705.12 (D) question

[jaggedben]

Another problem with your language is that seems to allow the sources, including the one excluded device, to add up to more than the rating of the busbar. This violates other code sections but would surely be confusing to many.

I also have to say, ErikS' example notwithstanding, I can't think of a situation where this would have ever helped me out. Usually the smallest breaker in a mixed use residential panel is going to be 15 or 20A, not enough to make much difference.

 

[wwhitney]

Another problem with your language is that seems to allow the sources, including the one excluded device, to add up to more than the rating of the busbar.

So does 705.12(B)(2)(3)(b), and it actually allows the total inflows to the panel to exceed the busbar rating, unlike my proposed revision to 705.12(B)(2)(3)(c).

I also have to say, ErikS' example notwithstanding, I can't think of a situation where this would have ever helped me out.

Simplest example I can come up with is a 125A AC combiner panel, (6) 20A breakers for inverters, you need another 15A or 20A breaker for some monitoring equipment. At present, you'd have to upsize to a physically much larger 200A panel under 705.12(B)(2)(3)(c), while my proposed change recognizes that the 125A bus could never be overloaded by the addition of the one extra breaker.

Cheers, Wayne

 

[wwhitney]

Question on 2017 705.12(B)(2)(3)(c) as currently written:

Say you have a 100A AC combiner panel for (6) inverter output circuits rated at 13A. 125% * 13A = 16.25A, so you use (6) 20A breakers. Is this configuration:

(a) disallowed under 705.12(B)(2)(3)(c) because, unlike the other subsections of 705.12(B)(2)(3), subsection (c) does not mention 125% of the power source output currents and only references the ratings of the breakers. 6 * 20A = 120A > 100A.
(b) allowed because 705.12(B)(2) states that 125% of the power source output current "shall be used in ampacity calculations for the following" and 705.12(B)(2)(3)(c) is a subsection of 705.12(B)(2). 6 * 16.25A = 97.5A < 100A.

Cheers, Wayne

 

[jaggedben]

So does 705.12(B)(2)(3)(b), and it actually allows the total inflows to the panel to exceed the busbar rating, unlike my proposed revision to 705.12(B)(2)(3)(c).

I didn't mean to include primary source, so they are not the same. A combiner panel needs to be rated at 125% of total inverter output; your language seems to allow exclusion of one inverter circuit from the calc. Inverters are much more likely than loads to all simultaneously run at maximum current.

Simplest example I can come up with is a 125A AC combiner panel, (6) 20A breakers for inverters, you need another 15A or 20A breaker for some monitoring equipment. At present, you'd have to upsize to a physically much larger 200A panel under 705.12(B)(2)(3)(c), while my proposed change recognizes that the 125A bus could never be overloaded by the addition of the one extra breaker.

Yes easy to describe an example but my point is I've installed many hundreds of residential systems and never run into a problem like that. Also the 200A panel might cost me $75, which would be annoying, but not a barrier to project completion on a 24kW system where that cost is about one tenth of one percent of contract price.

 

[wwhitney]

A combiner panel needs to be rated at 125% of total inverter output; your language seems to allow exclusion of one inverter circuit from the calc. Inverters are much more likely than loads to all simultaneously run at maximum current.

Yes, my proposed change to 705.12(B)(2)(3)(c) would allow the exclusion of one inverter circuit for determining whether busbar is protected. Not being too familiar with all of 690 and 705, I would expect there is another section that would require sizing the busbar based on 125% of the total inverter output, even just some generic "sufficient ampacity for the load to be served" language. Since 705.12(B)(2)(3)(c) doesn't distinguish inverter circuits from loads, I think it is enough for 705.12(B)(2)(3)(c) to focus on ensuring the busbar is protected, and leave the case of all inverter circuits to that other section. If necessary, an informational note referring to that section could be included.

As to my example of avoiding a 200A combiner panel, I meant that space constraints might be an obstacle to a 200A combiner panel, not cost. But I agree, usually there is room to just put in something bigger or reconfigure the connectivity to comply with the current rules.

Cheers, Wayne