705.12 (D) question

[jaggedben]

Thank you for your response.

That is the crux of the matter, does ErikS's original configuration comply with the spirit of 705.12(D)(2)(3)? If so, then creative interpretations would not be circumventing the spirit of the code language, only the letter of it.

I believe that in ErikS's original configuration, no more than 400A (nominal, assuming breakers trip at their rating) can flow into the busbar in any normal operating situation. Is there more to the spirit of 705.12(D)(2)(3) than that?


So do you think the procedure I described meets the intent and spirit of 705.12(D)(2)(3)? And do you see how to express it more simply and clearly?

Cheers,
Wayne

I think the 'spirit' of 705.12(D)(2)(3) is quite a bit different than what you're implying. From what I've gathered, it was put in the code to legitimize solar 'AC combiner' panels, which might also have the occasional related load such as a monitoring device and or weather station. It explicitly removed the requirement for such panels to meet the 120% rule ( 705.12(D)(2)(2)) which was recognized to be overkill for that situation. But I don't think that appealing to 705.12(D)(2)(3) to allow connections in non-dedicated ordinary load centers, where the sum of the load breakers typically exceeds the busbar rating, meets the spirit of that section if it doesn't meet the letter.

Is it a very conservative approach? Yes. That's part of the spirit, too.

 

[wwhitney]

I think the 'spirit' of 705.12(D)(2)(3) is quite a bit different than what you're implying.

Thank you for the explanation. I see that 705.12(D)(2)(3)(c) (the AC combiner rule) is new to the 2014 NEC, and prior to that the only option was 705.12(D)(2)(3)(b) (the 120% rule). Having only gotten interested in this section recently, I hadn't realized that 705.12(D)(2)(3)(c) was so young.

I don't see any substantive 2020 public inputs for broadening 705.12(D)(2)(3). So I'll have to remember to submit a public input for 2023.

Cheers, Wyne

 

[ggunn]

Thank you for your response.

That is the crux of the matter, does ErikS's original configuration comply with the spirit of 705.12(D)(2)(3)? If so, then creative interpretations would not be circumventing the spirit of the code language, only the letter of it.

I believe that in ErikS's original configuration, no more than 400A (nominal, assuming breakers trip at their rating) can flow into the busbar in any normal operating situation. Is there more to the spirit of 705.12(D)(2)(3) than that?


So do you think the procedure I described meets the intent and spirit of 705.12(D)(2)(3)? And do you see how to express it more simply and clearly?

Cheers,
Wayne

Actually, no, I do not. I am bound to design to the letter of the code irrespective of my views on the spirit of the code. Furthermore, I am at the mercy of inspectors' interpretation of the code; the more I push the envelope from letter into spirit the more likely I am to run afoul of an inspector's differing opinion of what the spirit of the code is. Failed inspections mean extra truck rolls and extra money spent on labor and materials, which are anathema to a business such as ours which runs on narrow margins. I interpret the code as conservatively as possible in order to avoid conflict.

That's not to say that I never go to the mat with AHJ's when I disagree with their interpretations of code. Occasionally I do and sometimes I actually prevail, but I choose my battles.

 

[wwhitney]

OK, fair enough. So let me reframe my inquiry into (2017) 705.12(B)(2)(3):

As I now understand it, the introduction of the current 705.12(B)(2)(3)(c) in 2014 for AC combiner panels was the first time that an allowable busbar protection methodology involved counting breakers that could be serving loads. The current methodology in (c) is very conservative, in that all sources have to be counted too, save for the primary source (what I assume is meant by the OCPD "protecting the busbar".)

So would any new problems arise if the allowance in 705.12(B)(2)(3)(c) were broadened to only count breakers serving loads? This would be the exact analogue of 705.12(B)(2)(3)(a), just for loads rather than sources. The relevant physics is symmetric with respect to reversal of power flow, is it not?

I need to remember to submit a public input in 2020 for the 2023 NEC.

Cheers, Wayne

 

[wwhitney]

Here's a quick example of the current illogic of 705.12(B)(2):

Suppose I have a 100A MLO AC combiner panel with (5) 20A breakers in it. Each breaker is connected to an inverter output circuit from a string of microinverters. A 100A feeder connected to the main lugs goes to a 100A breaker in some upstream distribution equipment. This panel satisfies 705.12(B)(2)(3)(c).

Now I'd like to add some monitoring equipment that requires a 15A breaker. If the inverter output currents add up to less than 85A/125% = 68A, I can put a 15A breaker in this AC combiner panel and the arrangement still satisfies 705.12(B)(2)(3)(c). But if the inverter output currents add up to more than 85A, that doesn't work.

So instead I get a double lug kit for the MLO panel and I run a feeder to a separately enclosed 15A breaker. If you want to avoid feeder taps, make the feeder extension 100A. [But if the new run is less than 10', a 20A feeder tap would be sufficient under 705.12(B)(2)(2).] The 100A feeder with extension still qualifies under 705.12(B)(2)(1)(b), and the whole arrangement complies with 705.12(B)(2).

But really, that is a lot of complication for no real purpose. Putting the 15A breaker in the AC combiner panel would be a lot simpler and certainly no less safe.

Cheers, Wayne

 

[jaggedben]

So would any new problems arise if the allowance in 705.12(B)(2)(3)(c) were broadened to only count breakers serving loads? This would be the exact analogue of 705.12(B)(2)(3)(a), just for loads rather than sources. The relevant physics is symmetric with respect to reversal of power flow, is it not?

I need to remember to submit a public input in 2020 for the 2023 NEC.

You would have to write your proposal so it doesn't accidentally allow an unlimited amount of source amps. Copying what I said in post #10, you could propose that "neither the sum of the loads nor the sum of the [non-primary] sources shall exceed the busbar." Wordsmith from that.

Now I'd like to add some monitoring equipment that requires a 15A breaker. If the inverter output currents add up to less than 85A/125% = 68A, I can put a 15A breaker in this AC combiner panel and the arrangement still satisfies 705.12(B)(2)(3)(c). But if the inverter output currents add up to more than 85A, that doesn't work.

So instead I get a double lug kit for the MLO panel and I run a feeder to a separately enclosed 15A breaker. If you want to avoid feeder taps, make the feeder extension 100A. [But if the new run is less than 10', a 20A feeder tap would be sufficient under 705.12(B)(2)(2).] The 100A feeder with extension still qualifies under 705.12(B)(2)(1)(b), and the whole arrangement complies with 705.12(B)(2).

But really, that is a lot of complication for no real purpose. Putting the 15A breaker in the AC combiner panel would be a lot simpler and certainly no less safe.

Point taken, although in reality I think you would just specify a combiner panel that was the next larger rating, which would be slightly more expensive, perhaps, but not more complicated.

 

[ggunn]

You would have to write your proposal so it doesn't accidentally allow an unlimited amount of source amps. Copying what I said in post #10, you could propose that "neither the sum of the loads nor the sum of the [non-primary] sources shall exceed the busbar." Wordsmith from that.



Point taken, although in reality I think you would just specify a combiner panel that was the next larger rating, which would be slightly more expensive, perhaps, but not more complicated.

I am not yet fully familiar with the intricacies of the 2017 NEC, so I will bow out at this point. I will only say that I will continue to comply with the letter of whatever code cycle the AHJ holds me to as conservatively as I can, whatever it directs me to do.

 

[wwhitney]

I am not yet fully familiar with the intricacies of the 2017 NEC, so I will bow out at this point.

Looking at the 2017, the only substantive change to 2014 705.12(D)(2) was the addition of subsection (d) allowing the 120% rule for center fed panelboards. The other changes appear to be minor, such as the relettering of the section to 705.12(B).

So if you haven't grown weary of the topic, you're not missing anything regarding the 2017 NEC.

Cheers, Wayne

 

[wwhitney]

You would have to write your proposal so it doesn't accidentally allow an unlimited amount of source amps. Copying what I said in post #10, you could propose that "neither the sum of the loads nor the sum of the [non-primary] sources shall exceed the busbar." Wordsmith from that.

The wording gets a bit tricky, we would have to account for the possibility that a breaker connected to the busbar can at times be a load and at times be a source. Otherwise, it could allow a scenario such as:

100A MB panel fed at 100A with
100A combined of load breakers and
(2) 50A breakers, each feeding a panel with 50A of load and 40A of inverter output current

If the 50A breakers are just counted as non-primary sources, this arrangement would meet the simplest wording of the "neither" rule. But if one 50A breaker has a net outflow of 40A from the panel, while the other 50A breaker has a net inflow of 40A, and the 100A breakers are fully loaded, we have 140 amps flowing in and out of the 100A panel.

This is probably the reason for the current conservative wording of 2017 705.12(B)(2)(3)(c).

Cheers, Wayne

 

[wwhitney]

OK, I've thought about 705.12(B)(2)(3)(c) some more and here's where I'm at:

1) It is probably too complicated to add a procedure to the NEC allowing analysis of all possible combinations of whether a breaker is a net source or net sink. So for the purposes of 705.12(B)(2)(3)(c) each breaker has to be treated as both a possible source and a possible sink.

2) If every connection to the busbar gets a rating based on the breaker protecting the connection (or 125% of the output source current as applicable), then it is safe for the sum of all these ratings to add up to 200% of the busbar rating. That is because at any given time current inflow to the panel equals current outflow from the panel, so 200% corresponds to an inflow/outflow equal to the busbar rating.

3) When the "breaker protecting the busbar" has a size equal to the rating of the busbar, rule (2) is equivalent to the current 705.12(B)(2)(3)(c): all of the other breakers must add up to at most 100% of the busbar rating.

Changing the NEC rule from (3) to (2) would be a little more flexible, but it's not clear to me that would ever actually be useful in the real world. However we can safely expand (3) in another way:

4) In computing the sum of the breakers under 705.12(B)(2)(3)(c), also exclude one breaker of the lowest size present.

Here's why (4) protects the busbar just as well as (3): I'm going to think of rule (3) as rule (2) with an assumed full size breaker protecting the busbar. Now at any given time we have "sum of current inflows = sum of current outflows". The full-size breaker is going to be one side of this equation. If it is alone on that side of the equation, then the sum equals the rating of the busbar, and the busbar is protected. If instead another breaker is on the side of the equation with the full size breaker, then by rule (4) the other side of the equation sums up to at most the rating of the busbar, and the busbar is protected.

Rule (4) would be enough to qualify ErikS's original configuration, and it is very simple to state. So I think it would be both useful in the real world and as safe as rule (3).

What do you think?

Cheers, Wayne

 

[ggunn]

OK, I've thought about 705.12(B)(2)(3)(c) some more and here's where I'm at:

1) It is probably too complicated to add a procedure to the NEC allowing analysis of all possible combinations of whether a breaker is a net source or net sink. So for the purposes of 705.12(B)(2)(3)(c) each breaker has to be treated as both a possible source and a possible sink.

2) If every connection to the busbar gets a rating based on the breaker protecting the connection (or 125% of the output source current as applicable), then it is safe for the sum of all these ratings to add up to 200% of the busbar rating. That is because at any given time current inflow to the panel equals current outflow from the panel, so 200% corresponds to an inflow/outflow equal to the busbar rating.

3) When the "breaker protecting the busbar" has a size equal to the rating of the busbar, rule (2) is equivalent to the current 705.12(B)(2)(3)(c): all of the other breakers must add up to at most 100% of the busbar rating.

Changing the NEC rule from (3) to (2) would be a little more flexible, but it's not clear to me that would ever actually be useful in the real world. However we can safely expand (3) in another way:

4) In computing the sum of the breakers under 705.12(B)(2)(3)(c), also exclude one breaker of the lowest size present.

Here's why (4) protects the busbar just as well as (3): I'm going to think of rule (3) as rule (2) with an assumed full size breaker protecting the busbar. Now at any given time we have "sum of current inflows = sum of current outflows". The full-size breaker is going to be one side of this equation. If it is alone on that side of the equation, then the sum equals the rating of the busbar, and the busbar is protected. If instead another breaker is on the side of the equation with the full size breaker, then by rule (4) the other side of the equation sums up to at most the rating of the busbar, and the busbar is protected.

Rule (4) would be enough to qualify ErikS's original configuration, and it is very simple to state. So I think it would be both useful in the real world and as safe as rule (3).

What do you think?

Cheers, Wayne

I think the rules as written are fine.

 

[jaggedben]

So keep in mind that thermal heating in panelboard busbars with current at or over their rating is not necessarily a thing that linearly follows Kirchoff's Law. Heating happens mostly where the contacts of each breaker plug or bolt onto the busbars. If you have maximum current flowing at multiple locations on the busbar, you have to know that the assembly can safely dissipate the heat from that without deforming.

Somewhere there's a John Wiles article where he describes UL testing of panelboards. Two breakers of the busbar rating are attached next to each other and the maximum current is passed through the assembly while the ambient temperature is raised to match the rating. (Paraphrasing from memory.) This is a good 'worst case scenario' for a panelboard being fed by one source in one location. It is not necessarily a good test for scenarios with maximum current flowing in multiple different places on the busbar. All of the current code rules prevent maximum busbar current from flowing in any location other than near the primary source connection. I think that unless you can find someone to fund comprehensive lab testing of various old and new panelboards in the kind scenarios you're talking about, you're unlikely to see significant change to the code.

 

[jaggedben]

I think I can demonstrate the issues with a simple example.

Consider a common 8-space panelboard. (No connection to or endorsement of that vendor.)

One is allowed to feed this panelboard with a 100A breaker and even use it as service equipment. However, no one would ever think it okay to feed it with two 100A source breakers in the top four spaces. If one then put two 100A load breakers in the bottom two spaces, then clearly the busbars in between could see 200A; not okay.

Now, is it really any better to put the two source breakerson the left side and the two load breakers on the right? Not really, because the issue isn't just how much current the busbar sees. The issue is the thermal heating from four adjacent breaker connections operating at 100A. The assembly isn't more able to dissapate that resulting heat in the second scenario. The difference in the amount of time it would take to damage the busbar at full load would be marginal between the two scenarios.

If one is just considering the busbar rating and Kirchoff's law, then top and bottom looks bad and side-by-side looks okay. But if one considers the real world physics of heat dissipation in a panelboard, they're nearly equally bad.

 

[wwhitney]

So keep in mind that thermal heating in panelboard busbars with current at or over their rating is not necessarily a thing that linearly follows Kirchoff's Law.

Right, I believe I follow that. That's why it's not sufficient simply to ensure that the current at every point on the busbar is less than the busbar rating.

Instead, both 2017 705.12(B)(2)(3)(a) and (c) ensure that the total of all currents flowing into the panel would be no greater than the busbar rating. I infer that enforcing that limitation is sufficient thermal protection.

My proposed revision to 705.12(B)(2)(3)(c) still enforces that same limitation, that the total of all the currents flowing into the panel would be no greater than the busbar rating. So I don't see how there's any heating issue. For any thermal scenario (with specific current inflows and outflows) that can result from a breaker arrangement that complies with my proposed revision, I can find a breaker arrangement complying with 705.12(B)(2)(3)(c) that would allow the same thermal scenario. The proposed expansion allows some additional breaker configurations where it is not possible to simultaneously load all the breakers at their full rating, because of Kirchoff.

Cheers, Wayne

 

[wwhitney]

Here's an example of what I mean, with small numbers to keep it simple. Suppose you have a panel with a 2A busbar, a 2A main breaker, and (3) 1A breakers, where each 1A breaker could be a load or a source. Is it physically possible to fully load each breaker?

Answer: no. At any given time, call the signed current on the main breaker M, and the signed currents on the other three breakers A, B, and C.

Kirchoff says M + A + B + C = 0. Fully loading each breaker would mean M would be +/- 2 and A, B, and C would be +/- 1. So the Kirchoff equation would be even + odd + odd +odd = 0. That's a contradiction, meaning it is not possible to fully load each breaker.

In fact, as I briefly described earlier, it is not possible for more than 2A to flow into the bus.

Cheers, Wayne

 

[ggunn]

Right, I believe I follow that. That's why it's not sufficient simply to ensure that the current at every point on the busbar is less than the busbar rating.

Instead, both 2017 705.12(B)(2)(3)(a) and (c) ensure that the total of all currents flowing into the panel would be no greater than the busbar rating. I infer that enforcing that limitation is sufficient thermal protection.

My proposed revision to 705.12(B)(2)(3)(c) still enforces that same limitation, that the total of all the currents flowing into the panel would be no greater than the busbar rating. So I don't see how there's any heating issue. For any thermal scenario (with specific current inflows and outflows) that can result from a breaker arrangement that complies with my proposed revision, I can find a breaker arrangement complying with 705.12(B)(2)(3)(c) that would allow the same thermal scenario. The proposed expansion allows some additional breaker configurations where it is not possible to simultaneously load all the breakers at their full rating, because of Kirchoff.

Cheers, Wayne

Two questions:

One, without delving too deeply into the details, can you tell me whether the adjustment to the code you are suggesting would make it more, or less, restrictive than the existing code? I am in favor of taking the more conservative approach even if it costs more, so I would hope sharper minds than mine would thoroughly vet a code change that would make it more permissive. I have no problem complying with the code as it stands.

Two, are you going to formally pursue a change to the NEC, or is this just idle musing on an internet forum? Simply posting opinions on what oughta be is commonplace but it's all smoke with no fire.

 

[jaggedben]

[Responding to Wayne]

OK. It seems to me that the most concise way to state your proposal would be:
- The sum of the ratings of overcurrent devices protecting any loads, minus the smallest device, shall not exceed the rating of the busbar.
- 125% of inverter source outputs shall not exceed the rating of the busbar
- The main supply shall not exceed the rating of the busbar

Now, another thing to keep in mind: this code section requires a warning label that precisely tells following electrical personnel what requirement they have to keep. I have one on my desk. It's 3"x4" with 23 words on it in 1/4" high lettering. What I typed above, when added to 'Warning: Equipment Fed by Multiple Sources,' is 53 words and a lot more confusing. hmy:

 

[wwhitney]

One, without delving too deeply into the details, can you tell me whether the adjustment to the code you are suggesting would make it more, or less, restrictive than the existing code?

Slightly more permissive. It would allow the configuration ErikS posted in this thread, which is what got me started thinking more about this.

Two, are you going to formally pursue a change to the NEC, or is this just idle musing on an internet forum? Simply posting opinions on what oughta be is commonplace but it's all smoke with no fire.

Yes, I definitely plan to. It's too bad I can't submit the 2023 public input now, apparently I have to wait for the 2020 code revision cycle to complete.

Cheers, Wayne

 

[wwhitney]

OK. It seems to me that the most concise way to state your proposal would be:

Here's my first draft in redline format:

705.12(B)(2)(3)(c) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, and excluding the rating of one overcurrent device of the smallest size present, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment displaying the following or equivalent wording:

WARNING: THIS EQUIPMENT FED BY MULTIPLE SOURCES. TOTAL RATING OF ALL OVERCURRENT DEVICES EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE AND EXCLUDING THE SMALLEST OVERCURRENT DEVICE SHALL NOT EXCEED AMPACITY OF BUSBAR.

I seem to have not yet clearly and simply explained the math behind the above, which I obviously need to do in order to get a change accepted. It's a combinatorial argument, sort of an odd-even thing, the one extra breaker the above rule allows can't actually cause more current to flow into the panel (compared to the current 705.12(B)(2)(3)(c)) unless there were a second extra breaker to allow more current to flow out of the panel.

Cheers, Wayne

 

[ggunn]

Slightly more permissive. It would allow the configuration ErikS posted in this thread, which is what got me started thinking more about this.

Thanks. I'll comply with it if it gets implemented before I retire.