125% factor in 705.12

[Fred B]

My understanding is the Enhanced Content is suppose to help clarify some aspect of the code. Maybe.

enhanced content from 2023- 705.12(B) explanation:

ENHANCED CONTENT
The required ampacity of a feeder or bus connected to the interactive inverter is based on the inverter output circuit current, rather than on the overcurrent protective device (OCPD) in the inverter. Except where 705.12(A)(2)(b) and (B)(3) apply, the required conductor ampacity is determined by adding the ampacity of the primary OCPD protecting a busbar or feeder and 125 percent of inverter output current.

Where a tap is made to a feeder supplied by the inverter and the normal source, the calculated sum is used as the rating of the overcurrent device to determine the ampacity of the tap conductors in 240.21(B).

Unlike in service equipment where the number or rating of overcurrent devices is not limited, 705.12(B)(3) places a limit on panelboard overcurrent devices. The sum of the ratings of all overcurrent devices (excluding the main overcurrent device) supplying and/or being supplied by the panelboard is limited to the busbar rating. In addition, the main overcurrent device also must be limited to the ampacity of the busbar.



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NFPA70 705.12 enhanced content

 

[don_resqcapt19]

Enhanced content is the same as Handbook commentary and not an official interpertation of what the rules say. It is only the opinion of NFPA paid staff.

 

[Fred B]

Enhanced content is the same as Handbook commentary and not an official interpertation of what the rules say. It is only the opinion of NFPA paid staff.

Right, but can help explain some of the codes. No different than any interpretation by anyone including forum namesake. Even intent from code panel discussion narrative is open to debate. All the added content just helps in the debate process, and anyone can take it for whatever level of "authority" one wants to give it, until AHJ makes a judgement. Even then it can be fought or attempted to get it changed.

 

[wwhitney]

IOW, I think you would need to make it clear that the exception applies to OCPDs that are in the same equipment that is being qualified under a particular part of 705.12, and I don't think you've done that.

Why does it matter where the OCPD is located? Any OCPD between the power source and the bus will limit current to the bus, so any intervening OCPD is fair game, and I wrote it as such. [In fact, that's an obviously unnecessary limitation in the current wording of (2023 NEC) 705.12(B)(3), the sum of breakers rule.]

In other words, I see no technical reason to say that either your example in post #10 or the diagram in post #5 should be considered to change the worst case bus loading compared to what the 2023 NEC currently allows. [It does change the possible bus loading during normal operation, but I'm thinking various failure modes control.]

But should the 'rating of [that overcurrent device' be permitted to be 'used' to qualify the existing panelboard? I would say not.

Why not? It was allowed in the 2011 NEC (certainly the post #5 case, and I would argue the post #10 case as well). I don't believe the intention behind the 2014 NEC change was to rule out this case; however the 2014 First Draft says the change came from the CMP and doesn't discuss the reasons for it.

BTW, the application of this change to the 120% rule has been the focus so far, but there are 4 other usages of a 125% factor in 705.12 currently. The proposed PI is obviously fine for the two cases in 705.12(A) and for 705.12(B)(1). I see no reason that 705.12(B)(2) and 705.12(B)(4) need special treatment in this regard.

Cheers, Wayne

 

[don_resqcapt19]

Right, but can help explain some of the codes. No different than any interpretation by anyone including forum namesake. Even intent from code panel discussion narrative is open to debate. All the added content just helps in the debate process, and anyone can take it for whatever level of "authority" one wants to give it, until AHJ makes a judgement. Even then it can be fought or attempted to get it changed.

or anyone posting here...far too many times the handbook has been totally off base in the commentary over the years.

 

[tortuga]

705 should not deal with feeder/service ampacity at all, that should be done in 215, or 230. Taps in 240.
215 and 230 should reference 705.28 for a 'output circuit' (supply) calculation (as opposed to a load calc) or move that to a new part of 220.
It would be preferable if other articles also used the term 'output circuit' like 445, 701 and 702 and standardize the sizing of the conductors from a electrical source to their first disconnect.

I'd say delete all of the following in the [2023] 705:
705.8 licensure / qualifications are not in the scope of the NEC (see 90.2).
705.11(D) already covered by 230 (see 90.3).
705.11(E) already covered by 250 (see 90.3).
705.11(F) move to 230 and delete.
705.12(A) as this is already covered in other parts of the code (see 90.3).
Delete the 125% from 705.12(B)(1). Temp rating are covered in 110.14(C) add a FPN to that.
Delete the 125% from 705.12(B)(4). Temp rating are covered in 110.14(C) add a FPN to that.
705.25 is already covered in chapter 3 (90.3).
705.28(B) can go also as there is no 'where not otherwise required', your connecting to either a feeder or a service conductor.
And no need to reference 110.14 as all of 110 is still required (90.3).
705.30 already covered by 240 9 (see 90.3).
705.45(B) change to 3 phase to 'polyphase', not just 3 phase out there.

 

[jaggedben]

Why does it matter where the OCPD is located? Any OCPD between the power source and the bus will limit current to the bus, so any intervening OCPD is fair game, and I wrote it as such.

The OCPD doesn't limit current to the bus, the inverter does. So conversely, why should an OCPD somewhere else determine how much current the bus is allowed to take?

Why not? It was allowed in the 2011 NEC (certainly the post #5 case, and I would argue the post #10 case as well).

As I said above, for me appealing to the 2011 NEC is just not a compelling argument. The 2014 was a big improvement in many ways.

I don't believe the intention behind the 2014 NEC change was to rule out this case; however the 2014 First Draft says the change came from the CMP and doesn't discuss the reasons for it.

It seems to me that it may have been exactly the intention to rewrite the rules in such a way that the allowed currents (for the typical case) didn't change.

BTW, the application of this change to the 120% rule has been the focus so far, but there are 4 other usages of a 125% factor in 705.12 currently. The proposed PI is obviously fine for the two cases in 705.12(A) and for 705.12(B)(1). I see no reason that 705.12(B)(2) and 705.12(B)(4) need special treatment in this regard.

Yes, and I think your original question of why the 125% factor is in there is still apt. I'm just not making sense of your PIs as a way to deal with it.

 

[wwhitney]

Yes, and I think your original question of why the 125% factor is in there is still apt. I'm just not making sense of your PIs as a way to deal with it.

I've concluded it has to be a 125% continuous use factor, a carryover from the OCPD sizing rules for the reason that, as you say, the allowances for the simple case of one inverter and one regular OCPD sized at exactly 125% of the inverter output current would be unchanged by the 2011 to 2014 rewrite.

The OCPD doesn't limit current to the bus, the inverter does. So conversely, why should an OCPD somewhere else determine how much current the bus is allowed to take?

The conventional approach to bus protection, at least for panelboards, is via OCPD, e.g. 408.36, or 705.12 up through the 2011 NEC. Apparently in the 2014 NEC we started to trust power sources like inverters enough so that we can rely on their maximum output current rating instead of the OCPD rating. But that doesn't mean that an OCPD wouldn't also protect the bus. If the OCPD rating is less than 125% of the inverter output rating, seems like we should be able to take credit for that protection.

Consider 408.36 Exception 2 (Exception 1 in the 2023 NEC), which allows two OCPD to protect a panelboard of up to 42 spaces if the sum of their ratings does not exceed the panelboard rating, and contrast that to 705.12(B)(1), where the sum of the power source ratings does not exceed the rating of the busbar. For the case of 100% rated breakers and just two sources, should 705.12(B)(1) be more stringent that 408.36 Exception 2? That makes no sense.

The same principle applies to all of 705.12, where it uses a 125% factor, it should have an exception for 100% rated OCPD.

Cheers, Wayne