuss using a formula for the generation limit that incorporates the real time grid input measured. It only discusses the 120% rule, which would give you a fixed maximum generation limit based on the busbar rating and the OCPD size protecting the busbar.

Cheers, Wayne

 

[tortuga]

Siemens and Eaton offer aluminum busbar 200A panels, so they already have a plastic busbar mold for thicker busbar, I wonder if they could make a copper busbar that fit the aluminum bus mold, then they could perhaps offer a 250A busbar residential loadcenter. Then you could have a 200A main and 100A of backfeed, that probably would eliminate the issue until we get personal home fusion reactors.

 

[jim dungar]

Siemens and Eaton offer aluminum busbar 200A panels, so they already have a plastic busbar mold for thicker busbar, I wonder if they could make a copper busbar that fit the aluminum bus mold, then they could perhaps offer a 250A busbar residential loadcenter. Then you could have a 200A main and 100A of backfeed, that probably would eliminate the issue until we get personal home fusion reactors.

Panel board bus bars are usually rated by heat rise and not physical dimensions. It is hard to dissipate heat in a 3.5" deep enclosure.

 

[solarken]

While the diagram shows monitoring the grid input to the premises wiring, the text you quoted does not discuss using a formula for the generation limit that incorporates the real time grid input measured. It only discusses the 120% rule, which would give you a fixed maximum generation limit based on the busbar rating and the OCPD size protecting the busbar.

Cheers, Wayne

As soon as I posted that I realized jaggedben was right and deleted it.

 

[electrofelon]

If you were to detune the inverter AC output would you possibly run afoul of the 150% DC:AC limit?

Where is there a 150% limit? Is that in one of these new sections?

 

[ggunn]

Where is there a 150% limit? Is that in one of these new sections?

Many (most?) inverter data sheets show a maximum DC connected power, and 150% is what I have mostly seen. What I don't know is if the manufacturers would require a decrease in the allowed maximum DC power if the AC output were to be reduced.